Conditions Associated with Marine Lipid-Induced Milk Fat Depression in Sheep Cause Shifts in the In Vitro Ruminal Metabolism of 1-13C Oleic Acid

Abstract

Simple SummaryDairy ewes, just like cows, suffer from the low-fat milk syndrome, also known as milk fat depression (MFD), when marine lipids are added to their diet to modulate milk fatty acid (FA) profile. Alterations in the ruminal metabolism of dietary unsaturated FA seem to be at the core of this MFD but its causative agent has not yet been confirmed. Although some alterations may derive from shifts in oleic acid metabolism, methodological constraints represent a major challenge: the fact that oleic and other dietary FA (e.g., linoleic and linolenic acids) have common intermediates hampers the identification of those metabolites deriving specifically from each one. Thus, we conducted an in vitro trial combining an isotopic tracer approach with the use of rumen inoculum from ewes fed a diet supplemented or not with fish oil (which is known to cause MFD in dairy animals), to provide new insights into shifts in oleic acid metabolism. Fish oil altered the relative contribution of specific pathways, with relevant increments in oxygenated FA, including a major candidate milk fat inhibitor (10-oxo-18:0). Changes in the concentration of trans 18:1 intermediates were also observed. Results are discussed in the context of their potential association with milk fat depression.AbstractShifts in ruminal oleic acid (OA) metabolism have received little research attention but recent studies have suggested their association with marine lipid-induced milk fat depression (MFD) in ewes and cows. Measurement of specific products of OA within the complex mixture of digesta lipids is however challenging. Therefore, this in vitro trial combined the isotopic labelling technique with the use of rumen inoculum from cannulated sheep fed a diet supplemented or not with 2% of fish oil (which has been demonstrated to cause MFD in dairy ruminants) to characterize the metabolism of OA in response to ruminal alterations associated with MFD. The products of 13C-OA after 24 h of incubation were analysed by gas chromatography-combustion isotope ratio mass spectrometry (GC-C-IRMS). Numerous 13C-labeled 18:1 intermediates and oxygenated FA were detected and no elongation or desaturation of 13OA occurred. Diet supplementation with fish oil (i.e., MFD conditions) resulted in no unique metabolites of 13OA but in relevant changes in the relative contribution of specific metabolic pathways. The inhibition of 18:0 saturation caused by this treatment appeared largely attributable to increased oxygenated FA proportion, in particular the candidate milk fat inhibitor 10-oxo-18:0, and warrants further research on the association between MFD and oxygenated FA. Changes in the concentration of 13C-labeled trans 18:1 intermediates but not in cis 18:1, were also observed.

Invited review: Role of rumen biohydrogenation intermediates and rumen microbes in diet-induced milk fat depression: An update

“…Several recent in vivo studies have observed a positive association between MFD and the milk fat proportion of oxo-FA, particularly 10-oxo-18:0, in cows, ewes and does fed fish oil (Bernard et al, 2015;Kairenius et al, 2015;Carreño et al, 2016). Those increments in 10-oxo-18:0 may derive from alterations in the relative contribution of specific pathways of ruminal cis-9 18:1 metabolism, as suggested in an in vitro study with rumen inoculum of sheep adapted to fish oil consumption (Toral et al, 2018b). Overall, these observations support the potential involvement of oxo-FA, and of 10oxo-18:0 in particular, in marine lipid-induced MFD.…”

“…For example, the increased milk fat proportion of cis-11 18:1 under fish oil-induced MFD conditions (Shingfield et al, 2003;Kairenius et al, 2015) might result from its supply of marine lipids and the concomitant inhibition of 18:1 saturation, rather than from an increased production in the rumen through C18 biohydrogenation. This hypothesis is supported by the in vitro incubation of 13 C-labeled UFA with rumen inoculum from cows (Klein and Jenkins, 2011) or from sheep adapted or not to fish oil consumption (Toral et al, 2018b. As such, PUFA originating directly from fish oil might contribute, at least in part, to MFD.…”

“…However, ruminal biohydrogenation of 18:2n-6, 18:3n-3, and cis-9 18:1 might also result in the formation of several other minor FA intermediates, such as trans-9,trans-11 CLA, trans-10 18:1, and cis-12 18:1 (Figure 2-4). In addition to the pathways shown in Figures 2-4, alternative pathways might exist, as supported by the identification of additional biohydrogenation intermediates in recent in vitro experiments using stable isotopes in cows (deuterium oxide; Honkanen et al, 2016) and sheep ( 13 C-labeled FA; Toral et al, 2018bToral et al, , 2019.…”

Effect of Dietary Supplementation with Lipids of Different Unsaturation Degree on Feed Efficiency and Milk Fatty Acid Profile in Dairy Sheep

“…Regarding the impact of OO on milk fat composition, it is worth highlighting some desirable effects, such as the decrease in medium-chain saturated FA and the increase in some potentially health-promoting compounds, specifically 4:0, trans -11 18:1, cis -9 trans -11 CLA, and cis -9 18:1 [ 1 , 39 ]. The large variation in the latter would derive not only from dietary cis -9 18:1 supply, but also from its extensive saturation in the rumen [ 45 , 46 ], enhancing the availability of 18:0 for mammary Δ 9 -desaturation [ 47 ]. Ruminal cis -9 18:1 metabolism also involves isomerization and hydration/oxidation processes [ 45 , 46 ], which would partly explain the increments in milk trans 18:1 and 10-oxo-18:0, respectively.…”

The effect of metabolism on the course of labor and the postpartum period in highly productive cows

“…Microbial digestion of fiber and other nutrients in the gum stomachs of the ruminants indicates the nature of the influence of the formed and used metabolites of the substrate [1]. As a result of the relative values of some key processes, primarily lipogenesis in ruminants, it differs significantly from similar processes and nonruminant ones.…”