What vitamin D is
Vitamin D is a fat-soluble vitamin that works somewhat like a hormone in the body. Unlike most vitamins, the body can make it: when skin is exposed to ultraviolet B (UVB) rays from sunlight, it produces a form of vitamin D that the liver and kidneys then convert to its active form. It is also found in a limited number of foods and in fortified products.
This article is general information, not medical advice. Whether you need more vitamin D, and how, is a decision for you and a doctor or other qualified professional, ideally informed by a blood test where appropriate. This article does not recommend doses, and it does not suggest that vitamin D prevents or treats any disease.
What it does
The best-established role of vitamin D is helping the body absorb calcium and phosphorus, which are important for building and maintaining bones and teeth. Our article on dietary minerals describes calcium and other nutrients that work alongside it. Vitamin D is also present in muscles and the immune system, and researchers continue to study its wider roles. Headlines sometimes link low levels to many conditions, but being associated with a condition is not the same as causing it, and trials of supplements for those conditions have often shown mixed or limited results. It is sensible to treat bold claims with caution.
Severe, prolonged deficiency can lead to bone problems, such as rickets in children and softened bones in adults. Milder low levels often have no obvious symptoms, which is one reason people cannot reliably tell their vitamin D status by how they feel.
Sunlight
Sun exposure is a natural source, but it is complicated. How much vitamin D your skin makes depends on:
- Latitude and season; in winter at higher latitudes, the sun may not be strong enough to make much
- Time of day and cloud cover
- Skin pigmentation; darker skin contains more melanin, which reduces production for a given amount of sun
- Age; older skin tends to make less
- How much skin is uncovered, and use of clothing or sunscreen
- Time spent indoors
At the same time, too much unprotected sun raises the risk of skin damage and skin cancer. Balancing these is something for a professional to discuss with you, and our article on sun safety habits covers protective routines. Time outdoors has other benefits, as discussed in time in nature, but no one should aim to get a tan or burn for vitamin D.
Food sources
Few foods contain much vitamin D naturally. Commonly mentioned sources include:
- Fatty fish such as salmon, sardines and mackerel
- Fish liver oils
- Egg yolks, in smaller amounts
- Some mushrooms, especially those exposed to ultraviolet light
- Fortified foods, which in many countries include milk, some plant milks, some cereals and some spreads
Fortification rules differ from one country to another, so check packaging. The guide on reading a nutrition facts label can help you see whether a food contains it. Because food sources are limited, many people get a good share of their vitamin D from sun exposure, and others get less than they might need.
Who may be at higher risk of low levels
Groups often described as more likely to have low vitamin D include:
- People who spend most of their time indoors or cover most of their skin
- People living at higher latitudes, especially in winter
- People with darker skin
- Older adults and those with limited mobility; see the article on healthy aging
- Breastfed infants, who often receive professional advice about this
- People with certain digestive, kidney or liver conditions, or who have had weight-loss surgery
- People with higher body fat, who may have lower circulating levels
Being in one of these groups does not mean a person is deficient, only that it may be worth asking.
Testing and advice
A blood test can measure the main circulating form of vitamin D, but interpretation depends on the lab, the person and the situation, and experts disagree on ideal target levels. Testing is not necessary for everyone. A doctor can tell you whether it makes sense in your case, and could suggest dietary changes, sensible sun exposure or a supplement if appropriate.
If a supplement is recommended, take it as directed by the professional. Vitamin D is stored in body fat, so taking too much over time can lead to harmful levels of calcium in the blood, with symptoms like nausea, weakness and kidney problems. It can also interact with certain medications. For general cautions, see the article on dietary supplements. Please do not guess at an amount.
Everyday habits
Without going beyond general good sense, you can:
- Include fish, eggs or fortified foods in your meals if they suit your diet, as part of a balanced diet
- Spend some time outside in a way that suits your skin type and local conditions, using sun protection when you will be out for long
- Mention your diet, sun exposure and any concerns at your next doctor visit
- Check with a professional if you follow a vegan diet or avoid dairy and fish, since options may be more limited
The takeaway
Vitamin D matters most for bone health and is involved in other body functions that research is still exploring. Sunlight, food and sometimes supplements all play a part, and individual needs vary a lot. Because the right approach depends on you, the best next step is a conversation with a healthcare professional rather than a self-directed plan.