Factors associated with the content of mammary-synthesized fatty acids in milk fat: A meta-analysis

Consumption of specific fatty acids (FA) that are synthesized in the mammary gland, namely de novo FA, has implications for human health. The objective of the present meta-analysis was to study the associations between milk fat content of de novo FA, with (1) diet composition, and (2) milk production and composition. Milk FA data from 96 peer-reviewed studies published between 1990 and 2016 that included 324 treatment means from 83 bovine experiments, 36 treatment means from 12 caprine experiments, and 40 treatment means from 12 ovine experiments were used in this analysis. Individual species models including the fixed effect of experiment were fitted using multiple regression to explain milk content of de novo FA as a function of diet composition and milk production and composition variables. We also evaluated replacing the effect of the experiment by the effect of the experiment nested in the laboratory at which the research had been conducted, and the effect of the laboratory. Butyric acid content in milk fat was positively but weakly related to dietary ether extract in does and ewes. Lauric, myristic, and palmitic acid contents in milk fat were negatively related to dietary ether extract in does and to a somewhat lesser extent in cows and ewes. The results confirm that the inclusion of lipids in the diet may not only affect the availability of preformed FA but also the profile of FA synthesized de novo in the mammary gland. Most of the variation in all prediction models was explained by the experiment or by the laboratory if the latter was included in the model. The ample variation in analytical methods reported by the different research groups suggests that differences in analytical protocols might explain a substantial proportion of the variation in de novo FA profile. A main conclusion of this study is the potential influence of differences in analytical procedures to explain the variation in de novo FA profile. Standardization of methods of FA analysis to improve reproducibility seems to be an aspect of importance to this area of research.