Vitamin D is widely known to play a role in musculoskeletal and immune health, and a deficiency is associated with a wide spectrum of disease states and impaired physical performance. Vitamin D is mainly synthesized by skin exposure to sunlight ultraviolet B radiation and only a small amount (~20%) of vitamin D is available from the diet. Consequently, vitamin D deficiency is commonplace during wintertime, in those living at high latitude (>35°), with dark skin, who stay indoors during sunlight hours and cover up from the sun (sun cream and clothing). Vitamin D status is determined from circulating 25-hydroxyvitamin D (25(OH)D), and levels ³ 50 nmol/L are considered desirable. 25(OH)D ¾ the most abundant and stable form of vitamin D ¾ is metabolized into the biologically active form 1,25-dihydroxyvitamin D (1,25(OH)2D), and the waste product 24,25-dihydroxyvitamin D (24,25(OH)2D). Emerging evidence points to a biological role of the dynamic relationship between 25(OH)D and these two metabolic products.
In our study published in the November 2022 issue of Medicine & Science in Sports & Exercise®, we looked at whether these vitamin D metabolites play a role in the physical performance of young healthy men and women. In a large group of military recruits (1,205 men and 322 women; 22.0 ± 2.8 years) starting basic training, we explored the relationship between the vitamin D metabolites and three measures of physical performance: endurance performance assessed by a best-effort 2.4 km run, muscle strength by a maximal dynamic lift test, and muscle power by maximal vertical jump height.
Our key finding is that participants who converted more 25(OH)D to 24,25(OH)2D than 1,25(OH)2D ran faster over 2.4 km, were stronger and had greater muscle power than those who did not. Genetic mutations in the vitamin D receptor did not affect physical performance in our study. Importantly, we observed only a small role of vitamin D metabolites in determining physical performance after controlling for confounding lifestyle habits, sex, age, season and BMI.
These findings support a dynamic role of the vitamin D metabolic pathway on physical performance, a hypothesis that warrants further validation and extension to health outcomes. Our findings challenge the role of 25(OH)D as a measure of vitamin D status, the traditional role of 1,25(OH)2D as a biologically active metabolite and 24,25(OH)2D as an “inert” metabolite. The biological effect of vitamin D supplementation in individuals with genetic mutations in the enzyme CYP24A1, which converts 25(OH)D to 24,25(OH)2D, and supplementation strategies that promote hydroxylation of 25(OH)D3 to 24,25(OH)2D and avoid increasing 1,25(OH)2D beyond its physiological range, should be explored further.
Julie Greeves, OBE, PhD, is the principal physiologist at the British Army and honorary professor at the University of East Anglia. Her research interests are musculoskeletal health in military personnel, and female health and performance.
Alexander Carswell, PhD, is a senior research associate within Norwich Medical School at the University of East Anglia. His research focuses on human health and physical performance, with an interest in the impact of nutrition and exercise.
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