Vitamin D & Muscle Function After 50: The Clinical Evidence

Vitamin D3 is not merely a vitamin — it functions as a steroid hormone, binding to vitamin D receptors (VDRs) expressed in virtually every tissue in the body, including skeletal muscle. A 2023 meta-analysis in the Journal of Clinical Endocrinology & Metabolism found that vitamin D supplementation (800–2000 IU/day) significantly improved muscle strength, balance, and physical performance in older adults — with the greatest effect in those who were deficient at baseline.

Deficiency affects an estimated 40–50% of adults over 50 in Northern Hemisphere populations, and prevalence rises to over 60% in women who avoid sun exposure or have darker skin pigmentation. Serum 25-hydroxyvitamin D levels below 30 nmol/L are associated with a 65% increased risk of frailty, a 30% increased fall risk, and significantly elevated all-cause mortality.

“Vitamin D is not merely a vitamin — it functions as a steroid hormone, with receptors in virtually every tissue in the body, including skeletal muscle.”

The mechanism is direct: vitamin D upregulates the expression of genes involved in protein synthesis within muscle cells and promotes the absorption of calcium essential for muscle contraction. Low vitamin D levels blunt the muscle's response to both resistance training and dietary protein.

Food sources of vitamin D3 are limited — fatty fish (salmon, mackerel), egg yolks, and fortified foods provide modest amounts. Most adults over 50 require supplementation of 1000–2000 IU/day to maintain optimal serum levels, particularly during winter months or with limited sun exposure.