Inverted U-shape association between urine equol levels and cancer: a national population-based cross-sectional study

Equol, a naturally occurring phytoestrogen derived from the fermentation of soy and soy-based products by gut bacteria, is recognized for its diverse health benefits. While there is speculation about its association with cancer prevention, the scientific community has yet to reach a consensus due to the variability in research findings. Our study aims to shed light on this topic by examining the correlation between urine equol concentrations and the cancer risk among the American population. The National Health and Nutrition Examination Survey (NHANES) is a national survey of U.S. civilians in which cancer participants are enrolled in a database by a sample questionnaire. This study included 2797 Americans aged 40 years and older in the NHANES database (2005-2010). The relationship between urine equol concentration and cancer was analysed using weighted logistic regression models, stratified analysis, smoothed curve fitting and threshold effect analysis were also performed. Among the 2797 participants in our study, 390 individuals received a cancer diagnosis. Our findings indicate a positive correlation between urine equol levels and the risk of cancer. Notably, individuals in the highest quartile of equol excretion exhibited a significantly elevated risk of cancer, with a 25.4% increase compared to those in the lowest quartile (POR = 1.254, 95% CI: 1.252, 1.256), after fully adjusting for confounders. Similar results were observed in other adjusted models. A non-linear relationship in the shape of an inverted U-shape can be observed by smoothed curve fitting, and the inflection point is 25.5. Urinary equol concentrations below 25.5 ng/ml were positively associated with cancer risk, while equol concentrations above 25.5 ng/ml showed a slight negative trend in cancer risk. However, further prospective studies are needed to provide more robust evidence and confirmed in large clinical trials.