How Vitamin D Works: Deficiency, Dosage, and What Research Shows

Billions of People Are Deficient in the Only Vitamin the Body Can Make Itself

Unlike most vitamins, vitamin D is produced by the human body — specifically in the skin during ultraviolet B radiation exposure from the sun. Yet the National Health and Nutrition Examination Survey (NHANES) found that 41.6% of American adults have deficient or insufficient vitamin D levels. Globally, the World Health Organization estimates that 1 billion people are vitamin D deficient. The paradox exists because modern life keeps people indoors, sunscreen use (while important for cancer prevention) blocks vitamin D synthesis, and dietary sources are limited. Vitamin D deficiency is simultaneously one of the most prevalent and most correctable nutritional deficiencies in the world.

How the Body Synthesizes and Activates Vitamin D

Vitamin D metabolism involves three tissues and two conversion steps before the molecule becomes biologically active.

The kidney-based second hydroxylation step is tightly regulated by parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23). When blood calcium falls, PTH rises and stimulates more calcitriol production. This regulatory precision means that serum calcitriol does not reflect vitamin D stores well — measuring 25(OH)D (the storage form from the liver) is the correct diagnostic test.

What Vitamin D Does in the Body

Vitamin D (as calcitriol) acts as a hormone rather than a traditional vitamin. It binds to the vitamin D receptor (VDR) — present in virtually every cell type — and regulates expression of over 1,000 genes. Its major known functions include.

• Calcium and phosphate absorption: Calcitriol upregulates intestinal transporters that absorb calcium from food. Without adequate vitamin D, only 10–15% of dietary calcium is absorbed (compared to 30–40% when replete). This is why vitamin D is inseparable from bone health.
• Bone mineralization: Calcitriol promotes the deposition of calcium and phosphate into bone matrix. Severe deficiency in children causes rickets; in adults, osteomalacia (soft bones).
• Immune modulation: VDR is expressed on T cells, B cells, macrophages, and dendritic cells. Calcitriol modulates both innate and adaptive immune responses — it enhances antimicrobial peptide production (cathelicidins, defensins) while suppressing excess inflammatory cytokine production.
• Muscle function: Vitamin D deficiency is associated with muscle weakness and falls in older adults; supplementation in deficient individuals reduces fall risk in randomized trials.

Deficiency: Causes, Prevalence, and Symptoms

Optimal vitamin D levels (25(OH)D): most experts consider 30–50 ng/mL (75–125 nmol/L) as optimal for most adults. Levels below 20 ng/mL are classified as deficient; 20–29 ng/mL as insufficient. Levels above 100 ng/mL risk toxicity (though toxicity from supplementation alone is rare and typically requires sustained doses above 10,000 IU/day).

Dietary Sources: Limited But Real

• Fatty fish: wild salmon (600–1,000 IU per 3.5 oz), mackerel, sardines, herring
• Cod liver oil: approximately 400–1,000 IU per tablespoon
• Fortified foods: milk typically fortified to 100 IU per cup; fortified OJ, cereals
• Egg yolks: 20–40 IU per yolk (highly variable; pastured hens exposed to sunlight produce more)
• Mushrooms exposed to UV light: can contain significant D2

Supplementation: What the Evidence Shows

Vitamin D supplementation clearly corrects deficiency and improves 25(OH)D levels. Its benefits beyond bone health are more contested. The VITAL trial (2019), the largest randomized controlled trial of vitamin D supplementation, enrolled 25,871 adults and found no significant reduction in cancer incidence or cardiovascular events from vitamin D supplementation (2,000 IU/day). However, secondary analyses suggested reduced cancer mortality in those who developed cancer, and reduced autoimmune disease incidence. A 2022 update to the VITAL data found 22% reduction in autoimmune disease (rheumatoid arthritis, psoriasis, thyroid disease) in participants taking 2,000 IU D3 plus omega-3s.

• The Endocrine Society recommends 1,500–2,000 IU/day for most adults who want to maintain levels above 30 ng/mL without sun exposure
• Vitamin D3 (cholecalciferol) raises 25(OH)D more effectively than D2 (ergocalciferol) — choose D3 supplementation
• Taking vitamin D with the largest meal (containing some fat) improves absorption by 50% according to a Cleveland Clinic study

This article is for informational purposes only. Consult a qualified professional before beginning supplementation.