While earlier clinical studies indicated that soy and red clover isoflavones effectively increased bone density and reduced hot flashes, the evidence is now debatable, and clear evidence is lacking according to new reviews of research.
This is the conclusion of a review of the clinical evidence from the Bruce W. Carter Veterans Affairs Medical Center and researchers from the University of Miami Miller School of Medicine.
The researchers pointed to "an extensive number of studies" that have tested soy-enriched foods, soy foods such as tofu and soy isoflavone supplements, many of which studied large populations of people. These included both Asian populations and Western populations of women.
They found that while they observed a huge variance in the design of these studies and some of the studies did show benefits of isoflavones, "the evidence points to a lack of a protective role of soy isoflavones in the prevention of postmenopausal bone loss."
Other studies find similar results
Another more extensive review of research comes from the University of Auckland. This Cochrane review of research analyzed 43 clinical studies that included 4,364 human participants.
The study found that when the results were combined, there was little solid evidence of soy isoflavones being helpful for hot flushes and other menopausal symptoms in women. They found that much of the evidence seemed to be related to a large placebo effect.
However, the researchers found some indication that genistein - one of the central isoflavones in soy and clover - may provide other benefits to postmenopausal women.
Both reviews found no negative effects of soy foods or isoflavone supplementation.
These results conflict another review of research, from Sichuan University. In their meta-analysis of the research, the medical researchers found that soy isoflavones increased bone density by 54% while reducing bone loss by 23% as measured by a bone loss marker called deoxypyridinoline, which is found in the urine, especially at higher levels among those women in postmenopause.
One possibility is related to dose. This analysis found that doses above 75 milligrams a day of soy isoflavones had the most effect among postmenopausal women.
Osteoporosis is the leading cause of disability among the elderly. Nearly half of women over 60 will have a hip fracture among industrialized nations.
The bottom line is that these foods are healthy but it is uncertain as to their definite therapeutic effects. The uncertainty was echoed by the Veteran's Affairs researchers:
"Although some women might experience a reduction in the frequency or severity of hot flashes, most studies point towards the lack of effectiveness of isoflavones derived from soy or red clover, even in large doses, in the prevention of hot flashes and menopausal bone loss."
The connection between isoflavones and S-equol
The error in these reviews relates to the limitations of soy isoflavones for those who do not produce equol - a metabolite of isoflavones produced by intestinal bacteria.Research has indicated that only a third to 50% of women produce equol in the presence of soy isoflavones. And it is equol that produces the benefits for menopause. So if more than half of all women do not produce equol, this would certainly skew the overall relationship between isoflavones and menopausal effects in the reviews discussed above.
The fact that the benefits of isoflavones is limited to equol producers has been illustrated in multiple studies, including a study from the Czech Republic’s Mlada Regional Hospital and another of 96 women from Taiwan. Each study found that after consuming isoflavones, only the equol-producing women had lower Kupperman index levels. The Kupperman index measures menopausal symptoms such as hot flushes, insomnia and others.
Other studies have shown that equol production is specifically related to populations of certain species of probiotics, such as Lactobacillus rhamnosus.
S-equol is chemically referred to as 4',7-isoflavandiol - and is also called 5-hydroxy-equol. It is produced by intestinal bacteria after consuming daidzein and genistein.
Isoflavones daidzein and genistein are contained in asparagus, beans - especially soy, fava beans, lupins, mung beans and lentils, sesame, linseed and flax seeds, yams, apples, pomegranates, whole wheat, black cohosh, licorice root, fennel, anise, hops and chaste berry.
Intestinal bacteria can differ among people depending upon the health and pH of the intestines. Probiotics can be damaged by antibiotic use and the use of other antiseptics such as antiseptic mouthwash.
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Photo by Pedro Ribeiro Simões