Skeletal health
Vitamin D's classical role is maintaining calcium and phosphorus homeostasis. Calcitriol (active vitamin D) increases intestinal calcium absorption from roughly 10-15% to 30-40% [1]. Without adequate vitamin D, the body cannot absorb enough calcium regardless of dietary intake, leading to secondary hyperparathyroidism, accelerated bone resorption, and ultimately osteomalacia in adults or rickets in children.
The relationship between supplementation and fracture prevention is nuanced. A Cochrane review found that vitamin D alone did not significantly reduce hip or other fractures, but vitamin D combined with calcium modestly reduced hip fracture incidence (RR 0.84, 95% CI 0.74-0.96), particularly in institutionalized elderly populations [5]. A 2019 JAMA Network Open meta-analysis of 11 trials (34,243 participants) confirmed that combined vitamin D and calcium reduced fracture risk, while vitamin D alone did not reach significance [1].
Immune function
Vitamin D receptors are expressed on most immune cells, including T cells, B cells, macrophages, and dendritic cells. Calcitriol modulates both innate and adaptive immunity: it stimulates antimicrobial peptide production (cathelicidin and defensins) while dampening excessive inflammatory responses [3].
The VITAL trial autoimmune substudy (25,871 participants, median 5.3 years follow-up) demonstrated that 2,000 IU/day of vitamin D3 reduced the incidence of confirmed autoimmune disease by 22% compared to placebo (HR 0.78, 95% CI 0.61-0.99) [3]. The effect strengthened over time, with a 39% reduction observed after excluding the first two years of follow-up. Conditions affected included rheumatoid arthritis, polymyalgia rheumatica, autoimmune thyroid disease, and psoriasis.
Mood and depression
Vitamin D receptors are present in brain regions involved in mood regulation, including the prefrontal cortex, hippocampus, and amygdala. A meta-analysis of 31,424 participants found a significant association between low vitamin D levels and depression (OR 1.31, 95% CI 1.0-1.71) [4]. When restricted to studies without biological flaws (proper assay methods, adequate sample size), the association strengthened.
The D2d trial, while designed to study diabetes prevention, found that 4,000 IU/day of vitamin D3 did not significantly reduce the risk of type 2 diabetes in adults with prediabetes (HR 0.88, 95% CI 0.75-1.04), though post-hoc analyses suggested benefit in participants who were vitamin D deficient at baseline [2].
Fracture evidence: resolving the controversy
The fracture prevention literature has been marked by conflicting results, largely because trials varied in dose, co-administration of calcium, baseline vitamin D status, and population characteristics. The Cochrane review by Avenell et al. (2014) analyzed 53 trials with 91,791 participants and concluded that vitamin D alone (in any form or dose) did not prevent fractures, but vitamin D with calcium showed small but statistically significant reductions in hip fracture and total fracture, primarily in institutional settings [5]. The number needed to treat was approximately 43 for hip fracture prevention over 24 months in this population.
Yao et al. (2019) reinforced these findings in a meta-analysis specifically designed to evaluate vitamin D with and without calcium. Their pooled analysis showed that combined supplementation reduced fracture risk (RR 0.94, 95% CI 0.89-0.99) while vitamin D monotherapy did not (RR 1.02, 95% CI 0.93-1.11) [1]. The clinical implication is clear: for fracture prevention, vitamin D supplementation should be paired with adequate calcium intake, whether from diet or supplements.
Autoimmune disease: the VITAL evidence
The VITAL autoimmune substudy by Hahn et al. (2022) represents the strongest randomized trial evidence to date for vitamin D's role in autoimmune disease prevention [3]. The 2x2 factorial design (vitamin D3 2,000 IU/day and/or omega-3 fatty acids 1 g/day vs. placebo) enrolled 25,871 adults aged 50 and older. Vitamin D supplementation reduced confirmed autoimmune disease incidence by 22%, with the effect strengthening after a two-year latency period (HR 0.61, 95% CI 0.43-0.86 after excluding the first two years).
Notably, the omega-3 arm also showed a trend toward reduced autoimmune disease (HR 0.85, 95% CI 0.67-1.08), and the combination appeared additive. These results provide a mechanistic rationale consistent with vitamin D's known effects on regulatory T cell induction and inflammatory cytokine suppression.
Depression and mental health
Anglin et al. (2013) conducted a methodologically rigorous meta-analysis that specifically addressed biological flaws in prior studies (use of inaccurate assays, failure to account for season of blood draw, small sample sizes) [4]. Across studies without such flaws, the association between low vitamin D and depression was robust (OR 1.31). However, the evidence remains largely observational. Intervention trials have produced mixed results, with some showing benefit in individuals who are deficient at baseline and others showing no effect in replete populations. The relationship may be bidirectional: depression leads to indoor behavior and poor diet, which in turn lower vitamin D levels.
The overall evidence supports maintaining adequate vitamin D status (at least 50 nmol/L) as a baseline health measure, with the strongest causal evidence for skeletal health when combined with calcium, and emerging but promising evidence for immune regulation and autoimmune disease prevention [2][3].