Walking in the Light: How History of Physical Activity, Sunlight, and Vitamin D Account for Body Fat—A UK Biobank Study

OBJECTIVE: High prevalence of vitamin D deficiency and obesity drive the need for successful strategies that elevate vitamin D levels, prevent adipogenesis and stimulate lipolysis. We provide a theoretical model to evaluate how physical activity and sunlight exposure influence serum vitamin D levels and regional adiposity. We hypothesized posteriori that sunlight is associated with undifferentiated visceral adiposity by increasing the ratio of brown to white adipose tissue. METHODS: Using ten-year longitudinal data, accelerometry, a sun exposure questionnaire, and regional adiposity quantified by DEXA imaging, a structural equation mediation model of growth curves was constructed with data-driven methodology. RESULTS: Sunlight and physical activity conjointly increased serum vitamin D. Changes in vitamin D levels partially mediated how sunlight and physical activity impacted adiposity in visceral and subcutaneous regions within a subjective physical activity model. In an objective physical activity model, vitamin D was a mediator for subcutaneous regions only. Interestingly, sunlight was associated with less adiposity in subcutaneous regions, yet greater adiposity in visceral regions. CONCLUSIONS: Sunlight and physical activity may increase vitamin D levels. For the first time, we characterize a positive association between sunlight and visceral adiposity. Further investigation and experimentation are necessary to clarify the physiological role of sunlight exposure on adipose tissue.