Epidemiologic studies are not designed to establish cause and effect relationships because it is not possible to control for all potentially confounding variables. Additionally, it is difficult to accurately assess dietary intake. Nevertheless, epidemiologic studies have played an important role in informing dietary guidelines. Although intervention studies carry the most weight within the scientific community, their expense and complexity limit their utility. For example, due to the necessary size and duration, a clinical study evaluating the effect of diet on the prevention of cardiovascular events in a healthy population is not feasible. In contrast, this type of endpoint is routinely examined in observational studies.
Prospective studies, as opposed to cross-sectional and case-control studies, are the most credible type of epidemiologic study. For this reason, a recently published scoping review of meta-analyses of prospective studies that examined the impact of soy consumption on a range of chronic diseases is especially notable. Yip et al.1 identified 28 meta-analyses published between 2000 and 2021 that analyzed associations between soy intake and cardiovascular disease, cancer, and type II diabetes. Six of these meta-analyses were judged to be of high quality, 10 as moderate quality and 12 low quality. Estimates from 11 reports were identified as the best-identified meta-analysis estimate for total soy intakes.
As shown in Table 1, of the 27 values reported, all show relative risks below 1.0 except for 1 outcome (gastric cancer and fermented soy intake) and of the 26 relative risks below 1.0, all were significant except 1 (total soy intake and cancer mortality). In other words, a very consistent pattern emerges which is that soy and isoflavone intake is associated with protection against several cancers, CVD, and diabetes. The positive association between fermented soy intake and gastric cancer is not a new observation. More than 20 years ago, this relationship was identified in a meta-analysis by Wu et al.2 A number of explanations for this association have been proposed including the salt content of and nitroso compounds in fermented soy and a possible loss of antioxidants with fermentation.3
In addition to the publication by Yip et al.,1 in a large recently published cross-sectional study involving nearly 30,000 Chinese adults, soy intake was associated with higher handgrip strength.4 Handgrip strength is increasingly viewed as an indication of overall frailty. Bohannon5 recently concluded that the use of grip strength can be recommended as a stand-alone measurement or as a component of a small battery of measurements for identifying older adults at risk of poor health status. Handgrip strength is inversely related to mortality.6,7
In this Chinese study by Wu et al.,4 after adjustments, handgrip strength increased in a stepwise fashion according to soy intake. More specifically, handgrip strength (kg) was 35.5, 36.1, 36.3, and 36.6 among men and women who consumed soy less than once per week, once per week, 2-3 times per week and at least 4 times per week, respectively. The odds ratios of having low handgrip strength (<28 kg, men; <18 kg, women) was inversely related to soy intake. Both findings were statistically significant.
When considering the research by Yip et al.,1 and Wu et al.,4 soy intake is associated with higher handgrip strength, an overall measure of frailty, and with protection against several chronic diseases. Is it biologically plausible that soy could exert such benefits? Certainly, the effect on handgrip strength could be related to the protein content of soyfoods, a possibility discussed by Wu et al.4 And the isoflavones in soy have pleiotropic effects that could result in their reducing risk of different types of cancers.8 It could also be that the lifestyles of consumers of soyfoods are different from those of non-consumers, and the overall lifestyle, not soy per se, accounts for these beneficial associations. Findings are typically adjusted for a considerable number of potentially confounding variables but as noted at the onset, it is not possible to control for all such variables.
On the other hand, the differences between soy-consumers and non-soy-consumers in Asia, where these studies were conducted, is likely to be a lot less than the differences among these groups in non-Asian countries. In any event, these recent epidemiologic findings certainly suggest that at the very least, soyfood consumption is compatible with better overall health.
Table 1. Relative risks when comparing high vs low intake
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2. Wu AH, Yang D, and Pike MC. A meta-analysis of soyfoods and risk of stomach cancer: the problem of potential confounders. Cancer Epidemiol Biomarkers Prev 2000;9(10):1051-8.
3. Kim J, Kang M, Lee JS, Inoue M, Sasazuki S, and Tsugane S. Fermented and non-fermented soy food consumption and gastric cancer in Japanese and Korean populations: a meta-analysis of observational studies. Cancer Sci 2011;102(1):231-44.
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This blog is sponsored by SNI Global and U.S. Soy.