Long-chain fatty acid metabolism in dairy cows: a meta-analysis of milk fatty acid yield in relation to duodenal flows and de novo synthesis.

This study is a meta-analysis of the response of milk long-chain fatty acid (FA) yield and composition to lipid supply, based on published experiments reporting duodenal FA flows or duodenal lipid infusions and milk FA composition (i.e., 39 experiments reporting 139 experimental treatments). Analysis of these data underlined the interdependence between milk yields of C18 and short- and medium-chain (C4 to C16) FA. Lipid supplementation (producing an increase in duodenal C18 flow) decreased linearly milk C4 to C16 yield (-0.26 g of C4 to C16 produced per gram of duodenal C18 flow increase) and increased quadratically milk C18 yield. When these 2 effects increased the percentage of C18 in milk FA up to a threshold value (around 52% of total FA), then milk C18 yield was limited by C4 to C16 yield, decreasing the C18 transfer efficiency from duodenum to milk with high-lipid diets. Moreover, for a given duodenal C18 flow, a decrease in milk C4 to C16 yield induced a decrease in milk C18 yield. Despite high variations in C18 transfer efficiency between duodenum and milk, for a given experimental condition, the percentages of C18 FA in milk total C18 could be predicted from their percentages in duodenal C18, and the percentages at the duodenum and in milk were very similar when mammary desaturation was taken into account (i.e., considering the sums of substrates and products of mammary desaturase). The estimated amounts of 18:0, trans-11-, and trans-13-18:1 desaturated by the mammary gland were a linear function of their mammary uptake, and mammary desaturation was responsible for 80, 95, and 81%, respectively, of the yield of their products (i.e., cis-9-18:1; cis-9, trans-11-, and cis-9, trans-13-18:2). Thus, mammary FA desaturation capacity did not seem to be a limiting factor in the experimental conditions published so far.