However, data from women able to produce equol – an isoflavandiol produce by select gut bacteria – indicated significant 6% and 7.2% reductions in ratios of total cholesterol to HDL and the ratio of LDL to HDL.
Individuals have different abilities to produce equol, which may have beneficial effects on prostate health, bone health, and for managing menopausal symptoms. Equol is produced in the gut by bacteria, but only 30 to 50% of people are reported to have the intestinal bacteria necessary to produce it.
“These data suggest that soy supplementation will not play a major clinical role in serum lipid management, especially among those lacking the ability to produce equol,” wrote researchers in Nutrition Research.
“If soy protein or isoflavones provide cardiovascular benefits, it would most likely be via a mechanism other than lipid alteration. Future studies should consider enrolling individuals on the basis of their ability to produce equol to better define the potential benefits of soy on lipids.”
Soy protein and heart health
The cardiovascular potential of soy protein has been reported many times, and the American Heart Association currently recommends consumption of at least 25 grams per day of soy protein with phytoestrogens, along with a low-fat diet, to reduce the risk of heart disease.
While some studies have examined the effects of soy proteins on the various inflammatory markers the evidence is said to be “limited and inconsistent”.
Earlier this year, scientists from the Tulane University School of Public Health and Tropical Medicine in New Orleans reported that supplements of soy protein may reduce compounds in the body linked to dysfunctional blood vessel health.
Writing in the European Journal of Clinical Nutrition, the scientists reported that a daily 40 gram dose of soy protein was associated with significant reductions in E-selectin and leptin, whereas no such reductions were observed in people taking the same dose of milk protein.
The study was said to be one of the largest randomized trials to investigate the effect of protein supplementation on multiple biomarkers of inflammation, endothelial dysfunction (the lining of the blood vessels), and adipocytokines (signaling molecules produced by fat tissue).
All about equol?
The new study adds to the inconsistency around soy and heart health. The Connecticut researchers’ one-year randomized, double-blind, placebo-control, clinical trial study involved 131 healthy women older than 60. The women were randomly assigned to one of four groups: The first group received a combination of soy protein (18 g/d) and isoflavone tablets (105 mg/d isoflavone aglycone equivalents), the second group received soy protein and placebo tablets, the third group received control protein and isoflavone tablets, and the fourth group received control protein and placebo tablets.
Results showed that, for the 97 women who completed the trail, there were no significant differences between the groups for serum lipids and inflammatory marker levels.
On the other hand, equol producers did show lower lipid measure.
A significant change in levels of the pro-inflammatory market interleukin-6 (IL-6) was observed for people who received soy protein, compared with the control protein groups, added the researchers.
“The ability of some individuals to produce equol from the soy isoflavone, daidzein, has been hypothesized to be responsible for some of the health benefits of soy,” they wrote.
“We hypothesized that individuals who produce equol would have greater cardioprotection compared with nonequol producers. Although comparison of equol producers with nonequol producers did not alter serum lipids or inflammatory markers, serum lipid ratios (TC/HDL and LDL/HDL) were significantly lower in equol producers.
“Whether this statistical difference is large enough to be biologically important is unknown.”
Source: Nutrition Research
Published online ahead of print, doi: 10.1016/j.nutres.2013.08.009
“Soy proteins and isoflavones reduce interleukin-6 but not serum lipids in older women: a randomized controlled trial”
Authors: K.M. Mangano, H.L. Hutchins-Wiese, A.M. Kenny, et al.