Once confined largely to health food stores, soy foods have become firmly entrenched in the American mainstream. Toddlers snack on edamame, soy-milk lattes are a staple at cafés, and soy-based yogurts, frozen desserts, cheeses, and meat substitutes (including the popular Impossible Burger) command large swaths of supermarket real estate. Sales of tofu and tempeh, two soy products, are rising as consumers increasingly seek plant sources of protein.
Soy’s popularity—and its commercial success—got a big boost from the FDA back in 1999, when the agency began allowing a health claim on food labels stating that consumption of soy protein may reduce the risk of coronary heart disease. Specifically, products that contain at least 6.25 grams of soy protein per serving, and are low in saturated fat and dietary cholesterol, are permitted to state: “A total of 25 g of soy protein a day, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease.”
Hundreds of individual food products now carry the claim, from cereals to nutrition bars to protein drinks to nondairy spreads. Its approval by the FDA led to “dramatic growth” in sales of soy foods, which more than quadrupled between 1996 and 2013, according to the Soyfoods Association of North America, a trade group. (More recent statistics weren’t available.)
But all of that is on the line: In October 2017, the FDA proposed a rule revoking soy’s heart-health claim, citing “inconsistent findings” in studies published since 1999 on the effect of soy protein on heart disease and a lack of “significant scientific agreement” to support the claim. A public comment period on the proposed change, which has been strongly opposed by the soy industry, closed in March 2018, but the FDA has yet to make its final ruling. (An FDA spokesperson told us the agency was still reviewing comments as of November 2019.)
In the meantime, makers of soy foods are bracing for a blow to the health halo that’s surrounded soy protein—and helped to lend a health-food aura to highly processed products like nutrition bars and protein-fortified shakes merely because of their soy content. And consumers are, understandably, confused.
Is soy still a healthy choice? Have its benefits been overhyped? Are there other, non-heart-related reasons to choose soy over other plant sources of protein? And is there any merit to safety concerns about soy—namely, that it may contribute to certain cancers? Here’s a look at the complicated and ever-evolving soy story.
Soy and the heart
The approval of soy’s heart-health claim was based on a spate of studies in the 1980s and 1990s that found that soy protein—either added to the diet or substituted for other sources of protein—could reduce levels of LDL (“bad”) and total cholesterol. Soy’s effect on LDL may stem in part from its content of sterols, plant substances that help to lower cholesterol. There’s also what’s called the “displacement effect,” by which soy foods (which contain mostly unsaturated fat) take the place of other protein sources that are high in LDL-raising saturated fat. In a landmark 1995 meta-analysis that looked at nearly 40 controlled clinical trials, researchers found that replacing 50 grams of animal protein a day with 50 grams of soy protein lowered LDL by nearly 13 percent.
But a 2006 scientific advisory from the American Heart Association (AHA), which looked at 22 studies published between 1998 and 2006, found a much smaller LDL reduction from soy protein—about 3 percent on average. It also pointed out that to get 50 grams of soy protein a day, the amount tested in many studies, a person would need to eat about a pound of tofu or drink a half gallon of soy milk. “Earlier research indicating that soy protein has clinically important favorable effects as compared with other proteins has not been confirmed,” the authors concluded, though they added that many soy products can still be good for cardiovascular and overall health “because of their high content of polyunsaturated fats, fiber, vitamins, and minerals and low content of saturated fat.”
Fast-forward to 2017. The FDA based its proposal to revoke the soy heart-health claim on a reevaluation of the existing evidence on soy and heart disease. The agency identified 46 controlled trials of soy’s effects on cholesterol in adults, published between 1981 and 2013, that it deemed of sufficient quality to include in the review. Of them, 19 trials showed a benefit from soy; the other 27 did not. Based on the inconsistent results of the studies (which varied widely in size, design, and dose of soy protein), the FDA concluded that “the relationship between soy protein and heart disease does not meet the rigorous standard for an FDA-authorized health claim.”
However, two papers published last year reached more positive conclusions about the same studies by using different methodologies. One of them, published in June 2019 in the Journal of the American Heart Association by Canadian researchers, looked at the same 46 trials that the FDA identified. It sought to determine if, at any time since the heart-health claim was established in 1999, the effect of soy protein on blood cholesterol lost its statistical significance when the studies up to that date were pooled and analyzed. This “cumulative meta-analysis,” as it was described, found that as more data accumulated over time, the decrease in LDL from soy protein remained consistent. The evidence thus “supports the rationale behind the original soy FDA heart health claim,” the authors wrote.
The second paper, by some of the same researchers and published in May 2019 in the Journal of Nutrition, similarly pooled the data from the 46 studies the FDA reviewed (rather than analyze them one by one). It found that consuming soy protein at an average dose of 25 grams per day (half the amount tested in many earlier studies) had a small but “highly significant” effect on LDL cholesterol, lowering it by 3 to 4 percent. In addition, the authors cited a 2010 study that found that eating soy protein in place of animal-derived protein sources higher in saturated fat (the displacement effect noted earlier) would cut LDL by an additional 4 percent beyond what soy protein would do alone. In other words, the total LDL reduction would be around 7 to 8 percent.
There are plenty of other reasons besides potential cholesterol reduction to eat soy foods, especially whole or less-processed ones like tofu, edamame (young soybeans in the pod), and soy milk. They are an excellent source of protein. One cup of shelled edamame, for example, has about 20 grams of protein, an amount comparable to that in 3 ounces of chicken, fish, or meat or about three eggs. Soy is rather unusual in that it’s a complete protein source, meaning it contains all the essential amino acids rather than just some of them, as most plant sources do.
Soybeans are high in fiber, with about 8 grams in a cup of cooked edamame, slightly more than the amount in a cup of bran flakes. They also supply B vitamins, iron, potassium, and magnesium, as well as the plant sterols mentioned above.
If you can’t or prefer not to drink dairy milk, a cup of soy milk contains nearly as much protein as the same amount of dairy milk and, if fortified with calcium, supplies as much as 40 percent of the daily calcium requirement. Not all soy milk is fortified; check the label if you want to see if calcium and vitamins A and D have been added. Other vitamins are often added to soy milk too, including B12, which can be helpful for vegetarians since that vitamin is found mostly in animal products. Also check the label for sugar content, since many soy milks have added sugar.
Other health benefits
Some research, much of it observational, has suggested that consuming soy may have health benefits beyond cardiovascular health. Many of these studies have focused on soy’s isoflavones, plant compounds that weakly mimic the effect of estrogen (these are also called phytoestrogens). Among the possible benefits:
- Strengthening bones. Because estrogen plays a role in bone health, it’s been suggested that soy isoflavones may reduce osteoporosis risk in women. But the evidence for this is mixed; some studies have found that women nearing menopause who consume soy are more likely to have increased bone density than women whose diets lack soy isoflavones, but other studies have found no impact.
- Relieving menopausal hot flashes. Research on soy’s effect on hot flashes has been similarly mixed. The North American Menopause Society’s position statement on nonhormonal treatment of hot flashes says that supplements of soy isoflavones may help, but more research is needed. And some evidence suggests the supplements may increase breast cancer risk (see next section). So we think it’s prudent to avoid them.
- Preventing prostate cancer. Men who consume soy foods may be at lower risk for prostate cancer, according to an updated systematic review and meta-analysis of 30 observational studies, published in the journal Nutrients in 2018. It included diet and health data from 267,000 men in Asia, Europe, and North America. Overall, total soy intake was associated with a reduced risk of prostate cancer. In further analyses, the association held up for unfermented soy foods (soybeans, tofu, and soy milk), while fermented soy (miso and natto) showed neither benefit nor risk. Note that the studies looked at intake of soy from food, not supplements. Several earlier meta-analyses found a similar risk reduction from unfermented soy foods (while fermented foods had no effect).
What about breast cancer?
Because soy can mimic the activity of estrogen in the body, there has been some concern that it might increase breast cancer risk. But the types of soy that are potentially worrisome are soy supplements and foods that are fortified with soy isoflavones, such as nutrition bars. Whole soy foods, in contrast, may actually help protect against breast cancer, according to the American Cancer Society.
In addition, soy foods appear to be safe, and possibly even beneficial, for women who have already had breast cancer, according to a study published in 2017 in the journal Cancer. Researchers at Tufts University followed more than 6,200 American and Canadian women with breast cancer and found that after nine years, those who consumed the most isoflavones from their diet had a 21 percent lower death rate than those who consumed the least. The benefit was confined to women with hormone-receptor-negative cancers, however, who account for only about 20 percent of cases. There was no effect on survival rates in women with hormone-receptor-positive cancers.
A hex on soy foods?
Hexane in processed soy foods is another concern. Hexane is a solvent used to extract oil from soybeans. If your nutrition bar or meat substitute has soy protein isolate, soy protein concentrate, or textured vegetable protein listed on its label, that ingredient likely has undergone hexane processing. Hexane is classified as a neurotoxin by the CDC, and chronic exposure in factory workers has been linked to neurological conditions. But it’s unclear whether consuming trace residues in food is hazardous.
If you want to avoid hexane-treated soy foods, look for “100% organic” products with the USDA seal, since hexane is banned in organic food production. Better yet, buy soy foods made from whole soybean ingredients that don’t undergo hexane processing, such as most tofu, tempeh, soy milk, and soy yogurt. Whole soybeans (edamame) are always a hexane-free and healthful option.
Whatever the FDA decides about soy and heart health, including soy foods in your diet, if you like them, is a healthful option, especially if you choose whole or minimally processed ones and eat them in place of protein sources that are high in saturated fat. And the phytoestrogens in soy may have other, non-cardiovascular benefits, though more research is needed to establish them. We don’t recommend supplements of soy isoflavones, since studies of supplementation have not supported health benefits and some research (mostly in animals) even suggests potential harm.
This article first appeared in the UC Berkeley Wellness Letter.
Also see Soy: Health Food Gone Mainstream.
Published January 08, 2020