Most of the studies evaluated the effects of soy on various biomarkers or measures, not clinical outcomes, although, several of the endpoints, such as blood pressure, LDL, and bone mineral density, do have known meaningful correlations with clinical outcomes. Cardiovascular surrogate endpoints were assessed by the largest number of studies. Overall, soy was found to have a small effect on lipids. However, the duration of these studies were generally short, and it is uncertain whether the results would be sustained. No study evaluated clinical cardiovascular disease.
Reduction of hot flashes by soy was seen in trials involving post-menopausal and peri-menopausal women. Most of the trials lasted only 3 to 4 months, thus the long-term benefits remain unclear. In addition, different measurements were used to assess benefits across studies making comparisons and synthesis difficult. Soy phytoestrogens are seen by some as an alternative to oestrogen therapy to treat post-menopausal symptoms. However, the estrogenic effect of soy in potentially promoting tumour recurrence raises the concern for its use by breast cancer survivors. The current literature provides no data to address this issue.
The evidence does not support an effect of soy products on endocrine function, menstrual cycle length, or bone health; although evidence was often limited and of poor quality. No study evaluated clinical endocrine or bone disease.
This report was limited to human studies, and thus was unable to fully respond to biological or biochemical hypotheses of benefits or harms of phytoestrogens suggested by various animal, in vitro, or assay detection studies: the correlations between specific nutrients and their effects remain unclear. While the evidence does suggest a greater likelihood of adverse events with soy consumption, these were mostly minor in nature. There were a limited number of studies with duration of 1 year or longer, thus the long-term adverse effect of soy in a large population is uncertain.
For all outcomes, including adverse events, there is no conclusive evidence of a dose-response effect for either soy protein or isoflavone. However, for LDL reduction, there is a suggestion of a possible dose-response effect for soy protein.
Balk E, Chung M, Chew P, et al. Effects of Soy on Health Outcomes. Summary, Evidence Report/Technology Assessment: Number 126. AHRQ Publication Number 05-E024-1, August 2005. Agency for Healthcare Research and Quality, Rockville, MD. http://www.ahrq.gov/clinic/epcsums/soysum.htm
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