An examination of vitamin D levels as a mediator in the association of ambient air pollution and incident type 2 diabetes mellitus in India

Background & Aim: Recent evidence suggests a role for air pollution and low vitamin D levels in enhancing the risk of diabetes. In India, both air pollution and a high prevalence of vitamin D deficiency exist alongside a high burden of diabetes. Particulate matter also reduces penetration of UVB light into earth’s atmosphere resulting in reduced synthesis of cutaneous vitamin D. We hypothesized that air pollution as measured by PM2.5 is associated with vitamin D levels as measured by 25-OHD in the body, which may mediate association between PM2.5 and incident type 2 diabetes mellitus. Methods: We used a nested prospective case-control study in the Center for Cardiometabolic Risk Reduction in South Asia cohort study in Delhi and Chennai. Cases (n=506) defined as those with incident diabetes mellitus, were matched with healthy controls/non-diabetics (n=506). Multivariable regression analyses were performed to evaluate the associations between varying lag exposures of PM2.5 and incident diabetes. We estimated the mediating effects of vitamin D levels associated with PM2.5 exposure on incident diabetes through causal mediation analyses and boot strapping. Results: 92.4% and 89.9% of cases and controls were vitamin D insufficient (< 30 ng/ml), respectively. We observed statistically significant negative correlation [-0.41 95%CI: -0.35, -0.46] between vitamin D levels and PM2.5 for all the lag periods (1-day, 7-day, 30-day, and 90-day) studied. The association between short-term exposure to PM2.5 and incident diabetes were evident among people who were obese, had higher waist-hip-ratio, and elevated triglyceride levels. The mediating role of low vitamin D was not supported by our dataset. Conclusions: Air pollution appears to interfere with vitamin D synthesis from natural sources. We, however, did not find evidence to support a mediating role of vitamin D on the association between PM2.5 and incident diabetes. Keywords: Air pollution, diabetes mellitus, vitamin D, mediation analysis