Isoflavones and their health effects
Isoflavones belong to the flavonoid family of polyphenols. Fruits, vegetables, chocolate, wine, and tea are some familiar sources of flavonoids.
Many different legumes contain isoflavones, but soybeans and other soy foods like tofu, miso, and tempeh are particularly rich sources.
Media outlets sometimes refer to isoflavones as a "super-nutrient," claiming they possess antioxidant, anti-inflammatory, anti-microbial, and anti-cancer properties.
However, there has been controversy around some of these claims.
It's difficult to interpret research into the benefits of soy-rich foods. Studies often use different types of soy foods or look at different groups of people, making them hard to compare with each other.
Equally, findings from non-human studies, or those with varying doses of isoflavones, can be tricky to translate into clear, actionable advice for people.
In this article, we'll explore the research behind the health claims. We'll explain where the benefits lie and if there are any risks associated with eating isoflavones.
At ZOE, we know that the same food doesn’t affect everyone the same way.
Our at-home test analyzes your unique blood sugar and blood fat responses to food so that we can provide you with personalized nutrition recommendations.
Isoflavones and hormones
Isoflavones are a type of phytoestrogen, a plant compound that exerts estrogen-like effects. Specifically, the chemical structure of isoflavones is similar to estrogen, a key sex hormone in the body.
Because they have a similar shape, isoflavones can bind to many estrogen receptors in the body.
Examples of soy isoflavones include small compounds called genistein, daidzin, and glycitin. These molecules are linked to many of the health outcomes associated with eating soy foods.
The effect of isoflavones on the body depends, in part, on how well you can absorb them.
After you eat soy foods, isoflavones are either quickly absorbed by the body or broken down by microbes in the gut.
These microbes break isoflavones down into either estrogen-like compounds or anti-estrogenic compounds, with opposite effects.
Evidence suggests certain microbes affect this process, possibly explaining why soy benefits vary from person to person.
For example, the microbes Asaccharobacter celatus and Slackia isoflavoniconvertens are associated with converting daidzein — an isoflavone — into equol, a compound with estrogen-like effects.
Interestingly, it appears that only 30–50% of people have these microbes that can convert daidzein to equol. This percentage is higher amongst Asian populations.
One of the most well-studied areas of isoflavone research is their potential impact on menopausal symptoms.
One small study also suggested that soy isoflavones may also help improve the mood changes and fatigue that some women experience with menopause. However, more research is needed to confirm this.
Historically, there have been mixed messages on the effect of isoflavones on cancer risk.
A recent, large-scale meta-analysis included a variety of studies from across the world and concluded that isoflavone intake could reduce breast cancer risk and is worth further investigation.
Soy and soy products are particularly popular in Asian countries, where people eat an average of 15–60 milligrams of isoflavones daily. Some scientists think that the lower rates of breast cancer in Asian countries may partly be due to their soy-rich diets.
A large, long-term observational study of women in China found that each 10-mg increase in daily soy intake was associated with a 3% reduction in breast cancer risk. This led them to suggest that high intake may provide benefits.
A similar large-scale study based in Japan found that eating a moderate amount of soy foods could reduce the risk of breast cancer.
The current research on isoflavones and prostate cancer is mixed, with different studies suggesting either increased or reduced risk associated with isoflavones.
Ultimately, there is not enough high-quality evidence to make strong claims about the effects of isoflavone consumption on prostate health.
Osteoporosis is a condition where bone density decreases, increasing the risk of breaking a bone. As women go through menopause and levels of estrogen in the blood decrease, the risk of developing osteoporosis increases.
Eating a diet rich in isoflavones could be beneficial for some women due to their estrogen-mimicking effects.
A recent trial included 325 women who have been through menopause. The researchers found that isoflavone supplementation was as effective as hormone replacement therapy (HRT) in preventing bone loss.
A meta-analysis of 26 randomized controlled trials examining the effect of isoflavone supplementation for perimenopause and postmenopause symptoms came to a similar conclusion.
Similarly, a 2021 meta-analysis backed up these findings and noted that the possibility of an isoflavone intervention is promising but requires more extensive trials.
Research looking at the effectiveness of isoflavones for heart health is generally inconclusive, and more research is needed in this area.
While one meta-analysis indicated that eating soy foods can reduce the risk of cardiovascular disease and stroke, isoflavones alone didn’t have the same effect.
However, in one study involving 200 women, researchers gave the participants a supplement of either soy protein with isoflavones or soy protein without isoflavones.
After 6 months, the researchers found that the group that had the isoflavones had significantly improved heart health measures compared with the non-isoflavones group.
Similarly, a long-term study of over 200,000 participants linked higher consumption of isoflavones with a moderately reduced risk of coronary heart disease.
A meta-analysis of randomized controlled trials also found that soy isoflavones can improve the function of blood vessels.
And multiple studies have shown that eating more than 25 grams of soy (or 100 mg of isoflavones) daily can help lower blood pressure after menopause.
Results from the Nurses Health Study found that an increased intake of isoflavones was associated with a slightly lower risk of developing type 2 diabetes (T2D).
Nevertheless, the jury is still out on their effectiveness in helping to manage T2D.
While research indicates soy isoflavones may improve cholesterol levels in people with T2D, a recent meta-analysis found no benefit on blood sugar or insulin levels. However, these results must be interpreted with care, as the studies varied considerably.
Inflammation is one of the hottest topics in health at the moment, and rightly so. Research has linked chronic inflammation to an increased risk of heart disease, diabetes, and cancer.
Scientists suggest that isoflavone consumption may trigger a number of anti-inflammatory mechanisms.
However, much of the evidence comes from animal studies or specific population groups. While these are important to research, they can be challenging to translate into general advice.
What foods contain isoflavones?
Isoflavones are found in several plant foods, including:
soybeans and edamame
For a delicious way of including isoflavones in your diet, try:
snacking on a handful of edamame beans
switching up your breakfast scramble by substituting eggs for silken tofu
adding a handful of chickpeas to your curry
dialing up the flavor on your roast salmon with a miso marinade
What about isoflavone supplements?
While there is plenty of research exploring isoflavone supplements, the recommended dosage and duration is highly variable depending on different health goals.
In general, eating a moderate amount of isoflavone-containing whole foods appears to be the safest way of getting the health benefits.
Are there any risks?
Eating soy foods in moderation, as part of a balanced diet rich in whole grains, fruit, and vegetables, is safe and likely beneficial for health.
Talk to your doctor if you're considering regularly eating large amounts of soy or taking a supplement.
Kawasaki disease is an uncommon disorder mainly affecting young children. Research is sparse, but a 2017 study linked a higher than average consumption of soy products to an increased risk of the condition.
Some people are allergic to soy. For these people, eating soy may result in itching or hives. In rare cases, this may cause anaphylaxis — a severe allergic reaction requiring immediate medical attention.
Additionally, some people report digestive symptoms like diarrhea and constipation after eating soy foods.
Isoflavones can interact with certain drugs. It’s worth consulting your healthcare provider first if you're taking any medication or have any of the following conditions:
Isoflavones are a type of polyphenol found in soy and soy products. Including isoflavones and other phytoestrogens in your diet may benefit your health.
Evidence suggests that eating a diet rich in soy foods may offer protection against certain cancers and cardiovascular disease.
Isoflavones also appear to play a role in preventing bone loss. But there's still uncertainty about the benefits of isoflavone supplementation.
While isoflavones in soy foods may offer benefits, they are just one component of a healthy diet. Eating a varied diet rich in plant foods such as vegetables, fruit, whole grains, and legumes is important for supporting your healthiest self.
At ZOE, we recognize the importance of a balanced diet for a healthy life.
With our at-home test, we can analyze your gut microbiome and show you how your body responds to different types of foods. From this, we’ll give you personalized nutrition advice tailored to your unique body.
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