Sources of substrate for milk lipid synthesis. The substrate for triacylglycerol synthesis depends on plasma sources of substrate. In the high fat fed animal, such as the usual lactating women who consumes up to 40% of her calories as lipid, fatty acids and glycerol for the synthesis of milk triglycerides (TAGs) originate in the chylomicra and very low density lipoprotein (VLDL) of the liver, whereas only about 10% of TAGs are derived from glucose. During a fasting state, fatty acids continue to be derived from the plasma, but now are transported to the mammary gland directly from the adipose tissue bound to albumin or indirectly as VLDL derived from the liver. In the animal fed a low fat diet, such as the laboratory mouse on the usual chow, a much larger proportion of the fatty acids for TAG synthesis are derived from glucose via the fatty acid synthetic pathways shown in Figure 4. BM, basement membrane; DHAP, dihydroxyacetone phosphate; ER, endoplasmic reticulum; FA, fatty acid; FABP, fatty acid binding protein; GLUT, glucose transporter; LPL, lipoprotein lipase.