Soy protein's ability to lower total and LDL (low-density lipoprotein or "the bad") cholesterol has been extensively studied, but the mechanism whereby soy protein lowers cholesterol remains unresolved. A new study published in the Journal of Clinical Lipidology last month shows that soy protein lowers total cholesterol and non-HDL (non-high-density lipoprotein) cholesterol significantly more than milk protein in patients with moderately high cholesterol levels.
"Non-HDL cholesterol has been shown to be a somewhat stronger predictor of cardiovascular disease and mortality risk than LDL cholesterol in population studies," said Elaine Krul, co-author of the study and nutrition discovery lead at Solae. "The fact that soy protein significantly decreased non-HDL cholesterol levels compared to milk protein in this study is very promising."
This randomized, controlled, parallel arm trial evaluated the effects of an insoluble fraction of soy protein, compared to total milk proteins with high calcium content, on the fasting lipid profile. It also assessed the potential contributions of increased excretion of bile acids and neutral sterols to their lipid-altering effects.
"The results of this study also showed that soy protein lowered non-HDL through a mechanism that does not involve increased bile acid excretion, but some yet to be determined mechanism," said Kevin Maki, lead author of the study. "Nonetheless, these results are supportive of the heart health claim for soy protein."
The Food and Drug Administration (FDA) heart health claim for soy protein established in 1999 states that "25 grams of soy protein a day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease." Currently, 11 other countries have approved health claims for soy protein's potential to lower blood cholesterol and lower the risk of coronary heart disease.
Solae's soy protein that was used in this study was a relatively insoluble fraction of soy protein isolate that had been shown to lower plasma cholesterol and increase fecal bile acid excretion in animals. The levels of isoflavones in the soy protein were lower than the average commercial soy protein isolate further supporting the notion that isoflavones do not play a role in the cholesterol lowering. The milk protein supplemented group also showed a modest cholesterol lowering.
Subjects for this study included men and women 18 to 79 years of age with elevated cholesterol, defined as fasting LDL-cholesterol concentrations of at least 100 mg/dL and less than 200 mg/dL while receiving no lipid altering therapy. Once recruited, participants were asked to follow a Therapeutic Lifestyle Changes diet throughout the study. Subjects that still met the inclusion criteria were then screened for their ability to lower their cholesterol in response to a bile acid binding drug, colesevelam. A majority of subjects responded and were then randomized to the test protein groups.