BACKGROUND: The longer healthy life expectancy observed in Japan may be partly attributed to the Japanese diet. The researchers sought to examine whether serum isoflavone levels are associated with disability and death. METHODS: The researchers used a nested case-control study to compare serum isoflavones (daidzein, genistein, glycitein, and equol) levels between 165 participants that died or were certificated as disabled (cases) and 177 controls. Disability was defined by certification of long-term care insurance. Conditional logistic regression models were used to calculate the risk of isoflavones for the composite outcome. RESULTS: The proportion of cases was lower in the group with the highest levels of equol (34/91, 37%) compared with equol nonproducers (84/161, 52%). The risk of disability or death among equol producers remained reduced after adjusting for age and sex (odds ratio: 0.55, 95% confidence interval: 0.33-0.93). In a multivariate model, this risk was also unchanged (odds ratio: 0.51, 95% confidence interval: 0.27-0.96). There were no significant associations between daidzein, genistein, and glycitein with the composite endpoint. CONCLUSIONS: Higher serum equol levels, but not any other isoflavones, were inversely associated with the composite endpoint of disability and death. Although it cannot be concluded that equol per se has preventive effects on disability or death, higher equol levels appear associated with better health.