Vitamin D

Vitamin D is a fat-soluble secosteroid hormone that functions broadly in calcium and phosphate homeostasis, bone mineralization, immune regulation, and gene expression. Unlike most vitamins, it is synthesized endogenously from 7-dehydrocholesterol in the skin upon UVB irradiation — making sun exposure the primary natural source — with dietary intake (fatty fish, fortified foods) and supplementation serving as secondary sources. Vitamin D status is measured as serum 25-hydroxyvitamin D [25(OH)D], the major circulating form. Deficiency and insufficiency are widespread in the U.S. and globally, driven by indoor lifestyles, sunscreen use, latitude, skin pigmentation, obesity (D is sequestered in adipose tissue), and age-related decline in cutaneous synthesis. National survey data consistently show large proportions of Americans with 25(OH)D below conventional sufficiency thresholds (20–30 ng/mL). However, the appropriate serum target — and whether supplementing to levels above 30 ng/mL confers additional benefit in people who are not severely deficient — is actively debated. The most robust evidence for vitamin D supplementation centers on two areas: bone health in older adults (reducing fracture risk and falls, particularly in deficient elderly populations) and acute respiratory tract infection risk reduction. The 2017 Martineau BMJ meta-analysis of individual participant data (n=11,321) found a statistically significant reduction in ARTI risk with daily or weekly vitamin D supplementation, with the largest effect in those severely deficient at baseline. Bolus (infrequent high-dose) dosing was less effective than regular lower-dose protocols, a clinically important distinction. For cancer and cardiovascular disease prevention — two areas of intense interest — the VITAL trial (Manson et al., 2019, n=25,871) provides the most rigorous large-scale RCT data. Supplementation with 2,000 IU/day of vitamin D3 did not significantly reduce primary cancer incidence or cardiovascular events in an adult population that was largely vitamin D-replete at baseline. Secondary analyses suggested a possible reduction in cancer mortality and advanced/metastatic cancer, but these require replication. An important limitation is that VITAL enrolled individuals who were not selected for deficiency — results may not generalize to those with low baseline 25(OH)D. An umbrella review of 137 meta-analyses (Theodoratou et al., 2014) highlighted the consistent gap between observational studies (which associate low vitamin D with myriad adverse outcomes) and RCT evidence (which generally shows weaker or null effects). This pattern suggests that low vitamin D is often a biomarker of poor health, reduced outdoor activity, or metabolic dysfunction rather than a direct cause of associated conditions — a critical interpretive caution. Mood, cognition, and immunity are areas of active research with mixed but suggestive findings. Vitamin D receptors are present throughout the brain, and deficiency has been associated with increased depression risk in observational data. RCTs of vitamin D supplementation for depression have yielded inconsistent results, with meta-analyses showing modest effects primarily in deficient populations. Safety at commonly used doses (1,000–4,000 IU/day) is generally favorable. Vitamin D toxicity — resulting in hypercalcemia — is rare but documented at sustained very high doses, generally above 10,000 IU/day over extended periods. Individuals with granulomatous diseases (sarcoidosis, tuberculosis) or primary hyperparathyroidism may be at risk for hypercalcemia even at moderate supplemental doses. Serum level testing before and during high-dose supplementation is considered good clinical practice.