Soy Protein Health Claim: Where Does the Evidence Stand?

By Mark Messina, PhD, MS

Meta-analyses of the clinical data consistently show that soy protein lowers circulating LDL-cholesterol (LDL-C) levels. The most recent meta-analysis demonstrating this finding to be the case was published in 2015. The first one was published in 1995. Four years later, after conducting its own analysis of the literature, the U.S. Food and Drug Administration (FDA) authorized a health claim for soy protein and reduced risk of coronary heart disease. Since 1999, similar claims have been approved in 11 other countries; the most recent country to do so was Canada in 2015.

Nevertheless, in December of 2007, the FDA indicated its intention to reevaluate evidence in support of the soy protein health claim. On October 31, 2017, the FDA announced that it is proposing to revoke the existing heart health claim. The current claim is an “unqualified” claim which indicates that the very rigorous significant scientific agreement standard has been met in support of the hypocholesterolemic effects of soy protein. The FDA announcement suggested that a qualified claim could be approved if the existing claim is revoked. Not surprisingly, a qualified claim requires less support than an unqualified one. A total of 23 qualified claims exist whereas there are only 12 unqualified claims and only five of those have been approved since the enactment of the Nutrition Labeling and Education Act of 1990.

It isn’t precisely clear why the FDA undertook its review of the soy protein health claim, although a 2006 science advisory from the American Heart Association (AHA) questioning the hypocholesterolemic effects of soy protein may have been a factor.14The AHA found that soy protein lowered LDL-C only by about 3 percent. However, the AHA didn’t actually conduct a meta-analysis of the data. When such an analysis was done four years later, Jenkins et al.1 found that soy protein lowered LDL-C by 4.3% using the same 22 studies the AHA used for its estimate.  This magnitude of reduction is similar to that of soluble fiber, which has an unqualified health claim. The 4.3% reduction noted by Jenkins et al.1 is much lower than the initial estimates reported by Anderson et al. in 1995,11 but it has been known for some time that the hypocholesterolemic effect of soy protein is more modest than initially thought.15

In the October announcement, the FDA cited inconsistency of the data as the reason to propose revoking the existing soy protein health claim. Some inconsistency is not at all unexpected as there is probably no nutrition area that has been rigorously investigated where clinical studies have produced entirely consistent findings. This fact is true even for the effects of sodium on blood pressure16,17and calcium on bone mineral density18,19.  Nevertheless, reducing the intake of sodium is routinely recommended by health professionals as a means of reducing the risk of heart disease and increasing calcium intake as a means of preventing osteoporosis.

The FDA found that only 19 of 46 studies showed soy protein statistically significantly lowered LDL-C. However, in nine of those 46 studies, the amount of soy protein ingested by study participants was <25 g/day; the threshold intake established by the FDA for the cholesterol reduction claim. Therefore, the results of these studies are of questionable relevance. Furthermore, a reasonable argument can be made that the binary approach (i.e., an individual study was judged to be either supportive or not supportive) adopted by the FDA that lacked a statistical analysis of the data, isn’t the optimal approach for evaluating the evidence. Before making a final decision about the claim, the FDA will consider any comments submitted during the 75-day comment period.

Regardless of what the FDA decides, from a practical perspective adding soyfoods to the diet as a means of reducing coronary heart disease (CHD) risk makes sense. To markedly reduce cholesterol levels and CHD risk requires adopting a comprehensive dietary approach. At the very least, soybeans provide high quality protein20 that considerable evidence suggests lowers cholesterol as well as heart-healthy fat.21 It is not surprising that soy has been a key component of comprehensive dietary approaches that have led to dramatic reductions in cholesterol.22-27

References

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2. Zhan S, Ho SC. Meta-analysis of the effects of soy protein containing isoflavones on the lipid profile. Am J Clin Nutr. 2005;81:397-408.
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