Abstract
Vitamin D is lipophilic and accumulates substantially in adipose tissue. Even without supplementation, the amount of vitamin D in the adipose of a typical adult is equivalent to several months of the daily reference nutrient intake (RNI). Paradoxically, despite the large amounts of vitamin D located in adipose tissue, individuals with obesity are often vitamin D deficient according to consensus measures of vitamin D status (serum 25‐hydroxyvitamin D concentrations). Thus, it appears that vitamin D can become ‘trapped’ in adipose tissue, potentially due to insufficient lipolytic stimulation and/or due to tissue dysfunction/adaptation resulting from adipose expansion. Emerging evidence suggests that exercise may mobilise vitamin D from adipose (even in the absence of weight loss). If exercise helps to mobilise vitamin D from adipose tissue, then this could have important ramifications for practitioners and policymakers regarding the management of low circulating levels of vitamin D, as well as chronically low levels of physical activity, obesity and associated health conditions. This perspective led us to design a study to examine the impact of exercise on vitamin D status, vitamin D turnover and adipose tissue vitamin D content (the VitaDEx project). The VitaDEx project will determine whether increasing physical activity (via exercise) represents a potentially useful strategy to mobilise vitamin D from adipose tissue.
Highlights
Vitamin D has effects far beyond its classical actions on calcium homeostasis and bone metabolism, and vitamin D insufficiency is thought to affect many physiological systems and a wide array of human health outcomes (Dobnig et al 2008; Ginde et al 2009; Semba et al 2010)
One reason for using serum 25 (OH)D as a measure of vitamin D status is that it has a half-life of ~2–3 weeks and, quantification of this metabolite is not influenced by transient changes in dietary vitamin D or acute sun exposure to the same extent as other vitamin D metabolites (Jones et al 2015)
Vitamin D can accumulate in large amounts in adipose tissue where it may become sequestered
Summary
Vitamin D has effects far beyond its classical actions on calcium homeostasis and bone metabolism, and vitamin D insufficiency is thought to affect many physiological systems and a wide array of human health outcomes (Dobnig et al 2008; Ginde et al 2009; Semba et al 2010). The same group reported that 5 weeks of progressive endurance cycling exercise (30 minute bouts, three bouts/week, 60–75% V_ O2 peak) during winter months prevented any reductions in serum 25(OH)D concentrations in a small group of elderly men (Sun et al 2018) (Fig. 2c,d), independent from any changes in adipose tissue mass. These recent studies indicate that exercise has a direct and causal effect on 25(OH)D concentrations – possibly through the mobilisation of adipose-derived vitamin D and/or 25(OH)D. Dietary intake will be recorded with a 3-day weighed diet record in the week prior to, and during the final week of, the intervention, along with a retrospective food frequency questionnaire at both time points to capture dietary sources of vitamin D habitually consumed
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