The Health & Human Services' Agency for Healthcare Research and Quality (AHRQ) has reported that across 68 studies that examined the impact of soy on cholesterol levels, consumption of soy products resulted in a 5 mg/dL (about 3 percent) reduction in LDL and an 8 mg/dL (about 6 percent) decrease in triglyceride levels in the populations studied.
Among these studies, a large variety of soy products, doses of soy protein, and doses of soy isoflavones were tested. The average dose of soy protein in the studies was equivalent to about one pound of tofu or three soy shakes daily.
According to the AHRQ, the studies suggest that soy consumption may be more effective at lowering LDL among people with higher LDL levels. Also, larger amounts of soy protein, but not soy isoflavones, are more effective in people with abnormally elevated LDL levels.
And according to new evidence supported by AHRQ, the isoflavones found in soy may also help reduce the frequency of hot flashes in post-menopausal women.
However, AHRQ has cautioned against over-optimism, pointing out that available studies on the health impacts of soy are limited in number, and that their duration is too short to lead to any definite conclusions.
In addition, the AHRQ found insufficient data among the 200 human studies examined as part of this analysis to suggest that soy had an effect on bone health, cancer, kidney disease, endocrine function, reproductive health, neurocognitive function, or glucose metabolism.
"This report shows us that there is a lot we don't know about soy, and that more research is needed to determine if soy has any impact on a number of health conditions," said AHRQ director Carolyn M. Clancy.
"An important role for AHRQ's Evidence- based Practice Centers is to identify gaps in evidence and knowledge that can be used to develop future research agendas."
The researchers who conducted the evidence review, which was supported by the National Institute of Health (NIH)'s National Center for Complementary and Alternative Medicine and Office of Dietary Supplements, considered the type of soy product used, amount consumed, frequency of consumption, and safety issues in their review of health effects.
"The AHRQ report provides valuable information about the studies that have been done on soy for a variety of health conditions," said Stephen E. Straus, M.D., NCCAM Director. "It also highlights research needs in this area and will help inform NIH's research agenda."
The AHRQ report notes that future studies of the health effects of soy need to better address the complex relationship between health and food components, including how variations in the diets, lifestyles, and health of participants might affect the results. Such studies should also be of better quality, include larger numbers of participants, and be of longer duration, the researchers said. In particular, studies that substitute practical amounts of soy products into people's diets would better address the question of whether people should make the effort to include more soy in their diet.
A wide variety of soy products were studied, including foods such as soybeans, soy flour, soy milk, tofu, miso, tempeh, natto, and okara; isolated and textured soy protein that is added to foods; and soy-derived isoflavone supplements.
The AHRQ's Effects of Soy on Health Outcomes can be found online here.