Vitamin D Supplementation in Healthy Adults in Prolonged Endurance Exercise

Participants received 5,000 IU of vitamin D orally twice daily for 4 weeks. If the serum 25(OH)D concentration exceeds 48 ng/mL, the intervention remains the same. However, if the serum 25(OH)D concentration is under 48 ng/mL, the dosage would increase by 2,500 IU. The principle remains the same at week 6.

Participants received the same amount of placebo matching vitamin D orally twice daily.