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Soy protein and metabolic syndrome

Studies have linked soy protein and isoflavones intake with improved lipid profiles, glucose homeostasis and blood pressure, but no epidemiological studies have investigated the effect of soy protein intake on metabolic syndrome, which is a combination of metabolic disorders that increase the risk of developing cardiovascular disease and diabetes. Metabolic syndrome affects one in five people and prevalence increases with age. It is characterized by obesity, high blood pressure, decreased HDL cholesterol and increased triglycerides. One study showed a beneficial effect on glycemic control and lipid profiles on 42 postmenopausal women when replacing red meat with soy protein and soy nuts, and inverse association between the intake soy foods and risk of glycosuria (the excretion of glucose into the urine) in postmenopausal women. Other studies found a positive association between soy protein consumption and better lipid profiles.
A Chinese study, that was part of the Nutrition and Health of Aging Population in China Project, investigated the effects of soy food and soy protein intake on metabolic syndrome. In overall, the researchers found no effect of soy protein intake and the risk of metabolic syndrome. But they found a sex-dependant effect: a higher intake of soy protein reduced the risk of metabolic syndrome in women but elevated the risk in men. In overall, soy protein intake was associated with increased risk of hyperglycemia and a marginally reduced risk of high blood pressure, but was not associated with obesity, hypertriglyceridemia and HDL cholesterol. However, soy protein intake was associated with increased risk hyperglycemia and lower blood pressure in men.

This study shows many weaknesses: the food frequency questionnaire was not verified and other confounders may affect the results. For example, the highest quartile of soy protein intake contradictory consumed 10% more fat, 20% more cholesterol and 6% more non-soy protein than the lowest quartile. The intake of cholesterol and non-soy protein increased with higher soy protein intake, which seems very contradictory.

The study concluded habitual soy protein intake showed a sex-specific affect on the risk of metabolic syndrome, with slightly reduced risk in women but elevated risk in men. Additional work is needed to confirm these results and to determine the exact mechanisms of the observed effects.

Source: Soy protein intake has sex-specific effects on the risk of metabolic syndrome in middle-aged and elderly Chinese. J Nutr. 2008 Dec;138(12):2413-21.

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