Hundreds of human intervention studies evaluating the health effects of soyfoods and various soybean components have been conducted over the past 20 years. But a recent study published in the journal Stroke is among the most important and exciting. Led by Dr. Howard Hodis from the USC Keck School of Medicine in Los Angeles, the Women’s Isoflavone Soy Health (WISH) trial is notable for its size, duration and primary health outcome of interest, which was the progression of subclinical atherosclerosis. In this 3-year study, 350 healthy postmenopausal women ages 45 to 92 were randomly assigned to groups consuming either 25 grams of soy protein per day or 25 grams of milk protein. The soy protein provided 99 milligrams of isoflavones, a type of phytoestrogen found in soybeans.
Subclinical atherosclerosis can be assessed using ultrasound to measure the thickness of the carotid arteries—which are located on both sides of the neck beneath the jawline and provide the main blood supply to the brain. The thickness of the carotid artery is referred to as carotid intima-media thickness or CIMT. In most people, the CIMT increases or progresses over time; the extent of progression reflects risk of future coronary events.
In the WISH trial, progression among the women consuming soy was 16% lower than in the milk group by the end of the study. While the difference between groups was not statistically significant, the difference increased steadily over the 3 year study period. This suggests that after 5 and 10 years, progression would have been reduced to an even greater extent in the soy group. And, if reducing progression by 16% translates into a 16% decrease in risk of a heart attack or stroke that would suggest an important benefit. But there is something even more compelling about the findings from this study, and that is that soy didn’t affect all women similarly.
The researchers sub-analyzed the results according to the age of the participants, or more precisely, they compared the women according to how many years beyond menopause they were. They found that soy was more effective in women who were in the early stages of menopause. Among women who were only 5 years post-menopause, CIMT progression was reduced by 68 percent. It was reduced by 17 and 9 percent respectively in women who were 5 to 10 years post-menopause and 10 years post-menopause.
It makes sense that soy was so markedly effective in early menopausal women because of something called the “estrogen window hypothesis.” Proposed about a decade ago, this hypothesis, which has continued to gain support, maintains that estrogen therapy leads to dramatic coronary and cognitive benefits when it begins soon after menopause, but has less effect in later years. This suggests that soy isoflavones were acting in a similar way to estrogen in reducing CIMT progression in postmenopausal women. This explanation makes even more sense given that soy had no effect on blood pressure and only modestly affected cholesterol levels, since both phytoestrogens and estrogen are thought to be protective against heart disease independent of any effect on cholesterol and blood pressure.
The results provide the strongest evidence to date that menopausal women, especially those in the early years of menopause, benefit from consuming soyfoods. The women in this study consumed the equivalent of about four servings of traditional soyfoods per day However, it is possible, and even likely, that fewer servings would have produced similar benefits.
Hodis HN, Mack WJ, Kono N, Azen SP, Shoupe D, Hwang-Levine J, Petitti D, Whitfield-Maxwell L, Yan M, et al. Isoflavone soy protein supplementation and atherosclerosis progression in healthy postmenopausal women: A randomized controlled trial. Stroke. 2011.