Biochemical basis of dietary influences on the synthesis of the macronutrients of rat milk.

In this contribution the regulation of the synthesis of the macronutrients (lactose, lipid, protein) of rat milk are briefly reviewed. Lactose synthesis and lipid synthesis in mammary gland have glucose as the common precursor, and the rates of both processes show diurnal changes that correlate with the variations in food intake. Short-term (6-h) removal of food results in large decreases in the rates of synthesis of lactose and lipid, despite any change in plasma glucose or insulin. Similarly, an intragastric load of fat rapidly decreases de novo lipogenesis in the mammary gland. The existence of a factor in the circulation that signals the dietary intake of carbohydrate is postulated. This putative factor may originate from the intestine. Synthesis of protein appears less sensitive to dietary intake and is regulated mainly by prolactin.