Oilseed lipid supplements and fatty acid composition of cow milk: a meta-analysis.

Numerous experiments have studied the use of oilseed supplements in cow diets to alter milk fatty acid (FA) composition, but no quantitative synthesis of these studies is currently available. This article reports a meta-analysis of the response of cow milk FA composition to oilseed lipid supplements from linseed, rapeseed, soybeans, and sunflower seed. First, from a database of 145 oilseed supplementation experiments, we collected the mean FA percentages observed with unsupplemented diets and diets supplemented with the 4 oilseeds given as seeds (after various types of processing), as oils (including Ca salts and amides), or in protected forms. Second, we studied the response of the major milk FA percentages to increasing amounts of supplemental lipids from the 4 oilseeds. Responses were nonsignificant, linear, or quadratic, depending on the FA studied and the supplement. Effects of interfering factors, such as supplement form, forage component of the diet, or lactation stage, were difficult to assess from the available data. Third, we studied the response of the major milk FA percentages to increasing dietary intakes of linoleic or linolenic acids, taken separately. Overall, these results confirm the high plasticity of milk FA composition, with the widest variations being observed in the percentages of medium-chain versus C18 FA, and among the C18 in 18:0, cis-18:1, and trans-18:1. The percentages of the polyunsaturated FA cis-9 cis-12-18:2 and 18:3 were less variable, except when protected lipids (mostly formaldehyde treated) were supplied. However, trans-18:1 and polyunsaturated FA (including conjugated linoleic acid) exhibited the greatest variations when expressed relative to their respective basal values (for unsupplemented diets). Oils, compared with seeds, induced greater percentages of trans-18:1 and tended to decrease C6 to C12 FA more. Intakes of 18:2- and 18:3-rich lipid sources did not differ greatly in their effects on short- and medium-chain FA and trans-18:1 percentages, although the profiles of individual 18:1 and 18:2 isomers in milk differed. This meta-analysis provides quantitative estimates, obtained from the extensive literature produced over more than 40 yr, of the impact of oilseed supplements on milk FA composition.