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Is Soy Good (or Bad) for You?

Proponents say that soy can help prevent heart disease, osteoporosis, hot flashes, breast cancer, and more. Detractors worry that soy might be harmful to breast cancer survivors, interfere with thyroid function, and block nutrient absorption. Who's right? An update on the latest science surrounding soy.

By
Monica Reinagel, M.S.,L.D./N
8-minute read
Episode #592
The Quick And Dirty
  • Soy is a good source of protein, fiber and isoflavones
  • Including soy foods in a balanced and healthy diet may reduce your risk of some diseases
  • Unless you're allergic to soy, consuming it is unlikely to cause any harm
  • Whole soy foods retain more beneficial nutrients than more highly processed extracts and isolates

Soy is one of those foods that seems to have an equal number of equally ardent proponents and detractors. Proponents tout soy as helping to prevent osteoporosis, heart disease, and hot flashes. Detractors claim that soy can bring on early puberty, feminize men, damage your thyroid, and prevent nutrient absorption. 

Today, we'll take a look at the claims on both sides of the soy debate, as well as questions about the healthiest types of soy and recommended intakes. 

What makes soy special? 

Soybeans are a member of the legume family. All legumes are good sources of plant-based protein, as well as fiber. But soybeans have some unique qualities that elevate them above other beans. Unlike most plant-based protein sources, soy is a complete protein, containing all nine of the essential amino acids. Soybeans are also particularly rich in isoflavones, compounds with a variety of biological effects. Many of the health claims for soy are centered on these isoflavones. 

Many of the health claims for soy are centered on isoflavones.

Another thing that sets soy apart from other beans is the following FDA-approved health claim:

Diets that include 25 grams of soy protein a day (and are also low in fat and cholesterol) may reduce the risk of heart disease.

However, this claim has recently come under scrutiny.

RELATED: Soymilk vs Cow's Milk

Does soy prevent heart disease?

The FDA approved the heart health claim for soy in 1999, citing significant scientific agreement on the issue. However, in 2017, the FDA announced that they had reevaluated the research on soy and heart disease and did not find it as convincing as it originally appeared. As a result, a proposal to revoke this health claim was introduced, pending a period of public comment. The debate over the science continues and a final ruling on the health claim has not yet been reached but is expected in early 2021.

Keep in mind that the heart health claim for soy also stipulates that your diet be low in fat and cholesterol.

For the soy foods industry, the heart health claim is a valuable marketing tool that confers a sort of health halo over all soy foods. But the real impact of soy food consumption on heart disease risk in Western countries is questionable. For one thing, few Westerners are consuming anywhere near the amount of soy protein that some (but not all) studies found to be preventive.   

In general, it’ll take you about four servings of whole soy foods like soymilk, tofu, or edamame to get to 25 grams of protein. With a more concentrated form, like a soy protein powder, it takes a lot less. Also, keep in mind that the heart health claim for soy also stipulates that your diet be low in fat and cholesterol. 

So, about the best we can say for soy and heart health is that eating quite a bit of soy in the context of a healthy diet might offer a modest reduction in heart disease risk. 

Many of the other health claims for soy are related to women’s health issues. But men, keep listening, because this next part relates to you as well.

Effects of phytoestrogens in soy

It’s been suggested that eating more soy can help prevent osteoporosis, breast cancer, and hot flashes. After all, it has been observed that Asian women, who eat a lot more soy than Western women, don’t seem to suffer from these things nearly as much as we do.

Of course, there are a lot of differences between Asian and Western women that could potentially explain this, such as environment, genetics, lifestyle, or other aspects of diet. These are what we call “confounding factors,” and researchers do their best to account for them when analyzing data. But you can only account for the confounders that you know about—and you never know about them all.

The isoflavones in soy are phytoestrogens, plant compounds that happen to be shaped very similarly to the human hormone estrogen.

Nonetheless, the soy theory is plausible because the isoflavones in soy are phytoestrogens, plant compounds that happen to be shaped very similarly to the human hormone estrogen. In fact, they’re close enough that they can actually fit into estrogen receptors in human cells.

Osteoporosis, hot flashes, and breast cancer are all closely linked to estrogen activity in the body—either too much or too little. The thought is that these weak plant estrogens (or, phytoestrogens) might protect you from either scenario. If estrogen levels were low, isoflavones might provide just enough estrogenic activity to prevent bone loss or hot flashes.

On the other hand, if estrogen levels are too high, which might increase your risk of breast cancer, phytoestrogens could help block excess estrogen from entering your cells by occupying the estrogen receptors and causing your cells to turn on the “No Vacancy” sign.

Although it’s an interesting theory, it’s been tough to confirm. Some studies have found that eating soy (or taking isoflavone supplements) helps improve bone density but some found that it didn’t. Likewise, in terms of hot flashes, soy helps some women but not others. (Incidentally, it seems to be much more effective in reducing hot flashes if you also exercise.)

High soy or isoflavone consumption combined with a healthy lifestyle might have a modest beneficial impact. Or it might have no impact at all.

It's also important to note that the studies that did observe a benefit involved consuming pretty high amounts of soy or taking higher dosages of isoflavone supplements. As with the heart health claims, the best we can say is that high soy or isoflavone consumption combined with a healthy lifestyle might have a modest beneficial impact. Or it might have no impact at all. 

Soy consumption and breast cancer risk

Forunately, we have a bit more clarity on the effects of phytoestrogens on breast cancer risk. Studies show that soy intake seems to protect healthy women from getting breast cancer, although it appears that this effect is greater when soy is regularly consumed from a young age.

For a long time, however, researchers were concerned about the effects of soy on women who had already had breast cancer or were at high risk. They worried that the phytoestrogens in soy foods might promote the growth of estrogen-sensitive cancer cells. 

There is now compelling evidence to show that eating soy foods not only poses no risk to breast cancer survivors; it appears to be beneficial.

Up until recently, breast cancer patients were often advised to avoid soy food out of an abundance of caution. However, there is now compelling evidence to show that eating soy foods not only poses no risk to breast cancer survivors. It appears to be beneficial, reducing the risk of recurrence of both estrogen-receptor positive and negative breast cancers.

The effect of phytoestrogens on men

Guys, are you still with me? If so, you may be wondering how the phytoestrogens in soy might affect you. Good question! Men have estrogen receptors, too. And, in fact, there are some concerns about how the estrogenic compounds in soy might affect men.

Soy consumption has been associated with a reduced risk of prostate cancer.

For example, some fear that eating too much soy could affect fertility or sexual function. Indeed, scientists have found that giving large amounts of phytoestrogens to animals can affect fertility.  However, human research has failed to detect any negative effects of soy consumption on male hormone levels or fertility.  Meanwhile, soy consumption has been associated with a reduced risk of prostate cancer. 

Could phytoestrogens harm children?

There are similar concerns about eating soy during pregnancy and giving soy formula to babies. The idea is that babies, both in the womb and out, are extremely sensitive to hormones and that phytoestrogens might affect their development. Of course, Asian women have eaten soy throughout their pregnancies and while nursing for centuries without apparent difficulty.

Soy formula can be a lifesaver for babies who have an allergy to cow’s milk.

Soy baby formula is a somewhat newer development, however, and some have expressed concerns about the effects of isoflavones on infants. I think we can all agree that human breast milk is by far the optimal food for babies. Sometimes, however, breast-feeding is not an option. Soy formula can be a lifesaver for babies who have an allergy to cow’s milk. And a 2014 meta-analysis evaluated growth patterns, reproductive hormones, and immune and neurological functions found no significant differences in infants given soy formula.  

Researchers have also looked for a link between soy intake and early puberty in boys or girls. One survey of about 250 boys found that those who had the highest consumption of soy reported reaching puberty about 6 months earlier (on average) than those who ate the least. However, all groups were well within the normal age range for the onset of puberty. Another study by the same author, involving almost 350 girls found that there was no relationship between the amount of soy they consumed and the age at which they reached puberty. 

Effects of soy on thyroid function

Another widely-repeated charge against soy is that it disrupts thyroid function. In fact, studies overwhelmingly show that it has minimal, if any, effect on thyroid function in human beings, except if they're deficient in iodine. Iodine deficiency is rare, thanks to our iodized salt supply, so I don’t think this is a major concern.

Soy and nutrient absorption

If you’ve read any of the anti-soy manifestos on-line, you’ve probably also read that soy contains “anti-nutrients,” which sound very sinister, indeed. In fact, there are compounds in soybeans called phytates which can impair your ability to absorb certain nutrients.

This is not a big deal if you’re eating soy in reasonable quantities as part of a nutritious diet

This is not unique to soy, however. Spinach contains compounds called oxalates, which do the same thing, and people don’t go around talking as if spinach were a vegetable from the dark side. Really, this is not a big deal if you’re eating soy in reasonable quantities as part of a nutritious diet. In fact, phytates have several beneficial effects in the body.

RELATED: The Other Side of Phytates

Best forms of soy

Minimally processed soy products (which include soymilk and tofu) retain more beneficial nutrients than more highly processed forms of soy used in bars and meat substitutes.

Fermenting soy removes or deactivates many of the phytates. Fermented soy foods include miso, tempeh, natto, and soy sauce. Many consider these to be the ideal way to consume soy. Fermented foods, of course, are also a source of probiotic bacteria. And as is the case with any food, minimally processed soy products (which include soymilk and tofu) retain more beneficial nutrients than more highly processed forms of soy used in bars and meat substitutes.  

So, should you eat soy or not?

In my view, both the benefits and risks of soy tend to be exagerated. Soy is a good source of protein, fiber and isoflavones. Including soy foods in a balanced and healthy diet may reduce your risk of some diseases and (unless you're allergic to it) is unlikely to cause any harm. In other words, it might help and it can't hurt. And as with any food, the more minimally processed, the better.

Soy-based supplements contain much more concentrated amounts of isoflavones than you'd get from soy foods. It's a good idea to check in with your health care professional before starting any nutritional supplement. They can help you evaluate whether or not the supplement is likely to provide the benefit you're hoping for as well as advise you on a safe and appropriate dosage.

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