Are phytoestrogens harmful to men?

Many plant foods contain phytoestrogens, compounds that are similar to the hormone estrogen.

Some people believe that eating foods rich in phytoestrogens can affect fertility in men, while others claim that these compounds are healthy.

This evidence-based review examines science.

What are phytoestrogens?

Phytoestrogens are a group of natural compounds found in numerous plant foods.

They have different functions in the plants. Many have strong antioxidant properties and some may play a role in the defense of plants against infection (1, 2).

They are called "phytoestrogens" because their chemical structure resembles the structure of the sexual hormone estrogen. The prefix "phyto" refers to plants.

Estrogen levels are higher in women than in men.

This hormone is responsible for the fertility of women and for maintaining female body characteristics, but it also plays an important role in men.

The similarity of phytoestrogens with estrogen means that they can interact with estrogen receptors in cells. These receptors mediate the functions of estrogen within the body (3).

However, the effects of phytoestrogens are much weaker than those of estrogens. Also, not all phytoestrogens work in the same way. Some block the effects of estrogen, while others mimic its effects (4).

Phytoestrogens are found in most foods derived from plants in varying amounts. All of them belong to a large group of plant compounds known as polyphenols (5, 6, 7, 8).

Some of the most studied phytoestrogens include:

  • Lignans: It is found in many plant foods rich in fiber, such as seeds, grains, nuts, fruits and berries. Flax seeds are an especially rich source (9, 10).
  • Isoflavones: These are the most widely studied phytoestrogens. They are abundant in soybeans and other legumes, and are also present in berries, grains, nuts and wine (7).
  • Resveratrol: It is found in fruits, berries, red wine, chocolate and peanuts. He is believed to be responsible for some of the health benefits of red wine.
  • Quercetin: This is one of the most common and abundant antioxidant flavonoids found in numerous fruits, vegetables and grains (4).

The knowledge of phytoestrogens is gradually expanding, and scientists are regularly discovering new types.

While some researchers are concerned that high doses of phytoestrogens may alter the body's hormonal balance, most studies have associated them with health benefits.

Summary: Phytoestrogens are plant compounds that are structurally similar to the sex hormone estrogen. They are found in most vegetable foods.

Are phytoestrogens healthy or harmful?

Most studies indicate that phytoestrogens can be beneficial for health.

However, some studies suggest that a high intake of isoflavones may cause problems in certain circumstances.

The next two sections discuss the possible benefits and drawbacks of phytoestrogens.

Benefits of health

Several studies show that phytoestrogen supplements can provide health benefits.

  • Reduction of blood pressure: Supplements of resveratrol and quercetin can reduce blood pressure (11, 12).
  • Improves blood sugar control: Resveratrol, flaxseed lignans and soy isoflavones may benefit blood sugar control (13, 14, 15).
  • Reduction of the risk of prostate cancer: Isoflavone supplements may reduce the risk of prostate cancer, but solid conclusions can not be reached without further investigation (16).
  • Low cholesterol levels: Soy isoflavone supplements can lower the levels of total cholesterol and "bad" LDL cholesterol (17).
  • Less inflammation: Soy isoflavones and lignans can reduce CRP levels, an inflammatory marker, in postmenopausal women with high CRP levels (18, 19).

None of the studies mentioned above reported that the phytoestrogen supplements they tested had serious side effects.

Adverse effects

Some scientists are concerned that a high intake of phytoestrogens may alter the body's hormonal balance.

In fact, phytoestrogens are classified as endocrine disruptors. These are chemicals that can interfere with the body's hormonal system when consumed in a high enough dose.

However, there is not much evidence that phytoestrogens have harmful effects in humans (20).

Some studies suggest that a high intake of isoflavones in infant formulas based on soy may suppress thyroid function when iodine intake is poor (21, 22).

They also indicate that isoflavones can suppress thyroid function in those who have poor thyroid function, known as hypothyroidism, to begin with (23).

However, most studies in healthy people have not found any significant association between isoflavones and thyroid function (24, 25).

Currently, no good evidence associates other common phytoestrogens with adverse health effects in humans (26, 27, 28, 29).

Summary: Phytoestrogen supplements do not seem to have serious side effects. But some evidence indicates that high doses of isoflavones can suppress thyroid function in children who have low levels of iodine.

Do phytoestrogens affect male fertility?

When it comes to men's health, scientists worry that excessive exposure to phytoestrogens may reduce male fertility.

A study in cheetahs indicated that a high consumption of phytoestrogens affected the fertility of males (30).

However, scientists have pointed out that phytoestrogens probably have different effects on carnivores, such as cheetahs, compared to omnivores, such as humans.

In fact, no solid evidence associates a high intake of phytoestrogens with fertility problems in humans (31, 32, 33).

The most studied phytoestrogens are soy isoflavones. An analysis of 15 controlled studies concluded that soy isoflavones, whether in food or supplements, do not change testosterone levels in men (34).

In addition, one study showed that taking 40 grams of isoflavone supplements per day for two months did not affect the quality or volume of men's semen (35).

An observational study showed that a soy-based infant formula was not related to self-reported male fertility or puberty, compared to a cow's milk formula (36).

However, not all observational studies agree. Another study showed that a high consumption of soy, which is rich in isoflavones, was associated with a lower sperm count, but the researchers did not know if the isoflavones were responsible (37).

In short, most evidence indicates that isoflavones do not adversely affect the fertility of men. Although a study in cheetahs suggested that a high intake of phytoestrogens can affect fertility, the same does not necessarily apply to humans.

However, scientists know little about the effects of other phytoestrogens or about the long-term intake of high-dose human supplements. More research is needed.

Summary: Isoflavones, a common group of phytoestrogens, do not seem to cause fertility problems in men.

The bottom line

No solid evidence shows that phytoestrogens cause problems in healthy men.

Phytoestrogens are abundant in many healthy plant foods. In most cases, the benefits of eating these foods outweigh the possible health risks.

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