Vitamin D supplementation reduced risk of acute respiratory tract infection (adjusted OR 0.88, 95% CI 0.81–0.96); greatest benefit in those with baseline deficiency (<25 nmol/L) and those receiving daily or weekly dosing rather than bolus doses.

Vitamin D at 700–800 IU/day reduced hip fracture risk by 26% (RR 0.74) and any nonvertebral fracture by 23% (RR 0.77); no significant benefit was observed at 400 IU/day.

Vitamin D insufficiency is prevalent among athletes, particularly indoor athletes and those in northern latitudes in winter. Insufficiency is associated with reduced muscle strength, power, and endurance; supplementation of 2,000–6,000 IU/day recommended to maintain 25(OH)D >40 ng/mL.

Magnesium supplementation reduced sleep onset latency by ~17 min vs. placebo; increased sleep time and efficiency. Evidence rated low-to-very-low quality; all trials at moderate-to-high bias risk.

250 mg elemental magnesium as bisglycinate modestly but significantly reduced Insomnia Severity Index score vs. placebo (−3.9 vs −2.3, p=0.049) by week 4; well-tolerated with minimal GI effects.