Effect of soy protein containing isoflavones on blood lipids in moderately hypercholesterolemic adults: a randomized...

Dietary intake of soy protein with isoflavones may be associated with reductions in serum cholesterol.

To compare the effects of a water-washed soy protein concentrate with a milk-protein based control on blood lipid levels in hyperlipidemic men and women.

A randomized, double-blind, controlled clinical trial including 159 subjects. After a 3-week run-in period during which all subjects consumed a milk protein-based supplement, participants were randomized into one of two groups: a control group (continued milk protein) and an intervention group (soy protein) for a five-week period. Fasting venous blood draws for lipid measurement were obtained at baseline, towards the end of the run-in period and at the end of the intervention. Blood isoflavone concentrations were measured at the end of the study.

Blood lipid levels were not significantly different between groups at any point in time; and there were no significant associations between blood isoflavones and lipid levels. Significant decreases in total cholesterol (19 mg/dL), and LDL-cholesterol (11 mg/dL), were observed during the run-in period, with no further decreases in lipids during the intervention period in either group.

These results do not support the hypothesis that water-washed soy protein has an effect on blood lipids. Several hypotheses are discussed, highlighting the selective nature of the effect of soy consumption in the population. The cholesterol-lowering effect during the run-in period may be explained by the "regression to the mean effect" and by other factors related to study participation, mainly nutrient displacement induced by the protein supplement.