Vitamin D
A fat-soluble vitamin synthesized in the skin from sunlight. Involved in bone metabolism, immune function, and muscle function. Deficiency is common in indoor lifestyles and low-sunlight environments. Athletes show high deficiency rates, and associations with muscle strength, immunity, and bone density are well-researched.
An ingredient with growing research
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Effects reported in research
Improved muscle strength and power in deficient individuals (confirmed with supplementation)
Maintained bone density and reduced fracture risk (effective combined with calcium)
Support for immune function (associated with lower infection risk)
Support for testosterone production (association suggested when correcting deficiency)
Dosage & timing
- 1,000–4,000 IU (25–100 μg)/day.
- Take with dietary fat for improved absorption.
- Supplementation is particularly effective when serum 25(OH)D is below 30 ng/mL.
Cautions
- •Fat-soluble, so excess accumulation is possible.
- •Long-term intake above 10,000 IU/day may cause hypercalcemia.
- •Periodic blood level monitoring is recommended.
Supporting research
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