Endocrinological response to soy protein and fiber in mildly hypercholesterolemic men

Soy protein and soy fiber may affect hormones involved in cholesterol metabolism. Therefore, plasma insulin, total thyroxine (T4), free thyroxine index (FTI), triiodothyronine (T3), and thyroid stimulating hormone (TSH) were measured in 17 hypercholesterolemic men consuming baked products containing 50 g/d of protein and 20 g/d of fiber from either soy flour (SF), soy protein isolate (ISP)/soy fiber (ISP/SCF), ISP/cellulose (ISP/Cell), or nonfat dry milk/cellulose (NFDM/Cell). A latin square design was used and included all four dietary treatments, each lasting four weeks. Hormones were analyzed at the end of each experimental period and after an initial baseline period of 10 days. Although the mean values of T4 showed an increase in all treatments relative to baseline, significant differences (p<0.05) occurred only between the ISP groups and baseline. A similar pattern was observed with FTI (p<0.01). Mean insulin concentrations significantly increased (p<0.05) when subjects were fed SF compared to the ISP/SCF treatment. No effects were seen with either T3 or TSH. Results are consistent with the findings of animal studies, as well as human studies investigating T3/T4 influences on lipid metabolism, and suggest that the mechanism for the hypocholesterolemic effect of soy protein is via a T4-mediation.