SOY PROTEIN-CONCENTRATE AND ISOLATED SOY PROTEIN SIMILARLY LOWER BLOOD-SERUM CHOLESTEROL BUT DIFFERENTLY AFFECT THYROID-HORMONES IN HAMSTERS

There is a wide variation in the hypo-cholesterolemic response to inge stion of soy protein in humans. One possible explanation is that the d ifferent soy protein preparations used contain different spectra of bi ologically active components. This could affect a number of indices in cluding thyroid hormone status. An increased level of thyroxine has be en proposed as an underlying mechanism of the hypocholesterolemic effe ct of soy protein. The objective of this study was to determine if ser um cholesterol and thyroid hormone concentrations differed because of feeding soy protein from different sources. Twenty-nine male weanling golden Syrian hamsters were fed rations containing 25 g/100 g protein from either isolated soy protein (ISP), soy protein concentrate (SPC) or casein for 35 d. Serum total cholesterol concentrations were lower in hamsters fed ISP and SPC compared with those fed casein (P < 0.05). No differences in cholesterol concentrations were observed in lipopro tein fractions. Serum thyroxine and free thyroxine were greater only i n hamsters fed ISP than in those fed casein (P < 0.05), whereas triiod othyronine concentrations were higher in casein-fed than in SPC-fed ha msters (P < 0.05). Results indicate that protein from ISP and SPC are both effective in lowering blood cholesterol concentrations, whereas o nly ISP increases thyroxine concentrations. Therefore, it appears unli kely that modulation of thyroid hormone status is responsible for the cholesterol-lowering effect of soy protein.