The origin of lipoprotein in the intestinal and hepatic lymph of unsuckled new-born calves.

1. Experiments have been conducted to determine the origin and character of the lipid in thoracic duct lymph of new-born, unsuckled calves. This involved the collection and analysis of intestinal and hepatic lymph from two bile-fistulated and seven non-fistulated animals.2. The output of total esterified fatty acid in intestinal and hepatic lymph in the unfed calf after recovery from the anaesthetic was about 0.6 and 0.025 g/hr, respectively.3. Three lipoprotein fractions were obtained by ultracentrifugation of intestinal lymph samples. These appeared to correspond to very low- (d < 1.005), low- (1.2 > d < 1.005) and high- (d = 1.20) density lipoprotein of human serum. These lipoproteins contained approximately 40%, 50% and 10% respectively of the total lipid.4. Very low-density lipoprotein was not found in blood serum or hepatic lymph and it was evident that very low-density lipoprotein and most of the low-density lipoprotein in intestinal lymph were derived from sources other than the capillary filtrate.5. The output of total esterified fatty acid in intestinal lymph fell steadily during the course of an 11-16 hr period of bile deprivation to values 55% of those in comparable calves without bile fistulae. This was accompanied by a virtual disappearance of very low-density lipoprotein and a decrease in low-density lipoprotein of approximately 50%. These effects could not be attributed to starvation.6. There was a general resemblance in fatty-acid composition between biliary lipid and the triglyceride of the lipoproteins, the predominant fatty acids being 16:0 and 18:1. The presence of appreciable levels of odd carbon number and branched-chain fatty acids were consistent with the transfer of significant amounts of free fatty acid from maternal blood to the foetus.7. It is concluded that most of the lipid in thoracic-duct lymph of unfed, new-born calves is derived from the intestines and that the fatty acids of the very low-density lipoprotein in intestinal lymph arise from the absorption of biliary phospholipid.