Vitamin D

Vitamin D is a fat-soluble vitamin that helps the body absorb calcium, maintain blood calcium and phosphorus levels, build bone mass and prevent bone loss.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 206.} The skin makes D3 when exposed to sunlight.^{2Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 154.} With low sun exposure, food sources become more important, including fatty fish and fortified cereals, dairy and orange juice.^{2Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 154.} Deficiency can cause rickets in children and osteomalacia in adults.^{3International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 517.} The IPTA textbook lists adolescents, people with dark skin, adult women and housebound older adults as groups at higher risk.^{3International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 517.}

Sunlight through a window doesn’t produce vitamin D.^{4Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 155.}

In practice

• Helms and colleagues describe insufficiency as commonly defined by serum 25(OH)D below 75 nmol/L (30 ng/ml), and report that it’s more common among indoor athletes and those in very overcast climates.^{4Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 155.}
• Refer clients with nutrition deficiencies to a dietitian.^{5International Sports Sciences Association, Fast Track Study Guide: Nutrition, p. 1.} Helms and colleagues advise supplementation only after a blood test confirms insufficiency, followed by retesting to set the dose.^{4Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 155.} Their cited range for insufficient athletes is 20 to 80 IU/kg of D3 a day.^{4Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 155.}
• Beyond correcting insufficiency, Helms and colleagues report no evidence of added benefit, and warn that excess intake may be harmful.^{4Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 155.}
• The IPTA textbook lists an adult recommended dietary allowance (RDA) of 600 IU (15 mcg) at ages 19 to 50 and an upper limit of 4,000 IU a day.^{3International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 517.}
• Research on vitamin D as a lifting performance aid is mixed.^{4Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 155.}