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Cholesterol Metabolism, Digestion Rates and Postprandial Changes in Serum of Swine Fed Purified Diets Containing either Casein or Soybean Protein

Anton C. Beynen^*,,1, Clive E. West^*, Caroline J. K. Spaaij^*,, Joop Huisman, Piet van Leeuwen, J. Ben Schutte and Wil H. L. Hackeng^**

^* Department of Human Nutrition, Agricultural University, Wageningen Department of Laboratory Animal Science, State University, Utrecht Institute of Animal Nutrition Research (ILOB/IGMB-TNO), Wageningen ^** Department of Endocrinology, Municipal Hospital, Rotterdam, The Netherlands

Swine fitted with re-entrant ileo-cecal cannulas were fed purified diets containing either casein or soybean protein to study possible relations between cholesterol metabolism, digestion of dietary constituents and postprandial patterns of various serum components. Compared with soybean protein, dietary casein produced an increase in serum total cholesterol in which the excess cholesterol was located in the low density lipoprotein fraction. Ileal and fecal excretion of neutral steroids was diminished in pigs fed casein, suggesting that cholesterol absorption was stimulated. The apparent ileal absorption of protein was increased in pigs fed casein. The enhanced absorption of cholesterol and protein was associated with a reduced rate of flow of chyme through the ileum. The output of bile acids in the feces of pigs fed casein was decreased, whereas the ileal output was not significantly affected. This could be attributed to increased uptake of bile acids from the cecum and/or colon, which may in part be related to the indirectly observed decreased formation of secondary bile acids. Postprandial serum concentrations of insulin and glucose were temporarily increased in pigs fed casein, whereas those of triglycerides were decreased. We suggest that the decreased excretion of cholesterol and bile acids is the major determinant of casein-induced hypercholesterolemia in swine.

KEY WORDS: • pigs • casein • soybean protein • serum cholesterol • steroid excretion

¹ Address correspondence to Anton C. Beynen, Ph.D., Department of Laboratory Animal Science, State University, P.O. Box 80.166,3508 TD Utrecht, The Netherlands (tel int. 31.30.532033).

Manuscript received 6 April 1989. Revision accepted 5 October 1989.