Recent Advances in Milk Fat Depression: 1. Time Course of Milk Fat Depression and 2. Adipose Tissue Lipogenesis during Milk Fat Depression

Mammalian milk fat concentration and composition are variable and responsive to nutritional factors. First described over one and a half centuries ago, diet-induced milk fat depression (MFD) is characterized by a decrease in milk fat yield of up to 50% with no change in milk yield or yield of other milk components. MFD is classically observed in ruminants fed highly fermentable diets or diets that contain plant or fish oil supplements. Dietary factors that cause MFD are known to alter ruminal fermentation and for many years experimental approaches were based on the hypothesis that the altered ruminal fermentation resulted in a limitation of substrate for milk fat synthesis. Investigations testing this hypothesis, however, provided no support for this theory (Bauman and Griinari, 2003). Key insight was provided by the discovery that changes in milk fat yield were correlated with changes in the profile of milk fatty acids (FA), more specifically the concentration of rumen-derived biohydrogenation intermediates. Recent investigations have established that MFD is associated with rumen production of unique FA from ruminal metabolism of dietary polyunsaturated fatty acids (PUFA). Referred to as the “biohydrogenation theory,” the basis for diet-induced MFD relates to an inhibition of mammary lipid synthesis by specific FA that are intermediates in the biohydrogenation of dietary PUFA, and these are only produced under certain conditions of altered rumen fermentation (Figure 1; Bauman and Griinari, 2003). Trans10, cis-12 conjugated linoleic acid (CLA) was the first of these to be recognized (Baumgard et al., 2000) and recently two additional inhibitory FA have been identified, trans-9, cis-11 CLA and cis-10, trans-12 CLA (Bauman et al., 2006). This paper will review two key questions related to MFD: 1. How quickly is milk fat synthesis decreased during milk fat depression and 2. Does milk fat depression alter the energy balance of a lactating dairy cow?