The effect of exercise on vitamin D metabolism and the role of adipose tissue

Abstract

Vitamin D deficiency is very common in the UK, with up to 40% of people classed as deficient during winter months. Vitamin D’s central involvement in numerous mechanisms in the human body beyond calcium homeostasis and skeletal function means that suboptimal status poses significant risk in the aetiology of numerous diseases. The association between obesity and vitamin D deficiency is well documented, with several theories such as limited sunlight exposure, blunted hepatic and renal vitamin D metabolism, volumetric dilution, and sequestration proposed. Owing to its lipophilic properties, vitamin D accumulates substantially within adipose tissue. In fact, the adipose tissue of a typical adult contains the equivalent to several months of the daily reference nutrient intake. In obesity, vitamin D may therefore become sequestered in adipose tissue, and this sequestration may give rise to suboptimal circulating concentrations.

There are several strategies that may tackle vitamin D deficiency, such as supplementation and biofortification. However, recent data has shown that since the updated recommendations on supplementation by the UK’s Scientific Advisory Committee on Nutrition in 2016, the nation’s vitamin D intake remains relatively unchanged. Emerging evidence shows that vitamin D may be mobilised from adipose tissue and into the circulation through the action of lipolysis during exercise. Therefore, exercise theoretically presents as a novel strategy for optimising vitamin D status.

The aim of this thesis was to study the effect of acute and chronic exercise on a wide range of both circulating and stored vitamin D metabolites, as well as utilising stable-isotope tracer methods to examine 25(OH)D3 half-life (and thus vitamin D metabolism) in participants of a range of adiposity and cardiovascular fitness abilities. In chapter 2, a randomised crossover trial showed that 60-minutes of moderate-intensity exercise (as compared to rest) transiently increased serum 25(OH)D and 1,25(OH)2D3 in 33 healthy adults. The VitaDEx study primarily aimed to examine the effect of a 10-week exercise intervention (versus lifestyle maintenance) on a range of circulating and stored vitamin D metabolites in overweight adults during winter months. The study consisted of two arms: a randomised controlled trial which examined the effect of a 10-week exercise intervention in overweight, sedentary adults, and a cross-sectional comparison arm comparing the effect of adiposity and habitual activity on serum 25(OH)D at baseline in lean, active and overweight, inactive adults. Chapter 4 presented the randomised control trial, which showed that although 10-weeks of exercise training had no significant effect on serum 25(OH)D, there was a trend towards significance and a moderate effect size observed. Exercise also mitigated against the decline in serum 1,25(OH)2D3 over the intervention period. Chapter 5 presented an exploratory cross-sectional comparison in 20 lean, active and 46 overweight, inactive adults. When adjusted for age and days of winter, serum 25(OH)D concentrations were found to be significantly greater in lean, active adults compared to overweight, inactive adults.

The work presented in this thesis shows that both acute and chronic exercise represent an effective potential strategy to enhance circulating 25(OH)D concentrations, therefore supporting the hypothesis that vitamin D is mobilised from fat tissue through the process of lipolysis with exercise.

Date of Award	26 Jun 2024
Original language	English
Awarding Institution	University of Bath
Supervisor	Dylan Thompson (Supervisor), Oly Perkin (Supervisor), Javier Gonzalez (Supervisor) & James Betts (Supervisor)