Sun exposure and type 2 diabetes mellitus: Can sun exposure lower type 2 diabetes risk?

Abstract

Background: Lower serum 25-hydroxyvitamin D (25OHD) levels have been consistently associated with increased type 2 diabetes mellitus (T2DM) prevalence and incidence in systematic reviews and meta-analyses of observational studies, however this association has not consistently been replicated in vitamin D supplementation trials. This disparity may be due to a number of different factors: lack of power in the trials due to small sample size, insufficient duration of dosing, baseline vitamin D or glycaemic status differing between studies, low supplementation compliance or insufficient vitamin D dose. Alternatively, lower 25OHD levels may be a product, rather than a cause of ill-health, or they may share pathology earlier in life or in disease progression so that supplementing with vitamin D in adulthood has no effect on disease outcome. This body of work presents another explanation: given that sun exposure is the most influential contributor to serum 25OHD levels, observational studies may be reporting an effect of sun exposure, rather than vitamin D, on T2DM. Therefore vitamin D supplementation trials may be failing to capture any additional benefits of sun exposure through non-vitamin D pathways.

The aim of this body of work was to investigate the possible association between sun exposure and T2DM endpoints reported in scientific literature as well as in an original analysis. A major objective of the original analysis was to determine whether or not any association found between sun exposure and T2DM incidence was through non-vitamin D pathways.

Methods: Following a literature review, a systematic review of observational studies reporting on associations between sun exposure variables and T2DM-related endpoints was conducted. The potential of an association between sun exposure- measured using ambient ultraviolet radiation (UVR), and five-year cumulative incidence of T2DM was explored using a prospective, national diabetes cohort (AusDiab). A causal mediation analysis was undertaken to explore whether or not there were effects of ambient UVR on cumulative T2DM incidence, via non-vitamin D pathways.

Results: The systematic review revealed that high-level evidence for an association between sun exposure and T2DM-related outcomes was lacking. There was moderate-level evidence for greater sun exposure reducing T2DM incidence. The opposite was found in the original analysis using the AusDiab cohort: ambient UVR was associated with increased T2DM incidence (OR=1.17, 95% CI: 1.01-1.36, p=0.04). This association was independent of an effect of age, sex, body mass index, physical activity, ethnicity, smoking status and serum 25OHD levels, but was likely to be confounded by area-level determinants of health due to the nature of the exposure variable.

The major limitations of this work were that the sun exposure measures were suboptimal. Time-of-year measures were the most common sun exposure variables contained in the systematic review, and ambient UVR at the site of participant recruitment was the proxy for sun exposure in the original analysis.

Conclusion: There is likely to be a complex relationship between sun exposure and T2DM. The direction of the association between sun exposure and T2DM incidence, as well as delineation of the mechanistic pathways through which this association may exist, are yet to be confirmed. Future studies are encouraged to use person-level sun exposure measurements, and findings from such studies may influence sun protection policy.