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Background: Vitamin D deficiency with related consequences to human health has growing interest to military specific researchers worldwide. Many specific conditions could put soldiers in to the higher risk of vitamin D deficiency. Under high level of physical pressure during military training might increase the need of vitamin D in human body and therefore supplementation with vitamin D could be crucial for general health and physical fitness.

Study Design: Longitudinal, triple-blinded, randomized, controlled trial.

Methods: 113 men conscripts from the Estonian Army were randomized into two vitamin D supplementation groups: 58 to an 4000 IU and 58 to an 600 IU group. The length of the follow-up was ten months, from July 2021 until May 2022. Physical fitness and hand grip tests were performed 3 times and blood serum (25(OH)D), parathyroid hormone, calcium, ionized calcium values were measured 4 times during the study period.

Results: The 600IU group had a significantly lower mean value of 25(OH)D in all time points during the study compared to 4000IU group (p<0.001) except baseline. Non of the study subjects in the Group A 600IU reached sufficient level of 25(OH)D in January and May. 61.3% in the 600IU and 30.6% 4000IU group had 25(OH)D levels under 50 nmol/L in May. No statistically significant differences found in the body weight and vitamin D correlation analysis in any time point on between study groups. No statistically significant differences in PTH, i-Ca were found between study groups at any time point except Ca results. No significant differences at any time points were revealed in the physical fitness test and hand grip strength tests except in non-dominant hand grip test in October.

Conclusion: Ten months vitamin D3 4000IU supplementation decreased risk of vitamin D deficiency but had no effect to physical fitness compared to low dosage vitamin D supplementation.

Trial registration: ClinicalTrials.gov Identifier: NCT04939636. Prospectively registered 13 October 2020.