Many plant foods contain phytoestrogens — compounds that are similar to the hormone estrogen.
Some people believe that eating foods high in phytoestrogens may impair fertility in men, while others claim these compounds are healthy.
This evidence-based review looks into the science.
Phytoestrogens are a group of naturally occurring compounds found in numerous plant foods.
They have various functions in plants. Many have strong antioxidant properties and some may play a role in the plants' defense against infections (, ).
They're called "phytoestrogens" because their chemical structure resembles the structure of the sex hormone estrogen. The prefix "phyto" refers to plants.
Estrogen levels are higher in women than men.
This hormone is responsible for women's fertility as well as maintaining feminine body features, but it also plays an important role in men.
Phytoestrogens' similarity to estrogen means they can interact with estrogen receptors in cells. These receptors mediate estrogen's functions within the body ().
However, the effects of phytoestrogens are much weaker than those of estrogen. Also, not all phytoestrogens work the same. Some block estrogen's effects, while others mimic its effects ().
Phytoestrogens are found in most plant-derived foods in varying amounts. They all belong to a large group of plant compounds known as polyphenols (, , , ).
Some of the most studied phytoestrogens include:
- Lignans: Found in many fiber-packed plant foods, such as seeds, grains, nuts, fruits and berries. Flaxseeds are an especially rich source (, ).
- Isoflavones: These are the most widely studied phytoestrogens. They're abundant in soybeans and other legumes, and also present in berries, grains, nuts and wine ().
- Resveratrol: Found in fruits, berries, red wine, chocolate and peanuts. It 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 ().
Knowledge of phytoestrogens is gradually expanding, and scientists are regularly discovering new types.
While some researchers are concerned that high doses of phytoestrogens may disrupt 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 plant foods.
Most studies indicate that phytoestrogens may benefit health.
However, a few studies suggest that a high intake of isoflavones may cause problems under certain circumstances.
The following two sections discuss the possible benefits and drawbacks of phytoestrogens.
Several studies show that phytoestrogen supplements may provide health benefits.
- Reduced blood pressure: Resveratrol and quercetin supplements may reduce blood pressure (, ).
- Improved blood sugar control: Resveratrol, flaxseed lignans and soy isoflavones may benefit blood sugar control (, , ).
- Reduced risk of prostate cancer: Isoflavone supplements may reduce the risk of prostate cancer, but strong conclusions cannot be reached without further research ().
- Lower cholesterol levels: Soy isoflavone supplements may lower the levels of total cholesterol and "bad" LDL cholesterol ().
- Less inflammation: Soy isoflavones and lignans may reduce levels of CRP, an inflammatory marker, in postmenopausal women with high CRP levels (, ).
None of the studies referenced above reported that the phytoestrogen supplements they tested had any serious side effects.
Some scientists are concerned that a high intake of phytoestrogens may disrupt the body's hormonal balance.
In fact, phytoestrogens are classified as endocrine disruptors. These are chemicals that may interfere with the body's hormonal system when consumed at a sufficiently high dose.
However, there's not much evidence that phytoestrogens have harmful effects in humans ().
A few studies suggest that a high intake of isoflavones from soy-based infant formulas may suppress thyroid function when iodine intake is poor (, ).
They also indicate that isoflavones may suppress thyroid function in those who have poor thyroid function, known as hypothyroidism, to begin with ().
Nevertheless, most studies in healthy people haven't found any significant association between isoflavones and thyroid function (, ).
Currently, no good evidence associates other common phytoestrogens with adverse health effects in humans (, , , ).
Summary: Phytoestrogen supplements seem to have no serious side effects. But some evidence indicates that high doses of isoflavones may suppress thyroid function in children who have low levels of iodine.
When it comes to men's health, scientists are most concerned that excessive exposure to phytoestrogens may reduce male fertility.
A study in cheetahs indicated that a high intake of phytoestrogens impaired the fertility of the males ().
However, scientists have pointed out that phytoestrogens probably have different effects in carnivores, such as cheetahs, compared to omnivores, like humans.
In fact, no strong evidence associates high phytoestrogen intake with fertility problems in humans (, , ).
The most studied phytoestrogens are soy isoflavones. An analysis of 15 controlled studies concluded that soy isoflavones, whether in foods or supplements, do not change testosterone levels in men ().
Additionally, one study showed that taking 40 grams of isoflavone supplements per day for two months did not impair men's semen quality or volume ().
One observational study showed that a soy-based infant formula was not linked with self-reported male fertility or puberty, compared to a cows'-milk formula ().
However, not all observational studies agree. Another study showed that a high intake of soy, which is rich in isoflavones, was associated with a lower sperm count, but the researchers didn't know whether isoflavones were responsible ().
Put simply, most of the evidence indicates that isoflavones do not adversely affect men's fertility. Although a study in cheetahs suggested that a high intake of phytoestrogens may impair fertility, the same doesn't necessarily apply to humans.
Yet, scientists know little about the effects of other phytoestrogens or about the long-term intake of high-dose supplements in humans. More research is needed.
Summary: Isoflavones, a common group of phytoestrogens, don't seem to cause fertility problems in men.
No strong evidence proves 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.