Diurnal Variations in Plasma Lipids*

The concentration of lipids in the blood of animals and man is commonly considered to be a labile value which is affected markedly by the ingestion of food. This belief is reflected in the usual practice of securing samples of blood for comparative lipid studies with the subject in a fasting state. The same belief underlies the insistence on blood obtained during fasting in performing other analyses, for example serum bilirubin, which are thought to be influenced by the supposedly increased blood lipids after meals. This popular opinion owes its origin apparently to experiments performed many years ago, such as those of Ahlfeld in 1877 (l), in which it was shown that if an animal be given a meal rich in fat the serum of that animal becomes milky due to an increased fat content. Subsequent investigations (2, 3, 11, 18) proved that an alimentary lipemia may be readily induced by giving a copious meal of fat to carnivorous animals but seldom occurs in herbivorous animals. Attempts to produce an alimentary lipemia in man have, in general, been disappointing (8-10, 13, 17, 19). In most of this work, the question of the amount of fat fed relative to the weight of the animal or human subject has been largely overlooked. Recognizing this, Man and Gildea (15) were able to show that an alimentary lipemia may be produced in man if 3.5 gm. of fat per kilo of body weight be given but that a smaller ration containing about 0.5 gm. of fat per kilo resulted in minor and inconstant variations in the blood lipids. It became apparent from these studies that an alimentary lipemia did not occur in man unless well over 200 gm. of fat were