Milk fat depression in C57Bl/6J mice consuming partially hydrogenated fat

Mice of the C57Bl/6J strain were maintained on diets in which the unsaturated fatty acids were all cis fatty acids (CFA) or a mixture of CFA and trans fatty acids (TFA). The fats used were mixtures of corn oil, olive oil, cocoa butter, margarine and shortening blended to yield similar fatty acid compositions, except for the ratio of the CFA to TFA and the percentage of linoleic acid (EFA). Regardless of the level of fat (20 or 40 energy %) or the level of EFA (2 to 12 energy %), diets with TFA decreased the percentage of fat in mouse milk. When lactating females raised on the CFA diets were crossed to the TFA diets, TFA appeared in the milk at 12 h postcross, and within 4 d postcross the percent of milk fat was decreased to levels similar to that of nursing females raised continuously on the TFA diets. Conversely, lactating females crossed from TFA to CFA diets produced milk with percentage fat values and fatty acid compositions that approached those seen in nursing females fed the CFA diets continuously. The possible involvement of TFA in the classical milk fat depression phenomenon in ruminants and its potential relevance in human lactation are discussed.