A vitamin that behaves like a hormone
Vitamin D is unusual among vitamins. The body can make it in the skin when ultraviolet B (UVB) light from the sun reaches it, which means it is not strictly something you must get from food. Once made or absorbed, it is converted by the liver and kidneys into its active form, which acts somewhat like a hormone. Because of this, the amount you have depends on sunlight, diet and the way your body handles the vitamin.
This is general information, not medical advice. Whether you need testing or a supplement should be decided with a clinician.
What it does for the body
The best-established role of vitamin D is helping the intestines absorb calcium and keeping calcium and phosphate at levels that support healthy bones and teeth. Severe, long-term deficiency can lead to rickets in children, in which bones soften and may bend, and to osteomalacia in adults, a softening of bones that can cause pain and weakness. Vitamin D is also important in later life, alongside calcium, as part of how the skeleton maintains its strength, which ties into the way bones and muscles work together.
Vitamin D is also found in cells of the immune system, muscles and other tissues, and researchers have studied possible links with many conditions, including infections, heart disease, cancer, mood and diabetes. Results are mixed. Observational studies often find that people with low levels have worse health, but low levels can also be a result of illness, indoor living or obesity rather than a cause. Large trials of supplements in generally healthy people have often not shown clear benefits for those conditions. For most claims beyond bone health, the evidence is not settled.
Sunlight
Skin makes vitamin D when exposed to UVB. How much it makes depends on:
- Latitude and season. At higher latitudes in winter, the sun is low and the atmosphere filters out most UVB, so skin may make little or none for several months. Closer to the equator, the sun is strong year-round. The way the Earth’s axial tilt shapes seasons is the underlying reason.
- Time of day. UVB is strongest around midday.
- Skin pigmentation. Melanin, which gives skin its color, absorbs UVB, so people with darker skin typically need longer exposure to make the same amount.
- Age. Older skin tends to make vitamin D less efficiently.
- Clothing, shade and time outdoors. Covered skin and long periods indoors reduce production.
- Sunscreen. Sunscreen reduces UVB reaching the skin, though in everyday use it is unlikely to block production entirely.
- Cloud, pollution and glass. Cloud cover and air pollutants reduce UVB, and ordinary window glass blocks most of it.
Sun exposure is a trade-off. Too much UV raises the risk of skin damage and skin cancer, so health authorities generally do not recommend deliberate, prolonged sun exposure or tanning beds as a way to get vitamin D. Advice about safe amounts differs by country, skin type and local UV level, so local guidance from a health authority or a doctor is the most reliable source.
Food sources
Relatively few foods contain much vitamin D naturally. Commonly mentioned examples are:
- Fatty fish such as salmon, mackerel, sardines and herring, along with fish liver oils
- Egg yolks
- Some mushrooms, especially those exposed to UV light during growing
- Beef liver and small amounts in some other animal products
Many countries also fortify foods with vitamin D, such as milk, some plant-based milk alternatives, some yogurts, breakfast cereals and margarine. Whether a food is fortified, and by how much, varies, so labels are worth checking. Cod liver oil is high in vitamin A as well as vitamin D, and too much vitamin A can be harmful, so it needs particular care.
Why needs vary
Because sunlight, diet, age and health all play a part, there is no single level of intake that suits everybody. Official recommended intakes also differ between countries and age groups, and some health authorities advise supplements for particular groups, such as breastfed infants or people who have little sun exposure. Groups sometimes mentioned as being at higher risk of low levels include:
- People living at higher latitudes, especially in winter
- People who spend most of their time indoors, are housebound, or cover most of their skin
- People with darker skin living away from the equator
- Older adults
- Infants who are exclusively breastfed, since breast milk is usually low in vitamin D
- People with conditions that affect fat absorption, such as celiac disease or inflammatory bowel disease, or after certain types of weight-loss surgery
- People with obesity, kidney disease or liver disease
- People taking certain medications that affect vitamin D metabolism
A rule of thumb for any of these groups does not tell you your own situation.
Testing
A blood test for 25-hydroxyvitamin D is the usual measure. Interpreting the number is less simple than it looks. Laboratories and health organizations use different cutoffs for deficiency or sufficiency, and the numbers vary by testing method. Routine screening of healthy people is not recommended in many guidelines, whereas testing is more commonly used for people with symptoms, bone disease or higher-risk conditions. Whether a test makes sense for you is a question for a clinician.
Supplement cautions
Vitamin D supplements come as D2 (ergocalciferol) and D3 (cholecalciferol). D3 is more widely used, and many studies suggest it raises blood levels more effectively than D2, though the difference is not always important in practice.
Since vitamin D is fat soluble, the body can store it, and too much for too long can cause a build-up. Taking very high doses can lead to high blood calcium, which can cause nausea, weakness, kidney problems and other effects. This is uncommon from sunlight or food and typically results from excessive supplementation. For this reason, it is best to stick to the doses on a label or recommended by a clinician, and to tell your doctor about all supplements you take. Some medicines interact with vitamin D or calcium, and people with conditions such as kidney stones or sarcoidosis should ask before taking supplements.
Putting it together
Getting enough vitamin D usually comes from a combination of sensible sun exposure, foods that provide it and, for some people, supplements. For many healthy people in sunny regions, little effort is needed. In places with long winters, or for people with limited sun exposure, it is more of a question. If you are unsure, a clinician or dietitian can help work out whether your situation calls for testing or a supplement.