We examined the effects of high-carbohydrate and high-fat diets on the serum lipid levels of distance runners. For seven days before each study, subjects consumed a diet containing 15% protein, 32% fat, and 53% carbohydrate. During 14-day experimental periods, a control group (n = 10) continued the same diet while two other groups consumed 69% of their calories as either carbohydrate (n = 13) or fat (n = 14). High-density lipoprotein (HDL)-cholesterol decreased 9% during the high-carbohydrate diet because of a 26% fall in the HDL2 fraction (1.063 to 1.125 g/mL). These changes were not accompanied by changes in the levels of apolipoproteins (apo) A-I or A-II. Total and low-density lipoprotein (LDL)-cholesterol initially decreased but subsequently exceeded pre-diet values while triglyceride concentrations increased 30% to 50%. Postheparin lipoprotein lipase activity (LPLA) fell 20%. Despite these dietary effects, HDL and HDL2 cholesterol concentrations in the athletes remained above values typical of sedentary men. The high-fat diet produced different effects on the serum lipids and lipoprotein levels of the athletes. HDL levels changed little during the study although HDL-cholesterol and apo A-I on the last diet day were both slightly above initial values. The high-fat diet provided 111 g of saturated fat per day but had surprisingly little effect on total and LDL-cholesterol whereas serum triglycerides fell by 10% to 20%. Postheparin LPLA increased 30% with fat feeding and the changes in LPLA correlated with alterations in triglyceride levels (r = -0.53, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)