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 ingestion of soy protein in humans. One possible explanation is that the different soy protein preparations used contain different spectra of biologically active components. This could affect a number of indices including thyroid hormone status. An increased level of thyroxine has been proposed as an underlying mechanism of the hypocholesterolemic effect of soy protein. The objective of this study was to determine if serum 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 lipoprotein fractions. Serum thyroxine and free thyroxine were greater only in hamsters fed ISP than in those fed casein (P < 0.05), whereas triiodothyronine concentrations were higher in casein-fed than in SPC-fed hamsters (P < 0.05). Results indicate that protein from ISP and SPC are both effective in lowering blood cholesterol concentrations, whereas only ISP increases thyroxine concentrations. Therefore, it appears unlikely that modulation of thyroid hormone status is responsible for the cholesterol-lowering effect of soy protein.