Vitamin D3

Vitamin D3 (cholecalciferol) is a fat-soluble vitamin that the body can produce endogenously when ultraviolet B (UVB) rays from sunlight strike the skin, and that is also obtained from a small number of foods and from dietary supplements. Vitamin D obtained from any source is biologically inactive and must undergo two hydroxylations in the body — first in the liver to 25-hydroxyvitamin D [25(OH)D, or calcidiol], then primarily in the kidney to 1,25-dihydroxyvitamin D [1,25(OH)2D, or calcitriol] — before it becomes physiologically active. The 25(OH)D form is what laboratory blood tests measure when assessing vitamin D status.¹

Vitamin D plays a central role in calcium absorption and bone mineralization. Long-standing public health interest in vitamin D supplementation has focused on prevention of rickets in children, osteomalacia in adults, and contribution to the prevention of osteoporosis in combination with calcium.¹ Beyond bone health, vitamin D has been studied for a wide range of additional outcomes including cardiovascular disease, cancer prevention, immune function, and metabolic health. The large randomized VITAL trial (VITamin D and OmegA-3 TriaL), which followed nearly 26,000 adults for over five years, found that 2,000 IU per day of vitamin D3 did not significantly reduce the risk of invasive cancer or major cardiovascular events compared with placebo in the general adult population studied.²

The 2024 Endocrine Society Clinical Practice Guideline on vitamin D for the prevention of disease, which evaluated the available randomized controlled trial evidence using GRADE methodology, concluded that empiric vitamin D supplementation above the Institute of Medicine Recommended Daily Allowance (RDA) is suggested for specific groups — children and adolescents, pregnant individuals, adults aged 75 years and older, and adults with high-risk prediabetes — but that healthy adults under 75 are unlikely to benefit from supplementation above the RDA, and routine 25(OH)D testing in healthy individuals is not recommended.³

Daily intake recommendations from the Institute of Medicine (used by NIH ODS and other authoritative sources):¹

• Infants 0–12 months: 400 IU (10 mcg) — Adequate Intake (AI)
• Children and adults 1–70 years: 600 IU (15 mcg)
• Adults 71 years and older: 800 IU (20 mcg)
• Pregnant and lactating individuals: 600 IU (15 mcg)

The Tolerable Upper Intake Level (UL) for adults is 4,000 IU (100 mcg) per day from all sources combined.¹ The 2024 Endocrine Society guideline notes that higher empiric doses may be appropriate in specific risk groups but does not recommend a single specific dose because the underlying clinical trial literature used a wide range of doses without head-to-head comparison.³

Vitamin D blood concentrations are measured as 25-hydroxyvitamin D [25(OH)D] in either nanograms per milliliter (ng/mL) or nanomoles per liter (nmol/L). The NIH ODS describes general status categories — deficiency, inadequacy, sufficiency — but exact thresholds vary by organization, and the 2024 Endocrine Society guideline explicitly declined to endorse a specific sufficiency threshold due to limited supporting evidence.¹³

Oral. Vitamin D is fat-soluble and absorption is improved when taken with a meal or snack that contains fat.¹

Standard storage in a cool, dry place out of direct sunlight. Vitamin D supplements typically do not require refrigeration. Expiration dates printed on the product label should be observed.

Several factors can affect vitamin D status independent of supplementation: skin pigmentation (melanin reduces UVB-mediated synthesis), latitude and season (UVB exposure is limited at higher latitudes and in winter), sunscreen use, body composition (vitamin D is sequestered in adipose tissue), and certain medical conditions affecting fat absorption (such as Crohn's disease, celiac disease, or post-bariatric-surgery states).¹

Vitamin D can interact with several medications. Examples documented by NIH ODS include orlistat (which can reduce absorption of vitamin D and other fat-soluble vitamins), statins (high-dose vitamin D may reduce statin effectiveness via shared metabolic pathways), thiazide diuretics (concurrent high vitamin D may increase risk of hypercalcemia), and steroids (chronic corticosteroid use can impair vitamin D metabolism).¹

Excessive vitamin D intake — generally only from supplements or fortified products, not from sun exposure or food — can cause hypercalcemia, with symptoms including nausea, vomiting, weakness, and in serious cases impaired kidney function. Toxicity has typically been reported with sustained intakes well above the 4,000 IU per day Upper Limit, but individual sensitivity varies.¹