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Soy: Health Food or Food to Avoid?

Part One

by Gary F. Zeolla

In recent years, soy has been promoted has a “health food” with many potential benefits proposed for soy consumption. However, the proposed benefits are not without controversy. A quick search of the Internet will produce many pages claiming there are more potential drawbacks to soy consumption that there are benefits. The main reason for this controversy is that soy contains phytoestrogens, which are plant based forms of estrogen. This article will present some of the claims on either side. For this article, I did extensive research about soy on the Internet. I did both general Internet searches using a standard search engine. I also did research on PubMed. This site indexes abstracts of studies reported in scientific journals.

Prostate Cancer

One important proposed benefit of soy consumption is a reduced risk of prostate cancer. And there is some evidence to support this claim. For instance, a study done on Chinese men concluded, “Our results indicate a reduced risk of prostate cancer associated with consumption of soy foods and isoflavones” (Soy and isoflavone consumption in relation to prostate cancer risk in China).

And another abstract I came across was titled, Emerging evidence on the role of soy in reducing prostate cancer risk.

It stated:
In vitro, the main soybean isoflavone, genistein, inhibits prostate cancer cell growth; in animals, most but not all studies show isoflavonel rich soy protein and isolated isoflavones inhibit prostate tumor development. Currently, although only limited epidemiologic data indicate soy intake reduces prostate cancer risk, results from a pilot intervention trial suggest isoflavones may be beneficial to prostate cancer patients. For several reasons, men concerned about their prostate health may consider incorporating soy into their diet.

However, soy is not the only food that is associated with a reduced risk of prostate cancer. Green tea and tomato products have also been shown to reduce the risk of prostrate cancer. So for those concerned about prostate cancer, there are less controversial dietary options available.

Breast Cancer

It is with breast cancer that the controversy really heats up. The Mayo Clinic sums up the controversy, “Does soy help prevent or promote breast cancer? Right now, it’s impossible to say, according to the February issue of Mayo Clinic Women’s HealthSource.”

Many Web sites can be found that warn of the potential of soy to increase breast cancer risk. For instance, one article was titled, Experts Concerned About Soy and Breast Cancer.

It stated:
While the U.S. Food and Drug Administration (FDA) allows food labels to display a health claim stating that soy products can lower blood cholesterol, the amount of soy foods necessary to achieve this benefit -- as many as four servings a day -- comes awfully close to the level that some researchers say might increase the risk for developing breast cancer. That's right: Some scientists are concerned that too much soy could promote the growth of malignant tumors rather than protect against them.

However, all of the abstracts I read on PubMed reported a reduced risk of breast cancer from soy consumption. None of the studies found an increased risk. For instance, “In a population-based, prospective cohort study in Japan, frequent miso soup and isoflavone consumption was associated with a reduced risk of breast cancer” (Soy, isoflavones, and breast cancer risk in Japan).

And another study found a reduced rate of breast cancer was associated with an increased intake of tofu among Asian-Americans. But it concluded by stating, “We cannot discount the possibility that soy intake is a marker of other protective aspects of Asian diet and/or Asian lifestyle” (Tofu and risk of breast cancer in Asian-Americans).

Also, as with prostate cancer, the consumption of green tea, along with black tea, can reduce the risk of breast cancer (Green tea and risk of breast cancer in Asian Americans). The consumption of vegetables is also associated with a reduced risk of breast cancer (Epidemiologic evidence of the protective effect of fruit and vegetables on cancer risk).

Other Cancers

So there seems to be some evidence that soy can reduce the risk of prostate and breast cancer. But what about other kinds of cancer? I didn’t research this area in too much depth. But I did come across some interesting abstracts on PubMed.

One abstract reported, “Several case-control studies have suggested that soy food consumption may confer a reduced risk of colorectal cancer although the findings are rather inconsistent” (Soybeans, Soy Foods, Isoflavones and Risk of Colorectal Cancer: a Review of Experimental and Epidemiological Data).

Another abstract concluded, “These data suggest that soy intake may reduce the risk of death from stomach cancer” (A prospective cohort study of soy product intake and stomach cancer death). So these two abstracts were encouraging.

However, an abstract titled Phyto-oestrogens and cancer cautioned in regards to such positive effects, “Whether these observed protective effects are caused by the presence of dietary phyto-oestrogens, or whether they are merely indicators of a healthy diet in general, has not been established.”

Moreover, another abstract reported, “High intake of soyfood was statistically significantly related to an ELEVATED risk of bladder cancer” (emphasis added, Dietary soy and increased risk of bladder cancer: the Singapore Chinese Health Study).

To conclude on soy and cancer, there is evidence that for several forms of cancer, soy can exert a protective effect. However, it is not always easy to say if the protective effect is due to soy consumption per se or to other healthy, related behaviors. Moreover, there is concern but only limited evidence that soy can actually increase the risk of some kinds of cancer. It also needs to be emphasized that there are other foods that can exert protective effects against cancer besides soy. Foremost amongst these are green tea, tomato products, and fruits and vegetables, and for colorectal cancer, yogurt.

Cholesterol and Heart Disease

A rather notable proposed benefit of soy is its ability to lower LDL cholesterol levels when included as part of a low-fat diet. This is notable as food labels are now allowed to include notices to this effect.

The University of Arkansas: Division of Agriculture reports the following:
More than 40 studies have shown that replacing all or part of the animal protein in the diet with soy protein significantly lowers LDL-cholesterol. The effects are greatest in people with moderate to severe high cholesterol and when soy protein is part of a diet that is low in fat and saturated fat. The evidence is so strong that the FDA approved health claims about soy protein reducing the risk for coronary heart disease for low-fat soyfoods containing at least 6.25 grams of soy protein per serving (Soy and Heart Disease).

So the evidence that soy lowers cholesterol levels is rather strong. But the question still remained if this translated into lowered rates of coronary heart disease (CHD). A study of Chinese women looked at this question, and concluded, “This study provides, for the first time, direct evidence that soy food consumption may reduce the risk of CHD in women” (Soy food consumption is associated with lower risk of coronary heart disease in Chinese women).

So there is even evidence that soy can reduce not just cholesterol levels but the actual risk of CHD. However, this benefit is only notable when included as part of a low fat diet, which in itself can reduce the risk of CHD. Also soy is not the only food that has been found to have this benefit. Also associated with a reduced risk of CHD is a high fiber diet with an emphasis on fruits and vegetables, nuts and seeds, whole grains, and legumes. Also associated with a reduced risk is the consumption of fish and green and black tea and the use of supplemental vitamin C, folate, and vitamin B6. Exercise and other factors are also important. So there are many steps that a person can take to reduce the risk of CHD.


Another proposed benefit of soy is in regards to osteoporosis. For instance, John Robbins, a noted vegetarian advocate, claims, "In a study in which people consumed the same amount of calcium and protein, those who consumed animal protein lost 50% more calcium from their bodies than those who consumed soy protein" (Spotlight on Soy). And in an article about breast cancer and soy intake, the same author mentions, “Soy may also protect against bone loss (osteoporosis)” (Soy and Breast Cancer). However, the evidence is not so clear-cut.

On study simply reviewed many different studies that had been done on this subject.

It concluded:
This review focuses specifically on the potential influence of phytoestrogens on bone by examining the evidence from 17 in vitro studies of cultured bone cells, 24 in vivo studies of animal models for postmenopausal osteoporosis, 15 human observational/epidemiologic studies, and 17 dietary intervention studies. On balance, the collective data suggest that diets rich in phytoestrogens have bone-sparing effects in the long term, although the magnitude of the effect and the exact mechanism(s) of action are presently elusive or speculative (Dietary phytoestrogens and their effect on bone: evidence from in vitro and in vivo, human observational, and dietary intervention studies).

One study I found on PubMed concluded, “These results indicate that soy protein may positively influence bone and calcium homeostasis in postmenopausal women, particularly those not on HRT [hormone replacement therapy] (Soy protein has a greater effect on bone in postmenopausal women not on hormone replacement therapy, as evidenced by reducing bone resorption and urinary calcium excretion).

However, another study on postmenopausal women found that soy did not reduced bone resorption. It concluded, “These findings suggest that soy may not have biologically significant oestrogenic effects on bone resorption …” (The effects of soy protein containing isoflavones on lipids and indices of bone resorption in postmenopausal women).

So there is some evidence that soy might be beneficial for the prevention or treatment of osteoporosis, but it is not completely compelling. Also, there are many other steps a person can take to prevent or improve osteoporosis. Among these steps would be to first avoid those foods or food elements that can contribute to osteoporosis. These include excessive phosphorus, especially from the consumption of soft drinks, caffeine, and excessive sodium.

The next step would be to consume foods and supplements containing the nutrients necessary for the building of bone. These include not just calcium, but also magnesium, manganese, boron, vitamin D, and vitamin K. The final step would be to engage in weight bearing exercise. Strength training is the best option here.

I should also mention one point that has often been cited in this regard, protein consumption. For some time, it was thought that excessive protein consumption would lead to bone loss. I even make this claim in my book Creationist Diet. However, recent evidence has proven this is not the case. In fact, the exact opposite is true. “Of concern are several recent epidemiologic studies that demonstrate reduced bone density and increased rates of bone loss in individuals habitually consuming low-protein diets.” The reason for this is that protein is required for the absorption of calcium. And laboratory studies have shown that a high protein diet increases the rate of calcium absorption. “The increased dietary protein was accompanied by a significant increase in intestinal calcium absorption …” (Dietary protein, calcium metabolism, and skeletal homeostasis revisited).

The greater levels of urinary calcium seen with the consumption of animal protein that Robbins and other vegetarians cite are not due to the loss of bone calcium. It is actually due to greater amounts of calcium being absorbed in the intestines that is later excreted. Unfortunately, despite this evidence, vegetarians and other will still often claim that excessive protein can lead to osteoporosis. But if I ever come out with a second edition of my book, I will definitely make a correction in this regard.

Hormone Levels in Men

A major issue among the anti-soy crowd concerns soy’s effect on hormonal levels in men. The claim is that soy can lower testosterone and androgen levels and raise estrogen levels. And this alteration in hormonal levels have detrimental effects on a man’s sex drive and performance and his ability to gain or maintain muscular strength and mass. However, the evidence for these problems seems to mainly come from studies done on animals.

For instance, in several different issues in 2003, Men’s Health magazine included short blurbs about studies done on mice. These studies found reduced testosterone levels and sex drive among soy-feed male mice.

And in an article titled Soy is Still Bad Protein, the online magazine Testosterone reports the following:
In intact adult male mice, genistein (2.5 mg per kg of body weight per day for only 9 days) reduced testicular and serum Testosterone concentrations and pituitary LH-content. These results suggest that genistein - in doses comparble to those that would exist in a soy-based diet - induced typical estrogenic effects. A second study showed plasma Testosterone and androstenedione levels were significantly lower in the animals fed a phytoestrogen-rich diet compared with animals fed a phytoestrogen-free diet.(9) These results indicated that consumption of dietary phytoestrogens over a relatively short period can significantly alter plasma androgen hormone levels. In a study of Japanese men, total and free Testosterone concentrations were inversely correlated with soy product intake. (5)The evidence continues. In rats that were fed a diet in which casein was replaced by soy protein isolate/isoflavones, both serum levels of Testosterone and weight of testes were significantly reduced.(7) ([Note: the parenthesized numbers refer to footnotes in which the studies are cited].

However, when this writer did a search of PubMed, I was unable to find any studies confirming these claims. I’m not saying these studies don’t exist; PubMed probably simply did not index them. However, all of the abstracts I read found little or not effect on hormonal levels from the consumption of soy.

For instance, one abstract concluded with the following:
Only three intervention studies reported hormonal effects of soy isoflavones in men. These recent studies in men consuming soyfoods or supplements containing 40--70 mg/d of soy isoflavones showed few effects on plasma hormones or semen quality. These data do not support concerns about effects on reproductive hormones and semen quality (Hormonal effects of soy in premenopausal women and men).

So it seems difficult to say one way or the other at this time if soy has adverse effects on hormone levels in men or not.

This article is continued at Soy: Health Food or Food to Avoid? - Part Two.

Soy: Health Food or Food to Avoid? Copyright 2003, 2004 By Gary F. Zeolla.

Disclaimers: All company and products names are registered trademarks of the respective companies.

The material presented in this article is intended for educational purposes only. The author is not offering medical or legal advice. Accuracy of information is attempted but not guaranteed. Before undertaking any diet, exercise, or health improvement program, one should consult your doctor. The author is in no way responsible or liable for any bodily harm, physical, mental, or emotional, that results from following any of the advice in this article.

The above article was posted on this site October 7, 2003.
It was updated February 19, 2004.

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