Both doses led to sufficiency concentrations during winter months .
Among athletes with baseline vitamin D suggesting sufficiency , serum 25(OH)D sufficiency was maintained from different doses at both latitudes .
Conclusion Despite achieving sufficiency in vitamin D concentrations from ≥3000 IU supplementation , physical performance did not significantly improve .
Background There is currently no systematic review examining the effects of vitamin D supplementation  among athletes .
A rigorous systematic review and meta- analysis is important to provide a balanced view of current knowledge on the effect of vitamin D on serum 25-hydroxyvitamin D [ 25(OH)D ] concentrations and physical performance .
Objectives This systematic review of r and omized controlled trials ( RCTs ) evaluated the effects of oral vitamin D supplementation on serum 25(OH)D concentrations and physical performance in athletes .
Low serum 25-hydroxyvitamin D is associated with increased risk of stress fracture during Royal Marine recruit training
Summary The aim of this study was to investigate vitamin D status and stress fracture risk during Royal Marine military training . Poor vitamin D status was associated with an increased risk of stress fracture .  Vitamin D supplementation may help to reduce stress fracture risk in male military recruits with low vitamin D status . Introduction Stress fracture is a common overuse injury in military recruits , including Royal Marine ( RM ) training in the UK . RM training is recognised as one of the most arduous basic training programmes in the world . Associations have been reported between serum 25-hydroxyvitamin D ( 25(OH)D ) and risk of stress fracture , but the threshold of 25(OH)D for this effect remains unclear . We aim ed to determine if serum 25(OH)D concentrations were associated with stress fracture risk during RM training . Methods We prospect ively followed 1082 RM recruits ( males aged 16–32 years ) through the 32-week RM training programme . Troops started training between September and July . Height , body weight and aerobic fitness were assessed at week 1 . Venous blood sample s were drawn at weeks 1 , 15 and 32 . Serum sample s were analysed for 25(OH)D and parathyroid hormone ( PTH ) . Results Seventy-eight recruits ( 7.2 % ) suffered a total of 92 stress fractures . Recruits with a baseline serum 25(OH)D concentration below 50 nmol L−1 had a higher incidence of stress fracture than recruits with 25(OH)D concentration above this threshold ( χ2(1 ) = 3.564 , p = 0.042 ; odds ratio 1.6 ( 95 % confidence interval ( CI ) 1.0–2.6 ) ) . Baseline serum 25(OH)D varied from 47.0 ± 23.7 nmol L−1 in February , to 97.3 ± 24.6 nmol L−1 in July ( overall mean 69.2 ± 29.2 nmol L−1 , n = 1016 ) . There were weak inverse correlations between serum 25(OH)D and PTH concentrations at week 15 ( r = −0.209 , p < 0.001 ) and week 32 ( r = −0.214 , p < 0.001 ) , but not at baseline . Conclusion Baseline serum 25(OH)D concentration below 50 nmol L−1 was associated with an increased risk of stress fracture . Further studies into the effects of vitamin D supplementation on stress fracture risk are certainly warranted
Both doses led to sufficiency concentrations during winter months .
Among athletes with baseline vitamin D suggesting sufficiency , serum 25(OH)D sufficiency was maintained from different doses at both latitudes .
Conclusion Despite achieving sufficiency in vitamin D concentrations from ≥3000 IU supplementation , physical performance did not significantly improve .
Background There is currently no systematic review examining the effects of vitamin D supplementation  among athletes .
A rigorous systematic review and meta- analysis is important to provide a balanced view of current knowledge on the effect of vitamin D on serum 25-hydroxyvitamin D [ 25(OH)D ] concentrations and physical performance .
Objectives This systematic review of r and omized controlled trials ( RCTs ) evaluated the effects of oral vitamin D supplementation on serum 25(OH)D concentrations and physical performance in athletes .