Why dermatologists suggest food and supplements for vitamin D, what studies show about sunscreen and vitamin D levels, and who may need a test.
Skin makes vitamin D when ultraviolet B (UVB) rays strike it. The same rays cause sunburn and damage the DNA of skin cells. People who protect their skin carefully often worry that they are trading one health problem for another. The research on that trade is more reassuring than many expect, and it still has gaps.
Vitamin D helps the gut absorb calcium, and bones need it to stay dense and strong. Severe shortage causes rickets in children and soft, painful bones in adults. Muscles, nerves, and immune cells use it too. Researchers have studied links between vitamin D and many other conditions, including cancer and heart disease. Large trials of supplements have so far not shown clear benefits beyond bone health, and that work continues.
The American Academy of Dermatology (AAD) advises people to get vitamin D from foods that contain it naturally, from fortified foods and drinks, and from supplements. Its reasoning is that UV is a proven carcinogen and that no level of exposure from the sun or a tanning bed has been shown to produce the most vitamin D without also raising skin cancer risk.
A vitamin D pill delivers a known dose in any season. The amount the skin makes from sunlight changes with latitude, time of year, cloud cover, air pollution, age, and skin color. Melanin, the pigment in darker skin, filters UVB, so people with darker skin make less vitamin D from the same exposure. In much of the northern United States, winter sun is too weak to produce much at all.
Tanning beds are a poor source for a separate reason. Their bulbs emit mostly UVA, and vitamin D production depends on UVB.
In theory, a sunscreen that blocks UVB should also block vitamin D production. A systematic review tested that idea (Neale and colleagues, British Journal of Dermatology, 2019). The authors gathered 4 laboratory experiments, 3 field trials (2 of them randomized), and 69 observational studies.
The laboratory experiments confirmed the theory. Under artificial UV lamps, with sunscreen applied thickly, vitamin D production dropped sharply. The field studies told a different story. In the 2 randomized trials, adults assigned to daily sunscreen had vitamin D levels no lower than the comparison group. Most observational studies found either no link or higher vitamin D levels in sunscreen users, probably because people who use sunscreen also spend more time outdoors.
The likely explanation is ordinary behavior. People apply less sunscreen than laboratories do, miss patches of skin, and reapply late, so some UVB still gets through. The reviewers concluded that there is little evidence sunscreen lowers vitamin D in everyday use.
They also flagged a limit. The trials used sunscreens of about SPF 16, and no trial has yet tested the SPF 30 to 50 products recommended now. More data is needed on that point. People who combine high-SPF sunscreen with clothing, hats, and shade, as many skin cancer survivors do, may get little UVB, and for them intake from food and supplements matters more.
The NIH Office of Dietary Supplements lists these recommended daily amounts, which assume minimal sun exposure:
Few foods are naturally rich in vitamin D. Fatty fish such as salmon, trout, tuna, and mackerel lead the list, and fish liver oils contain the most. Egg yolks, beef liver, and cheese supply small amounts. In the United States most dietary vitamin D comes from fortified foods: nearly all cow's milk, many plant-based milks, and some brands of orange juice and yogurt. Mushrooms treated with UV light are another source.
Too much vitamin D causes harm. The upper limit for adults and for children 9 and older is 100 micrograms (4,000 IU) a day unless a doctor advises otherwise. Very high blood levels, which come almost always from supplements, cause nausea, vomiting, muscle weakness, confusion, and kidney stones. Sun exposure does not cause vitamin D toxicity, because the skin limits how much it makes.
The test measures 25-hydroxyvitamin D. According to the Office of Dietary Supplements, 50 nmol/L (20 ng/mL) or above is adequate for most people, and levels below 30 nmol/L (12 ng/mL) are too low. Levels above 125 nmol/L (50 ng/mL) are too high. Some laboratories and specialty societies use different cutoffs, which explains why 2 doctors may read the same result differently.
Routine testing of healthy adults is not standard. In 2021 the U.S. Preventive Services Task Force found the evidence insufficient to recommend for or against screening adults who have no symptoms. Doctors are more likely to check a level in people at higher risk of deficiency:
Anyone who avoids the sun strictly because of a past melanoma, many prior skin cancers, an organ transplant, or a condition such as lupus can reasonably ask a doctor whether a vitamin D level or a daily supplement makes sense. A standard 600 to 800 IU supplement costs a few cents a day.
