Effects of feeding medium-chain triacylglycerols on maternal lipid metabolism and pup growth in lactating rats

To study the effects of medium-chain triacylglycerols (MCT) on maternal lipid metabolism and pup growth, MCT (200 g/kg) were incorporated into a commercial chow diet and fed to lactating rats for 8-10 d. The results were compared with similar diets containing sunflower oil (polyunsaturated fatty acids; PUFA), tristearin (saturated fatty acid) or triolein (monounsaturated fatty acid). There was decreased food and energy intake with the MCT diet and this was accompanied by decreased (35%) pup growth. All the high-fat diets inhibited lipogenesis in vivo in the lactating mammary gland, the order of effectiveness being PUFA > triolein > tristearin > MCT. Only the MCT diet increased the rate of hepatic lipogenesis (180%). Experiments feeding an MCT meal containing 1-14C]octanoate indicated that very little (3-4%) of the C was present in mammary gland lipid, unlike the findings with 1-14C]triolein meal (40%). The major portion (65%) of the absorbed 1-14C]octanoate was oxidized to 14CO2. There was no evidence for adaptation of the mammary gland to increased dietary lipid uptake on the triolein or MCT diets. It is concluded that the decreased pup growth on the MCT diet is due in part to the decreased energy intake and to the inability of dietary medium-chain fatty acids to provide substrates for milk lipid synthesis.