Effects of dietary C18 PUFA-rich oils on rumen function, growth performance, nutrient digestibility, ruminal lipid metabolism and milk and meat quality in ruminants: a review

1 Introduction

Dietary lipid supplementation is an important strategy for nutritional regulation in ruminants. Among these, oils rich in C18 polyunsaturated fatty acids (C18 PUFAs) have attracted sustained attention in the field of ruminant lipid nutrition because of their wide availability, high energy density, and potential to modify the fatty acid composition of milk and meat (Palmquist and Jenkins, 2017). The major plant-derived C18 PUFAs commonly found in ruminant diets are linoleic acid (LA) and α-linolenic acid (ALA), and related research has therefore mainly focused on oils rich in these two fatty acids.

Unlike monogastric animals, dietary lipids ingested by ruminants undergo lipolysis and biohydrogenation mediated by rumen microorganisms before entering the small intestine. Therefore, only a portion of dietary LA and ALA flows out of the rumen in an untransformed form, whereas the remainder undergoes varying degrees of transformation, forming various fatty acid intermediates and end products and thereby altering the composition and flow of fatty acids entering the post-ruminal digestive tract (Toral et al., 2024). At the same time, oil supplementation may also alter the rumen microbial community and fermentation processes, thereby affecting nutrient digestion, production performance, and the composition and quality of milk and meat products (Palmonari et al., 2024). Therefore, the actual nutritional effects of C18 PUFA-rich oils depend not only on the amount and composition of fatty acids supplied but also on the combined effects of processes such as ruminal microbial transformation, post-ruminal digestion and absorption, and tissue metabolism.

Existing reviews and meta-analyses have examined the effects of unsaturated fatty acids on rumen fermentation, methane emissions, production performance, or milk and meat quality, but most have focused on only one of these aspects (Ibrahim et al., 2021; Sun et al., 2022; Zhang et al., 2024; Xin et al., 2025). Comprehensive reviews that focus on oils rich in LA or ALA and simultaneously cover rumen function, animal production, and milk and meat products remain limited. Furthermore, existing studies differ considerably in terms of animal species and physiological stage, basal diet, supplementation level, and oil processing or protection form, which increases the difficulty of comparing and comprehensively interpreting the study results and also makes it difficult to clarify the effects of C18 PUFA-rich oils under different conditions and the sources of variation among studies. Accordingly, this review aims to comprehensively compare the effects of oils rich in LA or ALA on rumen microorganisms and fermentation, nutrient digestibility, production performance, and the fatty acid composition and related quality characteristics of milk and meat in ruminants. At the same time, by considering the ruminal biohydrogenation process, it explores the relationship between dietary oils and changes in the fatty acid composition of milk and meat, and analyzes the major factors influencing differences in responses among studies and the conditions under which beneficial or adverse effects are more likely to occur, with the aim of providing a theoretical basis and practical reference for optimizing lipid nutrition in ruminants.

2 Methodology

The literature search for this narrative review was conducted in the Web of Science Core Collection, Scopus, and PubMed databases. The search period covered the relevant literature indexed in each database from January 2010 to 12 February 2026. In accordance with the specific search requirements of each database, terms related to C18 PUFAs and relevant oil sources, ruminants, dietary supplementation, rumen microbiota and fermentation, nutrient digestibility, production performance, biohydrogenation, and milk and meat quality were combined and searched in the title, abstract, keyword, or topic fields, as applicable. The main search terms included “linoleic acid,” “alpha-linolenic acid,” “C18 PUFA,” “soybean oil,” “corn oil,” “sunflower oil,” “safflower oil,” “grapeseed oil,” “linseed oil,” “flaxseed oil,” “ruminant,” “cattle,” “sheep,” “goat,” “diet,” “supplementation,” “rumen microbiota,” “ruminal fermentation,” “biohydrogenation,” “digestibility,” “production performance,” “milk composition,” “milk fatty acid composition,” “meat quality,” and “meat fatty acid composition.” Boolean operators and search fields were adapted for each database. In addition to the database searches restricted to the specified publication period, a supplementary targeted search was conducted for a limited number of original studies published before 2010 that were highly relevant to the scope of this review. These earlier studies were included only when they provided important and otherwise difficult-to-replace evidence for the comparison and interpretation of subsequent research findings.

This review primarily included peer-reviewed original research, comprising in vivo studies in ruminants and in vitro studies conducted using rumen inoculum derived from ruminants. Eligible studies were required to evaluate oils rich in LA or ALA and to report at least one of the following categories of outcomes: rumen microbial composition, ruminal fermentation, nutrient digestibility, production performance, biohydrogenation, milk yield and composition, meat quality, or the fatty acid composition of milk and meat. For studies that compared multiple lipid sources, only the results for linoleic acid- or α-linolenic acid-rich oil treatments that fell within the scope of this review, together with those of the corresponding control or comparator treatments, were extracted and synthesized. Studies involving non-ruminant species, that did not evaluate supplementation with eligible oils, that did not report the relevant outcomes specified above, and non-original publications such as reviews and meta-analyses were excluded. The Web of Science Core Collection, Scopus, and PubMed searches identified 1, 924, 1, 755, and 634 records, respectively, yielding a total of 4, 313 records. After the records were merged and duplicates removed, 2, 590 records underwent title and abstract screening, followed by full-text assessment of potentially eligible articles. Ultimately, 39 original studies that met the inclusion criteria—including a small number of studies published before 2010 that were identified through supplementary targeted searches—were included in the narrative synthesis (Figure 1). Therefore, this review focuses primarily on oils rich in LA or ALA and comprehensively synthesizes their effects on rumen microorganisms and fermentation, biohydrogenation, nutrient digestion, production performance, and the fatty acid composition and related quality characteristics of milk and meat. Based on existing studies, it also discusses the possible roles of oil-related, diet-related, and animal-related factors in explaining differences in study results.

3 Structural characteristics, oil sources, and metabolic fate of C18 PUFAs

Based on the number of carbon–carbon double bonds, fatty acids can be classified into saturated fatty acids (SFAs) and unsaturated fatty acids (UFAs), with the latter further divided into monounsaturated fatty acids (MUFAs; one double bond) and PUFA (two or more double bonds). C18 polyunsaturated fatty acids are a class of fatty acids containing 18 carbon atoms and two or more carbon–carbon double bonds. Natural C18 PUFAs mainly include LA and ALA, whose double bonds are predominantly in the cis configuration and are typically methylene-interrupted. Because the first double bond from the methyl end occurs at carbon 6 in LA and carbon 3 in ALA, they are classified as n-6 and n-3 PUFAs, respectively, and are the parent fatty acids of their respective series (Ratnayake and Galli, 2009).

Both LA and ALA are widely present in vegetable oils, but the predominant C18 PUFAs differ markedly among oils. Conventional soybean oil, corn oil, sunflower oil, safflower oil, and grape seed oil typically contain LA as the predominant PUFA, whereas flaxseed oil typically contains ALA as the predominant PUFA (Alimentarius, 1999). The Codex Standard for Named Vegetable Oils (CXS 210-1999) provides product definitions and typical ranges of fatty acid composition for various edible vegetable oils and serves as an important reference for understanding the typical fatty acid composition of different oils. The fatty acid composition of a given vegetable oil is not fixed but may be influenced by factors such as the variety or genotype of the raw material, growing conditions, and processing conditions (Vaisali et al., 2015). In particular, high-oleic soybean, sunflower, and safflower varieties may have fatty acid compositions that differ markedly from those of conventional varieties. Therefore, the actual supply of LA or ALA cannot be accurately determined from the name of an oil alone, nor can its nutritional effects be reliably predicted on this basis; such variation in fatty acid composition may also partly explain the inconsistent effects observed among studies.

The portion of dietary LA and ALA that escapes ruminal biohydrogenation and reaches the small intestine as unchanged LA or ALA can enter different metabolic pathways after digestion and absorption. These fatty acids can undergo β-oxidation and participate in energy metabolism, providing energy for physiological and productive processes such as maintenance, growth, and lactation (Mashek et al., 2002). They can also be re-esterified into lipids such as triglycerides and phospholipids, contributing to energy storage, cell membrane composition, and the formation of milk fat and body tissue lipids. In addition, LA and ALA can undergo a series of metabolic reactions, including elongation and desaturation, and serve as precursors for the synthesis of their respective long-chain PUFAs (Saini and Keum, 2018). Specifically, LA can be further converted into n-6 long-chain PUFAs such as arachidonic acid (ARA; C20:4 n-6), whereas ALA can be further converted into n-3 long-chain PUFAs such as eicosapentaenoic acid (EPA; C20:5 n-3), docosapentaenoic acid (DPA; C22:5 n-3), and docosahexaenoic acid (DHA; C22:6 n-3). PUFAs containing 20 or more carbon atoms are commonly referred to as long-chain polyunsaturated fatty acids (LC-PUFAs) (Ruiz-Lopez et al., 2015).

4 Effects of C18 PUFA-rich oils on rumen microbiota function

4.1 Rumen microorganisms

Although C18 PUFA-rich oils can exert inhibitory effects on certain rumen microorganisms, the direction and magnitude of the microbial community responses reported across studies have been inconsistent (Table 1). These discrepancies may arise from differences among microbial taxa in their sensitivity and tolerance to free PUFA and biohydrogenation intermediates (Maia et al., 2007). In addition, the inherent resilience and functional redundancy of the rumen microbial community may buffer against external perturbations (Weimer, 2015). Therefore, the effects of C18 PUFA-rich oils on the rumen microbial community are unlikely to be determined by any single factor, but may instead depend on the interaction between the extent of ruminal exposure to free PUFA and the fermentation environment established by the basal diet.

Table 1. Effects of dietary C18 PUFA-rich oils on rumen microbial composition.

Arrows indicate direction of the effect (↑ increase; ↓ decrease).

Some studies have found that, after supplementation with C18 PUFA-rich oils, the abundance or numbers of some fiber degradation-related microbial groups usually show a decreasing trend, whereas some soluble substrate-utilizing microbial groups may increase, thereby reshaping the structure of the rumen microbial community and being accompanied by changes in some metabolic functional characteristics (Sato et al., 2020; Shang et al., 2020). For example, Shang et al. (2020) reported that, after soybean oil was added to the diets of Holstein bulls, the numbers of rumen fungi, protozoa, and methanogens decreased, the activities of fiber degradation-related enzymes such as carboxymethyl cellulase, cellobiase, and xylanase decreased, and the numbers of Ruminococcus flavefaciens and Ruminococcus albus were also reduced. In contrast, α-amylase activity and the numbers of soluble substrate-utilizing microbial groups such as Prevotella ruminicola and Ruminobacter amylophilus increased. These findings suggest that C18 PUFA-rich oils may alter the relative dominance of fiber-degrading microorganisms and non-structural carbohydrate-utilizing bacteria, thereby affecting rumen fermentation patterns.

Notably, the effects of C18 PUFA-rich oils on rumen microbial community structure vary with their inclusion level, basal diet composition, and supplementation form. In general, higher supplementation levels of C18 PUFA-rich oils exert stronger inhibitory effects on fiber-degrading bacteria, methanogens, and protozoa than lower supplementation levels (Sato et al., 2020). This may be because increasing supplementation levels raise the ruminal load of free PUFA, thereby exerting stronger inhibitory effects on sensitive microorganisms. Previous studies have shown that the effects of long-chain unsaturated fatty acids on rumen microorganisms vary with fatty acid concentration and among microbial taxa, with certain fibrolytic bacteria being particularly susceptible (Maia et al., 2007). This inhibitory response may be associated with metabolic stress induced by PUFA within microbial cells. However, direct mechanistic evidence is currently derived mainly from a limited number of biohydrogenating bacterial strains. Therefore, whether this mechanism is widespread among other rumen microorganisms remains to be established (Maia et al., 2010). Moreover, the decrease in protozoal abundance may weaken interspecies hydrogen transfer between protozoa and their symbiotic methanogens, thereby indirectly inhibiting methanogenesis (Toyber et al., 2024).

By contrast, when C18 PUFA-rich oils are provided in a protected form, or when the basal diet contains a higher proportion of forage, their adverse effects on certain fibrolytic bacteria and biohydrogenation-related microorganisms may be attenuated. Protective treatment can, to some extent, limit the dissociation and release of unsaturated fatty acids in the rumen, thereby reducing direct contact between free PUFA and rumen microorganisms (Mitchell et al., 2023b; Gao et al., 2025). A higher proportion of forage in the basal diet may, on the one hand, provide a greater supply of fibrous substrates, thereby supporting certain fibrolytic bacteria and microbial groups involved in both hemicellulose degradation and biohydrogenation. On the other hand, it may promote the incorporation of branched-chain volatile fatty acids into bacterial branched-chain membrane lipids, which may help these microorganisms maintain membrane homeostasis and substrate-utilization capacity and thereby partially mitigate the adverse effects of PUFA on their growth and metabolism (Mitchell et al., 2023a).

It should be noted that current studies on the modulatory effects of C18 PUFA-rich oils on rumen microorganisms mostly rely on conventional qPCR or high-throughput 16S rRNA gene sequencing methods. The former is usually limited to the quantification of a small number of specific microbial groups or functional genes and is difficult to fully reflect changes in the overall microbial community; the latter mainly provides relative abundance information and cannot directly determine the absolute numbers of microorganisms or the causal relationships between their functions and host phenotypes (Tsilimigras and Fodor, 2016). Therefore, current interpretations of changes in the structure of the rumen microbial community after C18 PUFA-rich oil supplementation are still mostly based on associative evidence, and further studies are needed to validate these findings using more comprehensive omics approaches.

4.2 Ruminal volatile fatty acids and pH

4.2.1 Volatile fatty acids

The effects of C18 PUFA-rich oils on the host are largely manifested through rumen microbial metabolites, with volatile fatty acids (VFAs) being the most commonly used and representative indicators. Volatile fatty acids are the major end products of rumen microbial fermentation and can provide approximately 70% of the energy supply for the host (Bergman, 1990). Changes in their total concentration and composition can well reflect the direction of rumen fermentation and the host’s utilization status of energy substrates. In terms of metabolic function, propionate is the main gluconeogenic precursor in ruminants (Ren et al., 2019), whereas acetate is one of the important substrates for de novo milk fat synthesis (Urrutia and Harvatine, 2017). Therefore, changes in the ratio between these two VFAs are not only related to energy utilization efficiency but also closely associated with milk fat synthesis. Existing meta-analyses and multiple studies have shown that, after supplementation with C18 PUFA-rich oils, the ruminal VFA profile usually shows an increased molar proportion of propionate, a decreased molar proportion of acetate, and a reduced acetate-to-propionate ratio (A:P) (Table 2) (Kholif et al., 2016, Kholif et al., 2018; Hassanat and Benchaar, 2021; Signor et al., 2024; Xin et al., 2025). From a microbial perspective, this shift may be associated with the inhibition of some fiber-degrading bacteria, many of which produce acetate as one of their main fermentation products (Miller and Wolin, 1995; Hua et al., 2022), and with the enrichment of some propionate-producing bacteria (Figure 2). For dairy animals, this shift in fermentation pattern may, on the one hand, help increase the supply of gluconeogenic substrates and improve overall energy utilization efficiency (Aschenbach et al., 2010; Ren et al., 2019; Rathert-Williams et al., 2023); on the other hand, it may adversely affect de novo milk fat synthesis because of the reduced acetate supply (Homan et al., 2026).

Table 2. Effects of C18 PUFA-rich oils on ruminal VFA molar proportions in ruminants.

Arrows indicate direction of the effect (↑ increase; ↓ decrease; ↔ no significant change); “×”=interaction term.

Meanwhile, changes in VFA composition may also indirectly affect some meat quality traits by influencing whole-body energy metabolism and intramuscular fat deposition (Smith and Crouse, 1984; Liu et al., 2026). However, compared with the relatively well-established effects of acetate supply on milk fat synthesis (Urrutia and Harvatine, 2017; Homan et al., 2026), evidence regarding how shifts in ruminal VFA composition affect intramuscular fat deposition and meat quality in ruminants remains limited. Therefore, the effects of changes in ruminal VFA composition on product quality are generally more readily defined in dairy animals, whereas in meat-producing animals, such effects are more often manifested as indirect effects mediated by changes in energy metabolism and intramuscular fat deposition.

The effects of C18 PUFA-rich oils on ruminal fermentation are influenced by multiple factors (Table 2). A few studies, however, have reported that supplementation with C18 PUFA-rich oils increased the ruminal molar proportion of acetate or the A:P ratio, whereas the molar proportion of propionate remained unchanged or decreased (Nur Atikah et al., 2018; Zhang et al., 2019). For example, Zhang et al. (2019) found that, when corn oil was added to goat diets at 3% of dietary dry matter (DM), the ruminal molar proportion of acetate and the A:P ratio increased, whereas the molar proportion of propionate decreased. Meanwhile, the ruminal concentrations of dissolved H₂ and dissolved CH₄ decreased, whereas the proportions of SFAs and C18:0 increased, suggesting that corn oil may have enhanced PUFA biohydrogenation and shifted part of the reducing equivalents from methanogenesis toward fatty acid hydrogenation. The authors indicated that the low ruminal H₂ level associated with corn oil supplementation may make acetate-producing pathways accompanied by H₂ formation thermodynamically more favorable. At the same time, the increased relative abundance of Lachnospiraceae and a tendency for Butyrivibrio_2 to increase suggested that microbial groups related to PUFA biohydrogenation, hydrogen utilization, and potential reductive acetogenesis may have participated in this process.

In addition, the predominant C18 PUFA in a given oil may alter ruminal hydrogen utilization and VFA composition by affecting the profile of fatty acids released during lipolysis and their subsequent biohydrogenation (Kholif et al., 2016). Even when the same PUFA predominates, oils from different sources may exert different effects on ruminal lipid metabolism and VFA composition because of differences in their overall fatty acid profiles and physicochemical properties (Signor et al., 2024). The animal’s physiological stage and the duration of fat supplementation may also modify the effects of dietary fat on VFA composition over time by influencing feed intake, nutrient requirements, baseline ruminal fermentation, and the extent of microbial adaptation to fat supplementation (Signor et al., 2024). Feeding frequency may alter meal timing and size, causing fat and fermentable substrates to enter the rumen in a more concentrated or more evenly distributed manner throughout the day, thereby affecting VFA composition and its diurnal variation (Mirzaei-Alamouti et al., 2020).

Overall, although most studies and meta-analyses indicate that dietary supplementation with C18 PUFA-rich oils tends to increase the ruminal proportion of propionate, their effects on VFA composition are not fixed but vary with oil characteristics, the fermentation environment established by the basal diet, the animal’s physiological stage, and feeding management conditions.

4.2.2 pH

Ruminal pH is an important parameter reflecting the stability of the rumen fermentation environment and is also one of the most commonly used indicators when discussing subacute ruminal acidosis (SARA). For SARA induced by high-concentrate diets, the more critical issue is the persistent imbalance among VFA production, absorption, and buffering in the rumen, rather than a transient decrease in pH at a single time point. Under continuous monitoring conditions, a commonly used definition is that ruminal pH remains below 5.6 for more than 3 h/d, or below 5.8 for more than 5.4 h/d (Mao and Wang, 2025). Although some reviews have suggested that C18 PUFA-rich oils may help mitigate the risk of ruminal acidification to some extent, the available evidence does not support a consistent increase in ruminal pH, and some studies have observed no significant change in ruminal pH (Table 3) (Jenkins, 1993; Benchaar et al., 2012; Golder and Lean, 2024; Lima et al., 2025). However, under some conditions involving relatively high supplementation levels or diets high in concentrate and rapidly fermentable carbohydrates, dietary supplementation with C18 PUFA-rich oils may slightly increase ruminal pH (Hassanat and Benchaar, 2021; Bahramkhani-Zaringoli et al., 2022). One possible explanation is that greater PUFA exposure suppresses the activity of some ruminal microorganisms and impairs fiber digestion, thereby reducing the amount of fermentable organic matter actually fermented in the rumen and decreasing VFA production, ultimately leading to a higher pH (Mizrahi et al., 2021; Xie et al., 2025).

Table 3. Effects of dietary C18 PUFA-rich oils on ruminal pH and total VFA concentration in ruminants.

Arrows indicate direction of the effect (↑ increase; ↓ decrease; ↔ no significant change).

It should be noted that an increase in ruminal pH is not necessarily accompanied by a synchronous decrease in total volatile fatty acids (TVFA). In the studies of Bahramkhani-Zaringoli et al. (2022) and Gomes et al. (2021), soybean oil supplementation increased ruminal pH in lambs and goats, respectively, but TVFA did not show a marked decrease. This suggests that changes in pH may not be explained solely by a reduction in total acid load, but may also be related to sampling time, VFA absorption, rumen buffering capacity, and changes in other acidic products such as lactate. In contrast, Kholif et al. (2018) and Zhang et al. (2019) observed a decrease in ruminal pH and an increase in TVFA concentration after supplementing goat diets with flaxseed oil and corn oil, respectively, suggesting that supplementation with certain C18 PUFA-rich oil sources may also be accompanied by increased ruminal acid load and decreased pH under specific dietary conditions. Overall, C18 PUFA-rich oils may help alleviate the risk of ruminal acidification in some contexts, but current evidence is still insufficient to generalize them as a potential strategy for preventing SARA.

5 Effects of C18 PUFA-rich oils on growth performance and digestibility in ruminants

5.1 Growth performance

In many studies, dietary supplementation with C18 PUFA-rich oils did not markedly alter dry matter intake (DMI), average daily gain (ADG), or feed efficiency in ruminants (Figure 2) (Sharifi et al., 2018; Shang et al., 2020; Gao et al., 2022; Roskam et al., 2025). However, as the supplementation level of C18 PUFA-rich oils increased, some studies observed a linear decrease in either DMI or feed efficiency, or a negative quadratic response (Table 4) (Hassanat and Benchaar, 2021; Lima et al., 2024). For DMI, such changes may be related to increased ruminal propionate production and enhanced fatty acid absorption following supplementation with C18 PUFA-rich oils, as well as the resulting changes in hepatic oxidation signals and energy load, which may further affect satiety and feeding behavior in animals (Albornoz et al., 2023). In addition, the stimulatory effect of C18 PUFA-rich oils on intestinal cholecystokinin (CCK) secretion may also be involved. However, under different animal types and feeding conditions, there is still insufficient evidence regarding the extent to which these mechanisms participate in the regulation of feed intake. In contrast, changes in ADG and feed efficiency are more likely to be related to the efficiency with which the body utilizes ingested energy and nutrients. On the one hand, supplementation with C18 PUFA-rich oils can increase dietary energy density and, under certain conditions, may benefit growth performance by improving fatty acid digestibility (Boerman et al., 2015), enhancing energy utilization efficiency, and altering nutrient partitioning (Palmquist, 1994). On the other hand, C18 PUFA-rich oils supplemented at higher levels may reduce ADG and feed efficiency by suppressing rumen fermentation, decreasing fiber digestibility, and reducing the efficiency of fat absorption and utilization (Palmquist, 1991; Alstrup et al., 2015). It is worth noting that a few studies have found that, in young ruminants, especially during dietary transition, supplementation with C18 PUFA-rich oils can simultaneously increase DMI and ADG and improve feed efficiency (Bahramkhani-Zaringoli et al., 2022). Compared with adult animals, the rumen of young animals is not yet fully developed, and their diets usually contain a higher proportion of concentrates and are relatively less dependent on fiber fermentation (Ghaffari et al., 2025). Therefore, theoretically, their growth performance may be less constrained by the adverse effects of C18 PUFA-rich oils on rumen fermentation.

Table 4. Effects of dietary C18 PUFA-rich oils on growth performance and apparent digestibility in ruminants.

Arrows indicate direction of the effect (↑ increase; ↓ decrease; ↔ no significant change); “×”= interaction term; NS, not significant; FE, Feeding efficiency; FCR, Feed Conversion Ratio; F/G, Feed: Gain; aNDFom, neutral detergent fiber assayed with a heat-stable amylase and expressed exclusive of residual ash; NR, not reported.

Overall, in practical production, the effects of C18 PUFA-rich oils on growth performance should still be evaluated case by case, taking into account fatty acid type, supplementation level, animal stage, and dietary structure. The supplementation dose should be maintained within an appropriate range to avoid depressed feed intake while maintaining production benefits.

5.2 Apparent total-tract digestibility

Some studies have reported that dietary supplementation with C18 PUFA-rich oils can reduce the extent of complete biohydrogenation in the rumen and increase the flow of some unsaturated C18 fatty acids and their intermediates to the small intestine (Kalscheur et al., 1997; Duckett et al., 2002; Dewanckele et al., 2020). Compared with C18:0, these long-chain fatty acids are generally more favorable for digestion and absorption (Bionaz et al., 2020; Daley et al., 2020), and can promote the release of bile and pancreatic juice by stimulating CCK secretion, thereby improving fat emulsification, micelle formation, and lipolysis (Iqbal and Hussain, 2009; Guo et al., 2021). Therefore, supplementation with C18 PUFA-rich oils is commonly associated with increased ether extract (EE) digestibility (Table 4) (Figure 2) (Nur Atikah et al., 2018; Pi et al., 2019; Shang et al., 2020; Bahramkhani-Zaringoli et al., 2022). In addition, the increase in EE digestibility may also be related to an increase in the proportion of truly digestible lipids in the diet. However, as fat intake continues to increase, the capacity of the small intestine to digest and absorb fatty acids may gradually approach saturation, leading to a decline in marginal true digestibility. Consequently, EE digestibility may show an initial increase followed by a decrease in some studies, potentially diminishing the nutritional benefits of further increasing the level of fat supplementation (Palmquist, 1991; Bionaz et al., 2020; Hassanat and Benchaar, 2021; Lima et al., 2025).

With respect to fiber digestibility, findings from existing studies are inconsistent. In some studies, neutral detergent fiber (NDF) digestibility remained stable or even increased after dietary supplementation with C18 PUFA-rich oils (Emami et al., 2016; Pi et al., 2019; Roskam et al., 2025), whereas other studies showed that C18 PUFA-rich oils reduced NDF digestibility; in some dose-response trials, this inhibitory effect became stronger as supplementation levels increased (Table 4) (Sharifi et al., 2018; Hassanat and Benchaar, 2021). This variation is related not only to the supplementation level of C18 PUFA-rich oils but also to the fatty acid content and composition of the basal diet (Cattle et al., 2021). The basal diet and supplemental oils collectively determine the total dietary fatty acid level and composition, thereby affecting the ruminal load of free C18 PUFAs.

Diet structure and the supply of effective fiber further modulate the effects of this fatty acid load on ruminal fiber degradation. When oil supplementation results in a high total dietary fatty acid level, particularly when relatively large amounts of unprotected C18 PUFAs are released in the rumen, or when the supply of effective fiber is insufficient, the inhibitory effects of free unsaturated fatty acids on fiber-degrading bacteria and the coating of feed particles by oils may interfere with microbial attachment and substrate degradation, thereby impairing ruminal fiber degradation (De Beni Arrigoni et al., 2016; Shang et al., 2020; Sears et al., 2024). In contrast, when the ruminal load of free C18 PUFAs is relatively moderate and the supply of effective fiber is adequate, the functional redundancy of the microbial community may, to some extent, buffer the disturbance to fiber degradation (Weimer, 2015). Meanwhile, some fiber that is not fully degraded in the rumen may continue to be fermented in the post-ruminal digestive tract, thereby helping to maintain relatively stable whole-tract fiber digestibility. The study by Ueda et al. (2003) also suggested that linseed oil supplementation reduced the ruminal degradability of NDF and acid detergent fiber (ADF) in dairy cows under high-concentrate conditions, whereas the opposite trend was observed under high-forage conditions, indicating that basal diet structure can alter the direction of the effect of the same oil supplementation level on ruminal fiber degradation. Meanwhile, after linseed oil supplementation, the differences between ruminal and total-tract digestibility of NDF and ADF were markedly narrowed, further suggesting that post-ruminal fermentation may partially compensate for the reduction in ruminal fiber degradation. However, when the suppression of ruminal fiber degradation exceeds this compensatory capacity, whole-tract NDF digestibility may still decline. Therefore, when evaluating the effects of C18 PUFA-rich oils on fiber digestion, ruminal degradation should be distinguished from whole-tract digestion, and the assessment should take into account the actual ruminal load of unsaturated fatty acids and basal diet structure.

Compared with the effects on EE and NDF digestibility, the effects of C18 PUFA-rich oils on crude protein (CP) digestibility are more difficult to summarize. In available studies, most trials did not observe marked changes in CP digestibility after supplementation with C18 PUFA-rich oils (Kholif et al., 2016; Van Gastelen et al., 2017; Bayat et al., 2018; Pi et al., 2019; Zhang et al., 2019; Shang et al., 2020; Bahramkhani-Zaringoli et al., 2022), whereas a few studies reported increases or decreases (Kholif et al., 2018; Nur Atikah et al., 2018; Mirzaei-Alamouti et al., 2020; Yousefinejad et al., 2021; Lima et al., 2024), indicating that C18 PUFA-rich oils do not have a stable and consistent direction of effect on CP digestibility (Table 4). Available evidence suggests that this discrepancy may be partly related to the effects of C18 PUFA-rich oils on the structure and metabolic activity of the rumen microbial community, which may in turn alter microbial protein synthesis efficiency and nitrogen flow to the small intestine (Castillo-Lopez and Domínguez-Ordóñez, 2019; Sun et al., 2022). When the load of free C18 PUFA in the rumen is high, such disturbances may be more likely to become evident.

In addition, the effects of C18 PUFA-rich oils on DMI and fiber degradation may also be accompanied by changes in rumen digesta retention time and passage rate, thereby further affecting the extent of feed protein degradation in the rumen and the balance between microbial protein synthesis and outflow (Krizsan et al., 2010; Bettero et al., 2013). However, this effect, whereby changes in fiber degradation and digesta passage further affect CP digestibility, may not be consistent across all oil sources and dietary conditions. Overall, current evidence remains insufficient to clearly distinguish the relative contributions of ruminal protein degradation, microbial protein synthesis and outflow, and subsequent intestinal digestion and absorption to changes in CP digestibility. Therefore, the related mechanisms still need to be further verified by integrating indicators such as ruminal nitrogen metabolism, microbial protein synthesis, and small-intestinal nitrogen flow.

6 Effects of C18 PUFA-rich oils on ruminal biohydrogenation and the fatty acid composition of milk and meat in ruminants

Biohydrogenation (BH) is a key process linking dietary intake of C18 PUFA-rich oils with changes in the fatty acid composition of milk and meat. After LA and ALA enter the rumen, they undergo microbial transformation to varying degrees. This process not only affects their post-ruminal flow but also affects the entry of some biohydrogenation intermediates into the small intestine, where they further contribute to changes in the fatty acid composition of milk and meat (Dewanckele et al., 2020). Therefore, this section first outlines the ruminal BH processes of LA and ALA, and then discusses their effects on the fatty acid composition and related quality characteristics of milk and meat.

6.1 Biohydrogenation

After dietary lipids enter the rumen, they are first hydrolyzed by microbial lipases into free fatty acids (FFAs) and subsequently undergo BH to varying degrees (Bionaz et al., 2020). The ruminal BH pathways of different C18 PUFAs are not completely identical. In the typical trans-11 pathway, LA first undergoes isomerization of the cis-12 double bond to the trans-11 configuration, producing cis-9, trans-11 conjugated linoleic acid (CLA), which is then further reduced to trans-11 C18:1 and finally hydrogenated to stearic acid (C18:0) (Toral et al., 2024). α-Linolenic acid is usually first isomerized to cis-9, trans-11, cis-15 conjugated linolenic acid (CLnA), and then sequentially reduced to trans-11, cis-15 C18:2 and trans-11 C18:1, ultimately forming C18:0. As an important common intermediate of the two pathways, trans-11 C18:1 can be converted to cis-9, trans-11 CLA by Δ9-desaturase (stearoyl-CoA desaturase, SCD) in the mammary gland and peripheral tissues (Keating et al., 2006; Smith, 2024). Therefore, biohydrogenation not only reduces the post-ruminal proportion of dietary C18 PUFA but also determines which fatty acid intermediates can enter the small intestine and further affect the fatty acid composition of milk and meat (Figure 2).

When the rumen environment is relatively stable and the supplementation level is appropriate, LA and ALA increase the accumulation of trans-11 C18:1 and related CLA intermediates in the rumen, or their duodenal flow, through the trans-11 pathway, thereby further affecting the deposition of beneficial fatty acids such as cis-9, trans-11 CLA in milk and meat (Hess et al., 2008; Pi et al., 2019). However, when the free PUFA load in the rumen is high, or when it is combined with dietary conditions characterized by low physically effective fiber, high concentrate, or high rapidly fermentable carbohydrates, the BH pathway may shift from the trans-11 pathway toward the trans-10 pathway. Under these conditions, the LA-derived branch may be characterized by the accumulation of trans-10, cis-12 CLA and trans-10 C18:1, whereas the ALA-derived branch tends to form trans-10, cis-12, cis-15 CLnA and trans-10, cis-15 C18:2, and eventually converges on the formation of trans-10 C18:1 (Dewanckele et al., 2020; Toral et al., 2024). This pathway shift is associated with changes in the composition and metabolic activity of rumen biohydrogenation-related microbial groups. However, it remains difficult to clearly determine the roles and relative contributions of different candidate microbial groups under practical production conditions.

6.2 Milk yield, composition, and fatty acid composition

Available studies indicate that dietary supplementation with C18 PUFA-rich oils generally does not markedly alter milk protein or lactose percentages. Only a few studies have observed a decrease in milk protein percentage or an increase in lactose percentage (Van Gastelen et al., 2017; Kholif et al., 2018; Hassanat and Benchaar, 2021). Lactose is an important osmotic component regulating milk yield, and several studies have shown that dietary supplementation with C18 PUFA-rich oils is usually accompanied by increased ruminal propionate production (Kholif et al., 2016, Kholif et al., 2018; Vazirigohar et al., 2018; Hassanat and Benchaar, 2021; Signor et al., 2024; Xin et al., 2025). This may increase the supply of glucogenic precursors and provide some support for lactose synthesis and the maintenance of milk yield (Rigout et al., 2003). Therefore, in some studies, milk yield did not decrease markedly after supplementation with C18 PUFA-rich oils (Bayat et al., 2018; Mirzaei-Alamouti et al., 2020; Yousef et al., 2022), and some studies even observed an increase in milk yield (Table 5) (Kholif et al., 2016, Kholif et al., 2018; Boland et al., 2020).

Table 5. Effects of dietary C18 PUFA-rich oils on milk production, milk composition, and milk fatty acid profile in lactating ruminants.

Arrows indicate direction of the effect (↑ increase; ↓ decrease; ↔ no significant change); FW, fresh weight; NR, not reported.

In contrast, supplementation with C18 PUFA-rich oils usually decreases milk fat percentage (Kholif et al., 2016, Kholif et al., 2018; Van Gastelen et al., 2017; Boland et al., 2020; Yousef et al., 2022), which may be related to the accumulation of trans-10 fatty acid intermediates caused by enhancement of the ruminal trans-10 biohydrogenation pathway (Dewanckele et al., 2020). After these intermediates leave the rumen, enter the small intestine, and are absorbed, they can further affect mammary lipid metabolism. Among them, the effect of trans-10, cis-12 CLA is the most clearly established, and both infusion studies and studies on the expression of genes related to mammary lipid metabolism have confirmed its ability to inhibit milk fat synthesis (Peterson et al., 2002; Viswanadha et al., 2003; Guo et al., 2024). However, milk fat depression (MFD) cannot be fully explained by trans-10, cis-12 CLA alone, and trans-10 C18:1, trans-10, cis-15 C18:2, and other BH intermediates may also be involved (Dewanckele et al., 2020). In addition, the decrease in milk fat percentage may also be related to a shift in rumen fermentation toward increased propionate production. This shift may reduce the relative supply of precursors for de novo milk fat synthesis, such as acetate, thereby exerting a certain dilution effect on milk fat percentage when milk yield is maintained or increased. In contrast, Emami et al. (2016) and Manso et al. (2022) reported that dietary supplementation with linseed oil increased milk fat percentage in ewes and goats. Considering that DMI, milk yield, or milk trans-10, cis-12 CLA did not change markedly before and after oil supplementation in these two studies, the increase in milk fat percentage was more likely related to the increased uptake of long-chain fatty acids and the increased supply of preformed fatty acids to the mammary gland induced by C18 PUFA-rich oil supplementation (Boerman et al., 2015).

Taken together, existing studies and meta-analyses indicate a high degree of between-study heterogeneity in the effects of oil supplementation on milk fat percentage and absolute milk fat yield. Some meta-regression and dose–response analyses suggest that the supplementation levels of certain oils may be linearly or quadratically related to milk fat percentage or milk fat yield (Hassanat and Benchaar, 2021; Xin et al., 2025). However, both the response patterns and the supplementation levels at which the responses change direction differ among oils (Xin et al., 2025). Therefore, a single threshold for milk fat depression applicable across different C18 PUFA-rich oil sources and basal dietary conditions cannot currently be established (Mannai et al., 2016). When assessing the risk of milk fat depression in practice, the proportion of supplemental oil in dietary dry matter should not be used as the sole criterion; the extent of C18 PUFA release in the rumen and the basal dietary conditions should also be considered. In particular, when relatively large amounts of unprotected C18 PUFAs are released in the rumen and this occurs in combination with high starch fermentability, a high proportion of concentrate, or an inadequate supply of effective fiber, the risks of abnormal biohydrogenation and milk fat depression may increase (Jenkins and Harvatine, 2014; Dewanckele et al., 2020).

Changes in milk fatty acid composition depend simultaneously on the supply of exogenous C18 PUFA-rich oils, rumen biohydrogenation processes, and the status of mammary fatty acid synthesis. In general, short and medium-chain fatty acids are mainly synthesized de novo in the mammary gland, whereas C18 and longer-chain fatty acids are more often classified as preformed fatty acids and are mainly derived from fatty acids that are absorbed in the small intestine and then transported to the mammary gland through the blood (Taormina et al., 2024). Therefore, when the proportion of exogenous unsaturated C18 fatty acids and their biohydrogenation intermediates entering the small intestine and being taken up and utilized by the mammary gland increases, or when de novo fatty acid synthesis in the mammary gland itself is inhibited, the proportion of long-chain PUFAs in milk fat usually increases.

Existing studies generally indicate that dietary supplementation with C18 PUFA-rich oils can reduce the proportion of total SFAs in milk and increase some nutritionally important unsaturated fatty acids (UFAs), especially trans-11 C18:1, cis-9, trans-11 CLA, total CLA, and some PUFAs, thereby improving milk fatty acid composition. However, when the trans-10 biohydrogenation pathway is markedly enhanced, the resulting increase in trans-10, cis-12 CLA may also suppress SCD expression and reduce the endogenous conversion of trans-11 C18:1 to cis-9, trans-11 CLA in the mammary gland, thereby limiting, to some extent, the enrichment of cis-9, trans-11 CLA in milk fat (Perfield et al., 2006). With regard to different C18 PUFA sources, LA-rich oils are more likely to increase the proportion of n-6 PUFAs in milk, accompanied by an increase in the n-6/n-3 ratio (Kholif et al., 2016; Bayat et al., 2018; Yousef et al., 2022). However, if the oil is provided in a form that reduces the extent of ruminal biohydrogenation, the enrichment of biohydrogenation intermediates such as trans-11 C18:1 and cis-9, trans-11 CLA in milk fat may be attenuated, while more unhydrogenated LA may be retained in milk fat (Yousef et al., 2022). In contrast, ALA-rich oils are more favorable for increasing the proportion of n-3 PUFAs in milk and reducing the n-6/n-3 ratio (Table 5) (Emami et al., 2016; Van Gastelen et al., 2017; Manso et al., 2022).

In summary, dietary supplementation with C18 PUFA-rich oils usually decreases milk fat percentage, whereas its effects on milk yield and the percentages of milk protein and lactose are not as pronounced as those on milk fat percentage. In some cases, the decrease in milk fat percentage may be related to the enhancement of the ruminal trans-10 biohydrogenation pathway and the inhibitory effects of related fatty acid intermediates on mammary lipid synthesis. However, under conditions where an obvious trans-10 biohydrogenation shift or milk fat depression is not induced, supplementation with C18 PUFA-rich oils may also maintain or even increase milk fat percentage by increasing long-chain fatty acid intake and the supply of preformed fatty acids to the mammary gland. In terms of milk fatty acid composition, C18 PUFA-rich oils generally decrease the proportion of SFAs and promote the enrichment of certain UFAs in milk fat. The effects on milk fatty acid composition differ among oils, with LA-rich oils tending to increase the proportion of n-6 PUFAs and the n-6/n-3 PUFA ratio, whereas ALA-rich oils are more favorable for promoting the enrichment of n-3 PUFAs and reducing the n-6/n-3 ratio (Figure 2).

6.3 Meat fatty acid composition and related quality characteristics

Compared with studies on milk fatty acid composition and milk quality, relatively few studies have examined the effects of C18 PUFA-rich oils on the fatty acid composition and related quality characteristics of ruminant meat. This may be due to the high cost of slaughter trials, the complexity of tissue sampling, and the high variability in meat quality traits. Among the limited existing studies, most conventional meat quality traits showed no marked changes following oil supplementation, with only a few studies reporting changes in individual traits (Table 6). For example, Han et al. (2025) reported that supplementation with soybean oil increased the EE content of the Biceps brachii and Semimembranosus in goats and decreased the shear force of the Longissimus thoracis et lumborum. In addition, conventional linseed oil supplementation often showed no marked changes in some meat quality indicators, whereas microencapsulated linseed oil reduced muscle lightness (L*) value and shear force (Sun et al., 2021; Gao et al., 2022). Microencapsulation may influence meat color and tenderness by reducing the ruminal lipolysis of dietary lipids and the subsequent biohydrogenation of the released fatty acids, thereby increasing their post-ruminal availability and altering intramuscular fatty acid composition. This suggests that, even when the fatty acid source is the same, different protection methods may result in different effects on meat quality.

Table 6. Effects of dietary C18 PUFA-rich oils on meat fatty acid composition and related quality traits in ruminants.

Arrows indicate direction of the effect (↑ increase; ↓ decrease; ↔ no significant change); IMF, Intramuscular fat; WBSF, Warner-Bratzler shear force; TBARS, Thiobarbituric acid reactive substances; LT, Longissimus thoracis; BB, Biceps brachii; SM, Semimembranosus; LTL, Longissimus thoracis et lumborum; LL, Longissimus lumborum; LD, Longissimus dorsi; BF, Biceps femoris; NR, not reported.

In terms of muscle fatty acid composition, supplementation with C18 PUFA-rich oils at low to moderate levels (approximately 2%–4% of dietary DM) generally does not markedly increase total muscle fat content but can alter its fatty acid composition, with the direction of change depending largely on the oil type (Sharifi et al., 2019; Roskam et al., 2025). Among these oils, ALA-rich oils are generally more favorable for increasing the proportions of C18:3 n-3 and total n-3 PUFAs in muscle and reducing the n-6/n-3 ratio, showing a relatively clear n-3 fatty acid enrichment effect (Ebrahimi et al., 2014; Gao et al., 2022; Roskam et al., 2025). In contrast, LA-rich oils are generally more favorable for increasing the proportions of trans-11 C18:1, cis-9, trans-11 CLA, or total CLA in meat (Bessa et al., 2005; Sharifi et al., 2019; Han et al., 2025). In addition, supplementation with oils rich in either LA or ALA is generally accompanied by higher total PUFA and lower SFA proportions.

Furthermore, a study in which linseed oil gradually replaced sunflower oil showed that, as the replacement proportion increased, C18:3 n-3 and total n-3 LC-PUFAs increased in the longissimus muscle of lambs, whereas C18:2 n-6, n-6 LC-PUFAs, CLA, and the n-6/n-3 ratio decreased (Jerónimo et al., 2009). This suggests that ALA-rich lipid sources are more favorable for shifting meat fatty acid composition toward n-3 fatty acids, but may not necessarily be favorable for maintaining a higher CLA level. This study further estimated, using a quadratic regression model, that when both n-3 LC-PUFAs and CLA were considered, the maximum combined amount of the two occurred when 52% of sunflower oil was replaced by linseed oil.

Changes in fatty acid composition may further affect the oxidative stability and storage characteristics of meat. Theoretically, an increased proportion of UFA in muscle would increase the risk of lipid oxidation and may affect shelf life, flavor, and overall color (Domínguez et al., 2019). However, if the supplemented oil itself is rich in antioxidant components, these adverse effects may be partially offset. Sharifi et al. (2019) found that although grape seed oil supplementation increased the proportions of PUFAs, trans-11 C18:1, and cis-9, trans-11 CLA in the longissimus lumborum of lambs, the levels of thiobarbituric acid reactive substances (TBARS) in this muscle decreased after 12 d of storage, indicating improved oxidative stability. This phenomenon may be related to the high content of antioxidant compounds, such as polyphenols and vitamin E, in grape seed oil (Carmona-Jiménez et al., 2022). Similarly, a sunflower oil–soybean oil mixture combined with natural or synthetic antioxidants increased total CLA in the longissimus muscle without affecting the shelf stability of lamb meat during display (Olivier et al., 2024). Nevertheless, optimization of fatty acid composition does not necessarily translate into marked improvements in meat quality or sensory attributes. Gravador et al. (2020) reported that although protected linseed oil altered the fatty acid composition of the longissimus thoracis et lumborum, it had no marked effect on pH at 25 h postmortem (pH_25h) or consumer acceptance.

Overall, in ruminant meat studies, the most consistent effect of C18 PUFA-rich oils supplementation is the alteration of muscle fatty acid composition, especially in terms of n-3 PUFAs, CLA, PUFA/SFA ratio, and n-6/n-3 ratio, whereas its effects on meat quality indicators are relatively limited (Figure 2).

7 Practical feeding considerations for C18 PUFA-rich oils

Based on the results of the in vivo studies included in this review, a relatively conservative starting supplementation range of 2%–3% of dietary DM is recommended for unprotected C18 PUFA-rich oils (Ueda et al., 2003; Emami et al., 2016; Van Gastelen et al., 2017; Sharifi et al., 2019; Gao et al., 2022; Manso et al., 2022). Dose-response trials with linseed oil in lactating dairy cows further provide evidence regarding changes in production performance and nutrient digestibility when the supplementation level is increased from 2%–3% to 4% of dietary DM. In a mixed silage-based total mixed ration (TMR) with a forage-to-concentrate ratio of approximately 50:50, supplementation with linseed oil at 3% of dietary DM did not markedly affect DMI or apparent total-tract nutrient digestibility, whereas milk yield increased with increasing supplementation level (Benchaar et al., 2012). When the supplementation level was further increased from 3% to 4% of dietary DM, milk yield continued to increase, but fiber digestibility showed a noticeable decline, and the proportion of C18:3 n-3 in milk fat did not increase further. These results suggest that, under these dietary conditions, the marginal benefit of increasing linseed oil supplementation from 3% to 4% of dietary DM for further enriching milk fat with C18:3 n-3 was relatively limited, while fiber digestion may begin to be adversely affected. In another trial in which corn silage was the primary forage source, accounting for 58% of dietary DM, supplementation with linseed oil at 2% or 3% of dietary DM did not reduce DMI or apparent total-tract fiber digestibility, and milk yield increased; however, both milk fat percentage and milk fat yield decreased (Hassanat and Benchaar, 2021). When the supplementation level was further increased to 4% of dietary DM, DMI and fiber digestibility decreased, and milk yield returned to a level similar to that of the control. Therefore, supplementation levels approaching 4% of dietary DM should be used cautiously according to specific dietary conditions and should not be regarded as a generally applicable recommendation.

In addition, the supplementation level of C18 PUFA-rich oils should also be considered in relation to the total dietary fatty acid content. Taking dairy cows as an example, National Academies of Sciences, Engineering, and Medicine (2021) indicates that the fatty acid content of diets composed of conventional forages and grain concentrates is typically already close to 3% of DM. With additional oil supplementation, the total dietary fatty acid content should generally not exceed 7% of DM (Cattle et al., 2021). For early-lactation dairy cows, particularly when oil supplementation may suppress DMI, maintaining the total dietary fatty acid content below 5% of DM may be a more prudent approach. Therefore, the actual inclusion level of C18 PUFA-rich oils should be calculated based on the fatty acid content of the basal diet, the fatty acid content of the product, and changes in fatty acid supply resulting from ingredient substitution. The ruminal release characteristics of the oil, basal diet structure, and the animal’s physiological stage should then be considered to determine whether the corresponding supplementation level is appropriate. For calcium salts of fatty acids, microencapsulated oils, nanoemulsified oils, and other processed or protected oil products, the inclusion rates used for unprotected oils should not be applied directly. Instead, these products should be evaluated separately according to their actual fatty acid supply, together with the ruminal release or exposure characteristics of the fatty acids and their post-ruminal digestion and utilization.

While determining an appropriate supplementation level, oil sources should also be selected according to the intended goals for modifying the fatty acid composition of milk and meat. If the primary goal is to increase n-3 PUFAs in milk and meat and reduce the n-6/n-3 ratio, ALA-rich oils may be prioritized; if the primary goal is to increase trans-11 C18:1 and cis-9, trans-11 CLA, LA-rich oils may be considered. In practical feeding, monitoring priorities should also vary according to the animals’ production stage and production goals. For growing or finishing animals, DMI, ADG, and feed efficiency should be closely monitored. When PUFAs are markedly enriched in muscle, attention should also be paid to the oxidative stability, color stability, and shelf life of meat during storage; for feeding strategies that may increase the risk of oxidation, appropriate antioxidant measures may be taken when necessary. For lactating animals, particular attention should be paid to DMI, milk yield, milk fat percentage, and milk fat yield, and, where feasible, fiber digestion should also be evaluated using indicators such as NDF digestibility. If oil supplementation is followed by persistent reductions in feed intake, impaired fiber digestion, reduced production performance, or milk fat depression, the supplementation level should be reduced, and the oil form, total dietary fatty acid supply, and basal diet structure should be reassessed.

8 Future perspectives

Although numerous studies have examined the effects of C18 PUFA-rich oils on rumen microorganisms and fermentation, nutrient digestion, production performance, and milk and meat quality, individual studies generally address only some components of this sequence. Consequently, it remains difficult to determine the extent to which oil-induced ruminal changes can explain the subsequently observed responses in nutrient digestion, production performance, and milk and meat quality. This gap is particularly evident in fatty acid metabolism. Although some of the microorganisms involved in the biohydrogenation of LA and ALA and their metabolic functions have been identified, the relative roles of different microorganisms in each step of biohydrogenation and their interrelationships have not yet been systematically elucidated. Meanwhile, there is still a lack of continuous tracking and quantitative analysis of LA, ALA, and their biohydrogenation intermediates along the pathway from ruminal outflow, through host absorption, to incorporation into milk fat or deposition in muscle. Future studies should integrate multi-omics approaches, including metagenomics, metatranscriptomics, and targeted lipidomics, to identify candidate microbial taxa, key enzymes, and metabolic pathways involved in the biohydrogenation of LA and ALA, and progressively validate their functions through isolation and cultivation and in vivo interventions. On this basis, stable isotope tracing, omasal or duodenal fatty acid flow measurements, and kinetic modeling could be further combined to quantitatively track changes in the amounts and metabolic fates of C18 PUFAs and their metabolites throughout these processes.

Furthermore, to enhance the value of research findings in guiding production practice, future studies should not stop at determining whether a particular oil can produce the expected effects, but should also clarify how its effects vary with animal species and physiological stage, supplementation level, basal diet, and other factors, and identify potential adverse effects and the conditions under which they occur. To improve the comparability of results across studies, key conditions, including the actual amounts of LA and ALA supplied, the total dietary fatty acid level, oil composition and protection form, basal diet structure, and trial duration, should be fully reported. At the same time, fatty acid transformation in the rumen, post-ruminal flow, and whole-tract apparent digestibility should be clearly distinguished to avoid interpreting indicators from different digestive and transport processes as equivalent. For oil supplementation strategies that have shown application potential in short-term studies, long-term dose–response trials should also be conducted to determine whether their effects on nutrient digestion and production performance can be sustained, while simultaneously assessing potential adverse outcomes related to animal health, milk fat depression, and reduced oxidative stability of the products.

9 Conclusions

Overall, existing studies indicate that C18 PUFA-rich oils exert relatively consistent effects on the fatty acid composition of milk and meat in ruminants, whereas their effects on rumen microbial ecology and fermentation, nutrient digestion, feed intake, production performance, and conventional indicators of milk composition and meat quality are more condition-dependent. Among these oils, LA-rich oils are more often associated with the enrichment of trans-11 C18:1 and cis-9, trans-11 CLA in milk and meat, whereas ALA-rich oils are more favorable for increasing C18:3 n-3 and total n-3 PUFAs and reducing the n-6/n-3 ratio.

In practice, oil sources should be selected according to the intended goals for modifying the fatty acid composition of milk and meat, while supplementation levels should be determined by considering the total dietary fatty acid supply, the ruminal release characteristics of the oils, and effective fiber supply. Based on the evidence from the in vivo studies included in this review, 2%–3% of dietary DM may be considered a relatively conservative starting supplementation range for unprotected C18 PUFA-rich oils. However, when the supplementation level approaches 4% of dietary DM, the suitability of such a level should be evaluated in light of the specific dietary conditions and changes in feed intake, fiber digestion, and production performance. Therefore, a supplementation level approaching 4% of dietary DM should not be regarded as a generally applicable recommendation. For oil products provided in processed or protected forms, the above supplementation range may not necessarily apply; instead, these products should be evaluated separately based on their actual fatty acid supply, ruminal release characteristics, and post-ruminal digestion and utilization characteristics. In summary, the use of C18 PUFA-rich oils should aim to achieve the intended goals for modifying the fatty acid composition of milk and meat while also taking into account rumen function, nutrient utilization, and production performance.

Correction note

This article has been corrected with minor changes. These changes do not impact the scientific content of the article.

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Footnotes

Edited by: Doctor Mziwenkosi Nhlanhla Mthiyane, North-West University, South Africa

Reviewed by: Mohamed El-Sherbiny, National Research Centre, Egypt

Amr El-Nile, City of Scientific Research and Technological Applications, Egypt