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How Soy Affects an Enlarged Prostate

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Soy has enjoyed a rapid, if controversial, rise in popularity over the last few years. Its adoption in the US was driven by the health benefits soy products give Asians who traditionally eat them. Since its use became widespread, different research has been conducted to investigate the medicinal benefits and dangers of soy consumption. One of the best results have been obtained while studying the effect of soy on the prostate gland. Can soy help shrink enlarged prostates? How does it work? Is it safe? Which soy products are the most beneficial for the prostate? This comprehensive article answers all your questions.

What is Soy?

Soy is obtained from the seeds of soybean or Glycine max. It is a popular species of legume native to East Asia but now widely grown all over the world especially in the United States.

Soy is a versatile food crop. When fermented, it is used to make foods such as soy sauce, soy paste, natto, and tempeh. Unfermented soy is used to make soy milk and tofu. Besides these, other soy food products include miso, edamame and TVP (textured vegetable protein) which is used to make substitutes for meat and dairy products for vegetarians and other people who eat special diets.

The oil obtained from soy can be used as a vegetable cooking oil and also in a number of industrial applications.

Soy is regarded as the most efficient use of land with regards to producing protein food sources for man.

Because soy contains appreciable amounts of all of the essential amino acids, it is considered a source of complete protein. In addition, compared to other protein sources, soy protein is relatively stable to cooking heat.

By the standard used to measure protein quality, soy ranks as an excellent source of nutritional protein along with meat and eggs.

Besides protein, the oil and carbohydrate contents of soybean are also high (40% protein, 35% carbohydrate and 20% oil). Other nutrients found in soybean include vitamins A, B6, B12, C, and K as well as iron, calcium, magnesium, potassium, sodium, zinc, phosphorus and choline.

Most of the micronutrients in soy can also serve as medicinal phytochemicals. For example, soy contains phytic acid, isoflavones, and the omega-3 fatty acid, ALA (alpha-linolenic acid) all of which can be both nutritional and medicinal.

Other important phytochemicals in soy are saponins and phytosterols such as sitosterol and stigmasterol.

The Health Benefits of Soy

Over the years, soy has been claimed to be healthful or harmful by different researchers and different interest groups.

Often, these claims are made regarding the same disease linked to soy consumption or the same soy phytochemicals. For example, while some studies suggest that soy may reduce the risk of developing breast cancer, other studies have reached quite the opposite conclusion.

One of the most popular therapeutic benefits of soy is the reduction of LDL (low-density lipoprotein or “bad” cholesterol) levels.

Different studies have confirmed this effect and it is estimated that soy can reduce LDL levels by 5 – 10%.

In addition, soy can reduce total cholesterol and triglyceride levels. However, it does not significantly increase the level of HDL (high-density lipoprotein or “good cholesterol”). Still, researchers agree that these effects are enough to reduce the risk of congestive heart disease.

Even the FDA (Food and Drug Administration) has officially approved the marketing of soy foods for its cholesterol-lowering property and health benefits for the heart.

The wide adoption of soy and soy products in the US was largely driven by this FDA approval.

Another popular health benefit of soy is its anti-cancer properties. Different studies have established that soy can indeed reduce the risks of cancers affecting organs in the endocrine system. Therefore, soy can prevent breast, uterine and prostate cancers.

However, most studies show that Asians benefit more from the anti-cancer effects of soy than Westerners. This is because Asians eat diets with high soy content for most of their lives while Americans are just adopting the protein source.

Other medical disorders that soy may help treat include osteoporosis during menopause and other menopausal signs such as hot flashes.

However, the results of studies investigating the benefits of soy for these disorders have been mixed.

Side Effects, Contraindications and Health Risks of Soy

Soy allergy is the most common adverse reaction to soy products. It occurs within minutes of eating soy and its symptoms include itching and hives. In rare cases, a full anaphylactic reaction may develop after soy ingestion.

In addition, food intolerance has been associated with soy products. In these cases, vomiting and diarrhea result even when skin allergy tests fail to detect soy allergy.

Soy products should also be avoided by women who have or have had breast and uterine cancers. Even though soy is known to reduce the risks of these cancers, it can stimulate the growth of cancerous cells if they are already present.

Furthermore, the isoflavones in soy can reduce iodine levels in the body. This affects the production of thyroid hormones and can cause hypothyroidism.

Soy can also affect the efficacies of certain drugs such as tamoxifen and raloxifene. Zinc and iron, on the other hand, can reduce the absorption and level of soy in the body.

Non-steroidal anti-inflammatory drugs such as aspirin and blood thinners such as warfarin should not be used with soy as the phytochemicals in soy can increase the risk of bleeding.

Lastly, MAOI (monoamine oxidase inhibitor) antidepressants are to be avoided when taking soy.

How Soy Affects the Prostate

The most solid evidence that soy can reduce prostate size is from population studies that establish that Asian men have a lower risk (and lower incidence) of prostate cancer than men living in Western countries. Furthermore, this protection was lost in the second generation of Asian men whose parents moved to Europe and the US and then abandoned soy-rich diets.

Prostate cancer is the second most common cancer affecting men but soy products such as soy milk, tofu and tempeh can reduce the risk of this cancer by as much as 70%. How does soy achieve this?

The exact mechanisms by which soy protects the prostate are not well understood but researchers are certain that the isoflavones in soy are involved.

Men who regularly eat soy-rich foods are known to have higher levels of isoflavones than those who do not. In addition, the isoflavones obtained from soy products are mostly found in the fluids filling the prostate.

Once absorbed, isoflavones preferentially accumulate in the prostate rather than in the blood.

This means that this group of soy phytochemicals can help promote prostate health more than increase the risks of all the health conditions associated with them.

Isoflavones are also phytoestrogens. This means that although they are obtained from plants, they have estrogenic properties. Therefore, they may cause hormonal imbalance although their ability to do this is rather limited.

Even then, isoflavones definitely work by influencing hormonal balance. They are known to be good for reducing the risks of hormone-dependent cancers such as breast, uterine and prostate cancers.

The 3 most important isoflavones in soy are daidzein, glycitein, and genistein (the most bioactive).

How Soy isoflavones Can Help
Being potent antioxidants especially known for removing reactive oxygen species particularly singlet oxygen
Influencing the metabolism of sex hormones; however, soy does not prevent prostate hyperplasia (enlargement) by reducing the levels of testosterone; rather it counteracts androgens with its estrogen-like activities
Influencing cellular growth via contributing to the regulation of enzymes within cells, protein synthesis and the effects of growth factors
Inhibiting the rapid increase of malignant cells and also the rapid growth of new blood vessels feeding tumors

Soy consumption is also known to reduce PSA levels in men.

PSA or prostate-specific antigen is naturally produced in the prostate gland to increase the motility of sperms. While all men have a small amount of PSA in their blood, a high level indicates a high risk of prostate cancer.

In addition, soy isoflavones may not increase the levels of male sex hormones such as testosterone but the estrogenic activities of these phytoestrogens can counterbalance the effects of male sex hormones.

One such effect of male sex hormones is the enlargement of the prostate. Therefore, soy phytoestrogens can neutralize androgens, inhibit the growth of prostate cells and, therefore, prevent the enlargement of the gland.

Studies on Soy and Prostate Health

Is it Effective?

Having established that genistein, the most active isoflavones in soy, can inhibit the growth of prostate cancer cells, a group of researchers investigated the clinical effects of soy isoflavones in the treatment of prostate cancer.

In a study published in the journal, Nutrition, and Cancer, in 2003, 41 patients were recruited, and divided into 3 groups:

3 Groups of the Study
Group 1 – newly diagnosed and untreated prostate cancer cases with rising PSA levels (4 patients)
Group 2 – treated cases but with the PSA levels still increasing even after basic therapy (18 patients)
Group 3 – treated cases but with the PSA levels still increasing even after hormone therapy (19 patients)

All of the participants were given 100 mg of Novasoy (an oral soy isoflavones supplement) twice daily for 3 – 6 months.

While the soy product did not reduce PSA levels in the participants, it stopped its increase and even stabilized its levels in 83% of the patients in group 2 and 35% of the patients in group 3. As expected, soy did not reduce the levels of testosterone.

These results show that soy isoflavones may provide some benefits for some prostate cancer patients even when standard treatments fail.

A 2011 study published in Southern Medical Journal, however, confirmed that soy can indeed reduce PSA levels. In that study, the researchers treated 10 men with rising levels of PSA even after prostatectomy and radiotherapy.

The men were given commercially available soy products for an average of 24 months. During this period, the PSA levels were reduced in half the participants.

Combination with Other Natural Products

In the Phase II clinical trial published in 2007 in the journal, Nutrition, and Cancer, a group of researchers determined whether the combination of soy isoflavones and lycopene was better than lycopene alone in bringing down the size of enlarged prostates.

For this study, 71 patients with high PSA levels were recruited and randomly assigned either 15 mg lycopene or 40 mg soy isoflavones plus 15 mg lycopene twice daily for a maximal duration of 6 months.

The results showed that 95% of the patients in the lycopene group experienced stabilization of PSA levels while 67% of patients in the drug combination group enjoyed the same benefits.

This study showed that even though soy isoflavones and lycopene were effective for reducing the size of an enlarged prostate and reducing the risk of prostate cancer, their combination was less effective than lycopene alone.

In a 2010 study published in the same journal, researchers investigated the benefits of soy isoflavones for prostate cancer patients receiving radiotherapy. It is believed that soy can reduce the side effects of radiation therapy by its antioxidant and anti-inflammatory properties. In addition, the benefits of soy for prostate health can also contribute to the positive outcome of the combination therapy.

For the study, the researchers gave each of the 42 prostate cancer patients receiving radiotherapy 200 mg of soy isoflavone or placebo for 6 months.

The results showed that soy isoflavone reduced urinary, gastrointestinal and sexual side effects of radiation therapy. Therefore, the patients in the soy group saw fewer of these side effects than those in the placebo group.

This study shows that even when treating prostate cancer with drastic measures such as radiotherapy, supplementation with soy can still provide additional benefits.

A 2011 study published in the Journal of Nutritional Biochemistry determined whether the combination of soy and green tea can help reduce prostate size in prostate cancer cases.

Chronic inflammation and nuclear factor-kappa B have been linked to prostate cancer and since Asian men have a lower risk of prostate cancer than men in Western countries, many believe that the difference in the traditional diets of the two cultures is responsible.

The major food sources of anti-inflammatory agents in Asian diets are soy and green tea.

In this study, the researchers induced prostate hyperplasia in a group of male rats and then divided them into 4 groups according to the test drugs given to them. One group received soy alone; another group got green extract alone; the third group was given a combination of the soy and green tea, and the last group received a placebo.

The results showed that the best response was obtained from the group that received soy and green tea.

In this group, the combination reduced the levels of inflammatory factors in the prostate gland, and also prevented prostate cancer by reducing the excessive proliferation of prostate cells (prostate hyperplasia).

While it is necessary to see if the same effects can be replicated in humans, the study suggests that the combination of soy products and green tea can significantly reduce the size of an enlarged prostate and even lower the risk of prostate cancer.


In a meta-analysis of 5 cohort studies and 8 case-controlled studies published in 2009 in the journal, Nutrition, and Cancer, a team of Korean researchers weighed the benefits of soy food consumption against the risk of prostate cancer.

From the studies selected, these researchers concluded that the soy isoflavones, genistein, and daidzein, were linked with a lowered risk of prostate cancer. They also agreed that soy food consumption can reduce the risk of prostate cancer although the consumption of natto, miso or soybean milk did not seem to help.

In the same year, a second meta-analysis was published on the same subject in The American Journal of Clinical Nutrition. Here, the researchers analyzed a total of 24 epidemiological studies on the link between prostate cancer risk and soy/isoflavone consumption.

They concluded that soy food consumption can indeed lower the risk of prostate cancer but the degree of protection offered depended on the type and quantity of soy food products consumed.





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