​Summary: This study examines the relationship between health determinant behaviors themselves, and their subsequent combined relationship with chronic illness (diabetes/impaired glucose regulation, depression). While numerous studies have proven the benefits of engaging in more healthy behaviors, the question has not been answered whether the effect of multiple healthy behaviors together is greater than the sum of the effects alone.

The study design is cross-sectional, using data on the adult population from the 2017 California Health Interview Survey (CHIS). A total of 21,116 participants were included in final analyses. Researchers used multivariable adjusted logistic regression to calculate odds ratios for diabetes and for depression at each subsequent level of a healthy lifestyle index (HLI). They also calculated the adjusted odds ratios between adjacent levels of the index.

Findings: The odds of having depression and, separately, of having diabetes each decreased with each additional healthy lifestyle behavior, with three of five depression ratios significant at p < 0.05, and four of five significant for diabetes. The magnitude of the association between the HLI level and odds for disease declines exponentially with each additional healthy lifestyle factor, contrary to the hypothesis, for depression, but fits the hypothesis for diabetes.

These results are important for health promotion, suggesting that even one healthy behavior may dramatically decrease the odds for having depression, regardless of the type of healthy behavior chosen. These findings also show an association between lower prevalence of depression and health behaviors historically only considered preventive for physical illness.