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Soy Protein, Thyroid Regulation and Cholesterol Metabolism¹

School of Home Economics, University of Southern Mississippi, Hattiesburg, MS 39406

The effects of dietary protein on plasma cholesterol concentrations are well documented: animal proteins (casein) are hypercholesterolemic compared with plant proteins (soy protein). Although this effect of protein source on plasma cholesterol has been shown in many species, the mechanism is not completely understood. This paper reviews the relationship between dietary protein source and plasma thyroxine concentration. The basic premise is that feeding soy protein lowers plasma cholesterol concentration by causing an increase in plasma thyroxine concentrations. The metabolic changes involving cholesterol that occur when soy protein is fed are discussed. These changes are consistent with changes induced by elevating thyroxine. Data are presented from animal studies showing that feeding soy protein to laboratory animals consistently elevates plasma thyroxine concentrations. Furthermore, this elevation in plasma thyroxine concentrations precedes the change in plasma cholesterol concentrations: a necessary requirement for hypothesizing a causative effect. Possible mechanisms as to how a dietary protein source affects plasma thyroxine are also presented.

KEY WORDS: • soy protein • casein • thyroxine • cholesterol

¹ Presented at the First International Symposium on the Role of Soy in Preventing and Treating Chronic Disease, held in Mesa, AZ, February 20–23, 1994. The symposium was sponsored by Protein Technologies International, the soybean growers from Nebraska, Indiana, and Iowa and the United Soybean Board. Guest editors for this symposium were Mark Messina, 1543 Lincoln Street, Port Townsend, WA 98368, and John W. Erdman, Jr., Division of Nutritional Sciences, University of Illinois, Urbana, IL 61801-3852.

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