Soy Protein May Protect Against Breast Cancer, but Possibly Not Heart Disease

The FDA confirmed that it proposes a rule to revoke a previously authorized health claim that soy protein protects against heart disease. “For the first time, we have considered it necessary to propose a rule to revoke a health claim because numerous studies published since the claim was authorized in 1999 have presented inconsistent findings on the relationship between soy protein and heart disease,” the agency noted in a statement.

While some evidence does continue to suggest a relationship between soy protein and a reduced risk of heart disease, the FDA says that the “totality of currently available scientific evidence calls into question the certainty of this relationship.” For example, studies published after the FDA authorized the health claim are inconsistent in demonstrating that soy protein can lower low-density lipoprotein cholesterol levels (LDL-C). The agency’s review of this combined evidence has led it to conclude that the relationship between soy protein and heart disease isn’t demonstrated enough to merit authorization.

Even if the rule is finalized and the authorization is revoked, qualified health claims will still be permissible as long as there is enough evidence to support a link between consuming soy protein and a reduced risk of heart disease. FDA notes that qualified health claims don’t require the same scientific proof as an authorized health claim. This would allow the industry to use qualifying language that confirms the limited evidence linking dietary soy protein with heart disease risk reduction.

The rulemaking process involves 75 days for open comment. Manufacturers can keep the existing authorized claim on their products until the agency decides whether to revoke the authorization. Since 1990, the FDA has evaluated health claims on packaged foods. To date, 12 health claims have been authorized, including the effects of calcium and vitamin D in helping to lower the risk of osteoporosis and the cancer-lowering risks of certain fruits and vegetables.

Meanwhile, researchers in the U.S. report that a compound found in soy protein may help to suppress the risk of breast cancer. The studies, led by Donato F. Romagnolo, Ph.D., and Ornella I. Selmin, Ph.D., at the University of Arizona (UA) Cancer Center, suggest that genistein, the primary isoflavone found in soy, may prevent methylation-mediated silencing of the tumor suppressor BRCA1 gene by the aromatic hydrocarbon receptor (AhR). They reported their findings in Current Developments in Nutrition. The paper is entitled “Genistein prevents BRCA1 CpG methylation and proliferation in human breast cancer cells with activated aromatic hydrocarbon receptor.”

The researchers explain that only a small proportion of breast cancers have a mutated BRCA1 gene. Many other breast patients have a normal BRCA1 gene, but methylation silences the gene and prevents it from acting as a tumor suppressor. The UA Cancer Center team is particularly interested in AhR, which can activate environmental carcinogens, including tobacco smoke and UV light exposure. When activated, AhR silences BRCA1, which inhibits its tumor suppressor activity and allows cancer cells to proliferate. And when AhR silences BRCA1, estrogen receptor alpha (ER-alpha) is also lost, generating resistance to ER-alpha-targeting drugs such as tamoxifen.

The researchers’ initial studies in patient-derived breast cancer cells have demonstrated that the soy protein genistein can target AhR. And having generated results in vitro, the team is now progressing their research into mouse models. “Lifetime intake of soy in Asian women has been linked to reduced risk of breast cancer,” comments Dr. Romagnolo, nutritional and cancer biology professor at UA Cancer Center. “Genistein is the predominant isoflavone found in soy and may block DNA methylation.”

There are still many questions to answer, including which types of soy foods, when, and for how long during a lifetime they should be consumed to have anticancer benefits. The UA Cancer Center team is also interested in whether exposure to soy genistein during pregnancy might provide the fetus with lifetime protection against breast cancer.