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Am Fam Physician. 1999;60(5):1492-1494

A large number of environmental factors contribute to a lower prevalence of cardiovascular disease, reproductive cancer and menopausal symptoms in Asian countries, compared with Western countries. Substantial differences in the consumption of soy-based products containing phytoestrogens have been postulated as contributing to the discrepancy. Many of the previous studies on phytoestrogens have been limited because of failure to document phytoestrogen levels in the soy-based treatment, failure of blind randomization or small sample size. Wash-burn and associates evaluated the effect of a soy protein dietary supplement on chronic disease risk and menopausal symptoms in a double-blind, randomized, crossover study of nonhypercholesterolemic, perimenopausal women with reported vasomotor symptoms.

After meeting eligibility requirements, women 45 to 55 years of age were enrolled in the study. Participants received one of three dietary supplements and were randomly assigned to the following diet order (first, second or third six-week period): (1) 20 g of complex carbohydrate supplement containing no phytoestrogens, (2) 20 g of soy protein supplement containing 34 mg of phytoestrogens consumed once daily and (3) 20 g of soy protein supplement containing 34 mg of phytoestrogens split into two equal doses consumed twice daily. Physical and laboratory assessments were obtained at baseline and repeated at six-week intervals. After six and 12 weeks, women were crossed over randomly to the other intervention diets. Results were evaluated to determine whether differences existed between the three treatment diets for cardiovascular disease risk factors, menopausal symptoms, compliance and potential adverse effects.

Lower levels of serum cholesterol were noted in participants on both soy diets compared with those on the carbohydrate diet (6 percent decline). Levels of low-density lipoprotein (LDL) cholesterol were reduced 7.5 percent. No marked differences were noted in high-density lipoprotein (HDL) cholesterol levels. Compared with participants on the carbohydrate diet, triglyceride levels were 28 mg per dL (0.32 mmol per L) lower in those on the once-daily soy diet and 15 mg per dL (0.18 mmol per L) lower in those on the twice-daily soy diet, but the differences were not statistically significant. No marked diff erences were observed in systolic blood pressure among participants in the three groups, while diastolic blood pressure levels were notably lower in those consuming the twice-daily soy diet compared with those on the carbohydrate diet. No significant decline in diastolic blood pressure levels was noted in the once-daily soy diet group. Weight did not differ among the women in the three groups.

An improvement in symptoms attributed to estrogen deficiency was observed in both soy diet groups but only reached statistical significance in the twice-daily soy diet group compared with the carbohydrate diet group. Hot flush severity was lower for participants on the split-dose soy diet. Severity of night sweats improved slightly in those on the soy diets, although the improvement was not statistically significant.

Statistically higher levels of blood urea nitrogen were noted in both soy diet groups, attributable to the increased protein intake. No differences were noted in liver function tests, serum protein or fasting glucose levels; however, alkaline phosphatase levels decreased in both soy groups. Adherence to the dietary supplements and to the overall study was excellent. Soy protein supplementation was extremely well-tolerated. Consumption of the indicated amount of soy protein had no short-term adverse consequences on any of the measured variables.

This study extends findings of previous investigators by documenting the effect of soy supplementation on lipoproteins in nonhypercholesterolemic women. Based on mortality data from the lipid research clinics study, a 6 percent decrease in total cholesterol levels would be expected to reduce coronary heart disease by approximately 12 percent, suggesting a substantial impact on coronary disease prevention from the use of dietary soy protein. Whether or not the lower levels of alkaline phosphatase noted in the soy-treated groups represent a marker of bone metabolism will need to be evaluated with caution. Reduction in the severity of vasomotor events in the soy-treated groups suggests that more consistent levels of circulating phytoestrogens resulting from a split-dose regimen might be more beneficial than the levels obtained from a single-dose regimen.

The authors conclude that soy protein supplementation in perimenopausal women may produce beneficial effects on cardiovascular risk factors and have the additional benefit of reducing the severity of vasomotor events. These data suggest the potential importance of soy supplementation in reducing chronic disease risk in Western populations.

editor's note: Women are independently selecting “natural” menopausal therapies, often without the knowledge of their physician. Although there have been reports in the alternative medicine literature, the mainstream scientific literature has reported on few randomized-controlled trials comparing natural therapies such as soy protein with traditional hormonal replacement therapy. A scientific comparison of the dietary supplements with estrogen should be a top priority. If beneficial effects of soy protein cannot be affirmed, patients need to be informed. If beneficial effects are confirmed, physicians will need to learn how to prescribe the supplement effectively and safely.—b.a.

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Copyright © 1999 by the American Academy of Family Physicians.

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