Effects of skim milk, skim milk yogurt, orotic acid, and uric acid on lipid metabolism in rats Khursheed P. Navder,* E. Beth Fryer, and Holly C. Fryer Departments of Foods and Nutrition and Statistics, Kansas State University, Manhattan, KS, USA The effects of feeding two milk products (skim milk and skim milk yogurt) and two proposed hypo- cholesterolemic factors (orotic acid and uric acid) on serum cholesterol (HDL, LDL, total, HDL/ Total and HDL/LDL), liver lipids (total liver lipids and liver cholesterol), and aortal cholesterol were studied. Ten groups, of nine rats each, were fed isocaloric Chow-based diets containing water, 45% skim milk (SM), 45% skim milk yogurt (SMY), and 0.0025% orotic acid (OA) or 0.001% uric acid (UA), without or with cholesterol. The SM diet (with cholesterol) resulted not only in lower total cholesterol (P < 0.10), LDL cholesterol (P < 0.05), aortal cholesterol (P < 0.01), and liver cholesterol (P < 0.I0), but also in increased HDL (P < 0.05) and HDL/LDL (P < 0.10) cholesterol ratio. The SMY diet, on the other hand, resulted in lowered total serum cholesterol (P < 0.05) and aortal cholesterol (P < 0.01) and in higher LDL (P < 0.05) cholesterol. The hypocholesterolemic effects were more marked for SM than for SMY. Addition of OA and UA to diets increased serum cholesterol, LDL cholesterol, and total liver lipids; the OA diet also increased liver cholesterol. Neither OA nor UA alone was the factor responsible for the hypocholesterolemic effects seen with SM and SMY feeding. Keywords: Skim milk; skim milk yogurt; orotic acid; uric acid; serum lipids; liver lipids; aortal lipid Introduction Total serum cholesterol has been identified as a major risk factor for coronary heart disease. Though many environmental and genetic variables influence serum cholesterol, a good correlation exists between total dietary fat intakes and mean serum cholesterol levels of different populations. Since dairy products make an appreciable contribution to saturated fat and choles- terol content of the diet, whole milk often has been implicated as a coronary health hazard. However, Mann and Spoerry, ~ reported a low serum cholesterol in East African Maasai men, even though their diets Address reprint requests to Dr. E. Beth Fryer at the Department of Foods and Nutrition, Justin Hall, Kansas State University, Manhat- tan, KS 66506 USA. Supported by Kansas Agricultural Experiment Station Contribution No. 90-245-J. *Present address: Department of Nutrition and Food Science, Hunter College, 425 E 25 Street, New York, NY 10010, USA. Received April 9, 1990; accepted July 25, 1990. consisted chiefly of fermented whole milk (as much as 8 1/man/d). That observation has prompted a series of investigations on the hypocholesterolemic effects of milk. Various dairy products, such as whole milk, fer- mented milk, skim milk, yogurt, and buttermilk have been reported to lower serum cholesterol in man 2"6and in experimental animals. TM In contrast, little or no hy- pocholesterolemic effect of milk products was found in other human ~7-2°and animal studies. 2tz3 Even those researchers who have confirmed the presence of a cholesterol-lowering effect are not cer- tain about the responsible factors. Several milk con- stituents such as orotic acid, 24-29 uric acid, 3°,31 cal- cium, 3'5'12'32 lactose, 33 milk fat globule membrane, 6 and products of bacterial fermentation 1°,13 have been sug- gested. This study was undertaken to determine the effects of two dairy products (skim milk and skim milk yo- gurt) and two proposed hypocholesterolemic factors (orotic acid and uric acid) on the cholesterol metabo- lism in normal and experimentally induced hypercho- lesterolemic rats. In an effort to ascertain the mecha- nism(s) responsible for the dairy product effect, serum 640 J. Nutr. Biochem., 1990, vol. 1, December © 199o Butterworth-Heinemann