Vitamin D: Benefits, Dosage, and Safety

What vitamin d is

Vitamin D (also called calciferol) is a fat-soluble vitamin naturally present in a few foods, added to fortified foods, and available as a dietary supplement. It is also produced in the skin when ultraviolet B (UVB) rays from sunlight strike 7-dehydrocholesterol. Vitamin D from any source is biologically inert and must undergo two hydroxylations to become active: the first in the liver forms 25-hydroxyvitamin D [25(OH)D, calcidiol], and the second, mainly in the kidney, forms the active 1,25-dihydroxyvitamin D [1,25(OH)2D, calcitriol]. The two dietary forms are D2 (ergocalciferol) and D3 (cholecalciferol), which differ only in their side-chain structure.

What it does

• Promotes calcium absorption in the gut and maintains adequate serum calcium and phosphate concentrations to enable normal bone mineralization
• Prevents hypocalcemic tetany (involuntary muscle contraction, cramps, and spasms)
• Supports bone growth and bone remodeling by osteoblasts and osteoclasts, and, with calcium, helps protect older adults from osteoporosis
• Contributes to reduction of inflammation and to modulation of cell growth, neuromuscular function, immune function, and glucose metabolism
• Modulates, in part, many genes encoding proteins that regulate cell proliferation, differentiation, and apoptosis (many tissues carry vitamin D receptors)

How much vitamin d per day

These are population reference values from the NIH and the Dietary Reference Intakes. They are targets for healthy people, not personal prescriptions. The last column is the Tolerable Upper Intake Level, the most you should get from all sources combined.

Deficiency and who is at risk

Vitamin D deficiency causes rickets in children, in which bone tissue fails to mineralize, producing soft bones and skeletal deformities; severe rickets can cause failure to thrive, developmental delay, hypocalcemic seizures, tetanic spasms, cardiomyopathy, and dental abnormalities. In adolescents and adults, deficiency causes osteomalacia, in which bone is incompletely or defectively mineralized during remodeling, producing bone pain, muscle weakness, and weak bones. Long-term insufficiency of vitamin D and calcium contributes to osteoporosis. The Food and Nutrition Board considers serum 25(OH)D below 30 nmol/L (12 ng/mL) to place people at risk of deficiency, and 30 to 50 nmol/L (12 to 20 ng/mL) potentially inadequate for bone and overall health.

Who may need more

• Exclusively or partially breastfed infants (the AAP recommends 10 mcg/400 IU per day starting shortly after birth)
• Older adults, especially those with limited sun exposure or low dietary intake
• People with dark skin or little sun exposure
• People with fat-malabsorption conditions (celiac, Crohn's, ulcerative colitis, cystic fibrosis, some liver disease)
• People with obesity or a history of gastric bypass surgery
• People following vegan or ovo-vegetarian diets or who avoid dairy due to lactose intolerance or milk allergy

Food sources

Few foods naturally contain vitamin D. Best natural sources are the flesh of fatty fish and fish liver oils: cod liver oil (1 tablespoon) 34.0 mcg (1,360 IU); cooked farmed rainbow trout (3 oz) 16.2 mcg (645 IU); cooked sockeye salmon (3 oz) 14.2 mcg (570 IU). UV-exposed white mushrooms (1/2 cup) 9.2 mcg (366 IU). Fortified foods provide most dietary vitamin D in the U.S.: fortified 2% milk (1 cup) 2.9 mcg (120 IU); fortified soy, almond, or oat milks (1 cup) 2.5 to 3.6 mcg (100 to 144 IU); fortified ready-to-eat cereal (1 serving) about 2.0 mcg (80 IU). Smaller amounts: canned sardines (2) 1.2 mcg (46 IU); 1 large egg (yolk) 1.1 mcg (44 IU); braised beef liver (3 oz) 1.0 mcg (42 IU); canned light tuna (3 oz) 1.0 mcg (40 IU); cheddar cheese (1.5 oz) 0.4 mcg (17 IU).

Forms and absorption

Two main supplement forms: D2 (ergocalciferol), made by UV irradiation of ergosterol in yeast, and D3 (cholecalciferol), typically made by UV irradiation of 7-dehydrocholesterol from lanolin (sheep wool); an animal-free D3 from lichen is also available. Both are well absorbed in the small intestine and both raise serum 25(OH)D, but most evidence indicates D3 raises 25(OH)D to a greater extent and sustains higher levels longer than D2. Because vitamin D is fat soluble, it is best absorbed when taken with a meal or snack containing some fat. Some studies have used 25(OH)D3 (calcifediol), which per equivalent microgram is about three to five times as potent as D3, but no 25(OH)D3 supplements appear to be available on the U.S. consumer market. Conversion: 1 mcg vitamin D equals 40 IU.

Safety, excess and interactions

Too much

Excess vitamin D (almost always from high-dose supplements, not sun or food) causes hypercalcemia. Serum 25(OH)D above 375 nmol/L (150 ng/mL) can cause nausea, vomiting, muscle weakness, neuropsychiatric disturbances and confusion, pain, loss of appetite, dehydration, excessive urination and thirst, and kidney stones. Extremely high levels can cause kidney failure, calcification of soft tissues, irregular heartbeat, and death. The Food and Nutrition Board notes that serum concentrations above 125 to 150 nmol/L (50 to 60 ng/mL) may be linked to adverse effects. You cannot get too much vitamin D from sunshine because skin limits how much it makes.

Interactions to know

• Orlistat (Xenical, alli): this weight-loss drug can reduce absorption of vitamin D from food and supplements
• Statins (for example atorvastatin/Lipitor, lovastatin/Altoprev, Mevacor, simvastatin/FloLipid, Zocor): may not work as well when taken with high-dose vitamin D supplements
• Corticosteroids such as prednisone (Deltasone, Rayos, Sterapred): can lower blood levels of vitamin D
• Thiazide diuretics (for example chlorthalidone/Hygroton, indapamide/Lozol, hydrochlorothiazide/Microzide): combined with vitamin D supplements can raise blood calcium too high

How status is measured

Vitamin D status is measured by serum 25-hydroxyvitamin D [25(OH)D], the main indicator, which reflects both endogenous production and intake and has a circulating half-life of about 15 days. Results are reported in nmol/L or ng/mL (1 nmol/L = 0.4 ng/mL; 1 ng/mL = 2.5 nmol/L). FNB/NASEM interpretation: below 30 nmol/L (12 ng/mL) at risk of deficiency; 30 to under 50 nmol/L (12 to under 20 ng/mL) inadequate; 50 nmol/L (20 ng/mL) or above adequate for most people; above 125 nmol/L (50 ng/mL) potential adverse effects. Assays vary between laboratories, so results can be falsely high or low. Circulating 1,25(OH)2D is not a good status marker because of its short half-life and tight regulation.

Regulated by the FDA as a dietary supplement ingredient and food fortificant, not as a drug; supplements are not FDA-approved to diagnose, treat, cure, or prevent any disease. The FDA-established Daily Value (DV) for vitamin D is 20 mcg (800 IU) for adults and children age 4 and older, and food labels must list vitamin D in mcg per serving (IU optionally in parentheses). The FDA mandates fortification of infant formula and has approved UV-treated mushroom powder as a food additive source of vitamin D2.

What the evidence supports

We grade the strength of evidence behind each common reason people take vitamin d, mirroring how the NIH describes it.