Background: Vitamin D is a steroid-hormone involved in a wide range of physiological
processes in the human body. It has more recently been proposed that muscle cells are able to
store vitamin D and secrete the hormone into the blood stream during muscle contractions. Few
studies have previously investigated the acute responses on vitamin D levels after exercise,
however evidence suggests an exercise-induced elevation of vitamin D parameters. It is
additionally proposed an association between vitamin D levels and various fitness parameters.
Aim and Hypothesis: We sought to investigate the acute response on circulating levels of
vitamin D, immediately (0 hours), 3 hours and 24 hours after one session of high-intensity
interval training (HIIT) or strength training, and to compare the response between these two
modalities. Further, it was explored if an association between vitamin D response and fitness
parameters was present, along with possible gender differences. It was hypothesized that serum
levels of vitamin D would increase after both modalities, and that the exercise-induced response
was correlated with fitness level.
Methods: 36 young, healthy participants completed one session of both HIIT and strength
training. Blood samples were collected at baseline, 0 hours, 3 hours, and 24 hours after both
sessions and later analysed for 25-hydroxyvitamin D3 (25(OH)D), 1.25-dihydroxyvitamin D3
(1.25(OH)2D), and vitamin D binding protein (DBP) at all time points. All participants were
tested for maximal oxygen consumption (VO2max), maximal strength (1RM) and rate of force
development (RFD).
Results: Strength training induced a significant elevation in 25(OH)D both 0 hours (p<0.001)
and 3 hours (p<0.001) after exercise compared to baseline. HIIT elevated 25(OH)D levels
immediately after the exercise session (p<0.001). There was a rise in circulating levels of
1.25(OH)2D from 0 hours to 3 hours after endurance exercise (p<0.001), that even exceeded
baseline levels (p<0.006). The levels sustained elevated in blood samples drawn 24 hours
after(p<0.013). DBP was additionally elevated 0 hours after endurance exercise(p<0.047).
Conclusion: One session of either HIIT or strength training was determined to have a direct
impact on circulating levels of vitamin D metabolites. Both sessions displayed a significant
elevation in 25(OH)D levels immediately after the exercise, with strength training being
seemingly more effective. Present findings might enlighten exercise as an additional
intervention strategy when discussing vitamin D influencing factors.