The Effects of Soy Intake

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Recently in the U.S., soy has become an increasingly popular ingredient in people’s diets. People commonly think that soy has properties that can reduce the risks of breast cancer and prostate cancer. On the other hand, people also think that soy intake can lead to an increase in estrogen in men, causing them to have a lack in sex drive, be over-emotional, and even grow breasts. The purpose of this post is to investigate the true effects of soy based off of credible studies.

In 1998, Chrisato Nagata, M.D., of the Department of Health and the Gifu University of Medicine in Japan, and a group of four other Japanese scientists created a study to analyze soy’s effects on breast cancer. The logic behind the soy-breast cancer connection is that many people think estrogens cause the development of breast cancer and that the isoflavones (phytoestrogens) in soy can reduce the estrogen levels. In order to test whether soy reduces estrogen levels, Nagata et al. randomly assigned 29 premenopausal women to the control group on their regular diet and 31 premenopausal women to a soymilk diet and ran the study for the time it took for three menstrual cycles to end in all the participants. Before starting the study period, the participants filled out a questionnaire on their menstrual histories and basic demographic information. The participants were also weighed before and after the study period. The answers to the questionnaire showed no significant differences in height, weight, smoking frequency, or other demographic variables between the two groups of women, which helped to root out confounding variables. They were also weighed after the experiment. The soy-supplemented diet consisted of a daily intake of 400 mL of soymilk. At the end of the first and third menstrual cycles for both of the groups of study participants, follicular-phase blood samples were taken.

At the end of the study period, the study found a 23% decrease in estrone levels in the soymilk-supplemented group and a 0.6% increase in the control group. The study also found a 27% decrease in estradiol levels in the soy group and a 4% increase in the control group. The participants also kept record of the menstrual cycle length that showed that the control group’s cycles were shortened by 1 day whereas the supplemented group’s cycles on average increased by 2 days. Based off of statistical analysis, though, these results were deemed statistically insignificant and it was concluded that a larger study is necessary to confirm the results of this study that support the theory that soy intake increases menstrual cycle length while reducing estrogen levels in women, therefore preventing breast cancer.

Jill M. Hamilton-Reeves, Ph.D., of the Department of Food Science and Nutrition and the University of Kansas, along with a group of four other scientists ran a 6-month long, single-blinded, placebo-controlled study to test the theory that soy products can reduce prostate cancer risk. Steroid hormones have commonly been associated with increasing the risk of prostate cancer and the theory being tested is that soy can reduce the amount of those hormones. The study consisted of 58 male participants who were randomly divided into three groups. One group was instructed to consume a soy protein isolate daily that contained 107 mg of isoflavones. Another group consumed an alcohol-washed soy protein isolate daily that was made up of less than 6 mg of isoflavones and the final group drank a milk protein isolate daily that contained 0 mg of isoflavones. Serum samples that were tested for a variety of hormones were taken from the participants at the beginning of the study, halfway through (3 months), and at the conclusion of the study period. Also, prostate biopsy samples were taken before and after the study period and analyzed for estrogen receptor-β expression and androgen receptor (AR) expression. Relative to the first samples taken, the group consuming less than 6 mg of isoflavones had a significant increase in serum estradiol concentration. However, the most significant results were that the group consuming 107 mg a day of insoflavones was found to have a significant decrease in AR expression levels in comparison to the other groups, which indicates that soy and other foods that are rich in isoflavones may in fact be capable of reducing the risk of prostate cancer.

In an attempt to test whether soy can actually reduce serum estrone concentrations and possibly verify Hamilton-Reeves’s results, Christo Nagata and the same group of scientists in Japan made a randomized dietary intervention study of 35 men between the ages of 22 to 50. The men were divided into 2 groups, one given a soy-supplemented diet of 400 mL of soymilk daily and the other asked to stick to their regular no-soy diet. In order to find the estrone concentrations, Nagata et al. took the blood samples of the participants every two weeks for 12 months. After the study period, there were notable differences in the serum estrone samples taken between the two groups. The serum estrone decreased in the soy-supplemented group and increased in the control group, as shown in the graph below. By lowering the circulating estrone concentrations in men, this difference supports the hypothesis that soy reduces prostate cancer risk, but it is still not proven.

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In addition to these studies, there have been experiments on male rats that have rejected the hypothesis that soy products cause decreases in testosterone in men. Mindy S. Kurzer of the Department of Food Science and Nutrition at the University of Minnesota, did a meta-analysis in 2002 and cited these experiments. Other cited examples include a dietary study done on 14 men between 18 and 35 years of age in which after soymilk intake of 40 mg per day for 3 months, there were no differences found in sperm count or quality compared to before the study period. There is the possibility that the results from this meta-analysis were affected by the file drawer problem, meaning that studies finding no significant effects from soy on the human body were put into the file drawer and therefore not used for this meta-analysis, but that would only support the hypothesis that soy does not “feminize” men. Overall, all the experiments above, while inconclusive, suggest that soy may reduce breast and prostate cancer risk while also not reducing testosterone or other hormones in men.

 

 

One thought on “The Effects of Soy Intake

  1. Taylor Harrington

    This post is very well done. I liked how you found one example of an experiment, noted that it did not have the information you needed to make a concrete conclusion and looked for a larger study. I do want to note in your last paragraph, you suggest that the meta-analysis may have suffered from the file drawer problem. Due to the fact that this study is published on The Journal of Nutrition, like you cited, it cannot suffer from the file drawer problem. This study was made public. The results were not hidden and tucked away somewhere. Soy milk is often compared to almond milk, which is becoming increasingly popular, even though it has its own set of possible negative side effects. It would be a very interesting follow-up post to analyze almond milk’s negative side effects. I’d be interested to what you would conclude. Which type of milk should we drink? Does it depend on the person and what kinds of health issues he or she already has? These are important questions to ask.

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