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Comparison of the Hypocholesterolemic Effects of Dietary Soybean Protein with Those of Formaldehyde-Treated Casein in Rabbits

Clive E. West^*, Caroline J. K. Spaaij^*,, Welmoed M. Clous^*, Henk P. Twisk^*, Marij P. H. Goertz^*, Richard W. Hubbard, Marja W. Kuyvenhoven^*,1, Roelof van der Meer^°, Wojciech F. Roszkowski^¶,2, Albert Sanchez and Anton C. Beynen^*,

^* Department of Human Nutrition, Wageningen Agricultural University, 6700 EV Wageningen, the Netherlands Department of Laboratory Animal Science, State University, 3508 TD Utrecht, the Netherlands Department of Pathology, School of Medicine, Loma Linda University, Loma Linda, CA 92350 ^° Department of Nutrition, Netherlands Institute for Dairy Research, 6710 BA Ede, the Netherlands ^¶ Institute of Human Nutrition, Warsaw Agricultural University, 02-766 Warsaw, Poland Department of Nutrition, School of Public Health, Loma Linda University, Loma Linda, CA 92350

Treatment of casein with formaldehyde changes its tertiary structure and decreases its hypercholesterolemic properties in rabbits. To investigate whether formaldehyde-treated casein exerts this hypocholesterolemic effect in the same manner as soybean protein, rabbits were fed high or low cholesterol diets containing soybean protein, casein, formaldehyde-treated casein or a mixture of casein and formaldehyde-treated casein. Formaldehyde-treated casein was hypocholesterolemic when fed in a low, but not in a high, cholesterol diet. The hypocholesterolemic effect of soybean protein was independent of the amount of cholesterol included in the diet. In contrast to rabbits fed soybean protein, steroid absorption in those fed formaldehyde-treated casein did not differ from that in rabbits fed native casein. Furthermore, the absorption of phosphorus and nitrogen was lower in rabbits fed formaldehyde-treated casein than in those fed native casein, whereas the absorption found in rabbits fed soybean protein resembled that of their casein-fed counterparts. The diets containing soybean protein and formaldehyde-treated casein produced a comparable ratio of lysine to arginine in serum. The results presented in this paper indicate that the hypocholesterolemic action of dietary formaldehyde-treated casein does not resemble that of soybean protein.

KEY WORDS: • soybean protein • casein • formaldehyde-treated casein • serum cholesterol • fecal steroid excretion • phosphorus absorption • nitrogen absorption • serum amino acids

¹ M. W. Kuyvenhoven was supported by a grant from the Netherlands Institute for Dairy Research.

² W. F. Roszkowski was supported by an International Agricultural Centre fellowship through the Netherlands Ministry of Agriculture and Fisheries and a grant from the Office for International Relations of the Wageningen Agricultural University.

Manuscript received 6 July 1988. Revision accepted 13 January 1989.