The short answer
Vitamin D is a nutrient your body needs to keep bones, teeth and muscles healthy. It helps regulate the amount of calcium and phosphate in the body, the NHS says. Unlike most vitamins, you do not get most of it from food. Your body makes it from direct sunlight on your skin; food and supplements are the back-up. That is why vitamin D becomes a winter topic: in the UK, the sun is not strong enough for most people to make what they need from October to early March. This article explains what it does, where it comes from, and what official guidance says.
What vitamin D actually does
Vitamin D helps regulate the amount of calcium and phosphate in the body, and those nutrients are needed to keep bones, teeth and muscles healthy. That explains why low levels matter most for bones: without enough vitamin D, the body cannot use calcium properly. The CDC calls it essential for good bone health, and says it may also help muscle strength. Vitamin D is fat-soluble, so the body stores it in fat tissue and the liver — which is why it is a seasonal story: summer stores carry you for a while, but not forever.
Where the body gets it: sunlight
Sunlight is the main source. The body creates vitamin D from direct sunlight on the skin when outdoors, the NHS says. The relevant part is ultraviolet B, or UVB, which triggers the skin to make it. The CDC adds that the vitamin is then carried to the liver and converted into the form doctors measure in a blood test. Several things change how much vitamin D your skin can make. The CDC lists latitude, season, ageing, sunscreen use and skin pigmentation as factors that influence production. Two limits are worth knowing: the NHS says you cannot overdose on vitamin D through exposure to sunlight, but sunlight still damages skin, so it advises protecting your skin if you are out for long periods.
What food gives you
Food is the second source, but a modest one: vitamin D is found naturally in only a small number of foods, the NHS and CDC agree. The foods the NHS names are oily fish such as salmon, sardines, trout, herring and mackerel; red meat; egg yolks; fortified foods such as some fat spreads and breakfast cereals; and liver. The CDC’s list is similar: fish-liver oils, fatty fish, mushrooms, egg yolks and liver. In the United States, most food vitamin D comes from fortified milk, breakfast cereals and orange juice, the CDC says. In the UK, cows’ milk is generally not a good source, because it is not fortified as it is in some other countries, the NHS notes. The NHS is direct about the limits of food: it is difficult for people to get enough vitamin D from food alone.
Why winter is the problem
From about late March or early April to the end of September, most people should be able to make all the vitamin D they need from sunlight, the NHS says. Between October and early March, the sun in the UK is not strong enough for the body to make enough vitamin D. In mid-November, we are already in those months. The reason is the angle of the sun: in the winter months the sun sits lower in the sky, and the UVB that drives vitamin D production does not reach the ground with enough strength. A bright, cold, clear winter day can look sunny and still not deliver what your skin needs. The same pattern applies across the northern United States and Canada, though the exact dates vary with latitude. In Australia the seasons are reversed, so the UK dates do not apply there. Winter also keeps people indoors, in clothing that covers the skin.
Who is more likely to be low
Some people are more likely than others to run low on vitamin D, and the NHS lists the groups plainly. You may not make enough vitamin D from sunlight if you are not often outdoors, for example if you are frail or housebound; if you are in an institution such as a care home; or if you usually wear clothes that cover up most of your skin when outdoors. Skin tone matters too. If you have dark skin — for example, an African, African-Caribbean or south Asian background — you may not make enough vitamin D from sunlight, the NHS says. Melanin reduces how much vitamin D the skin makes from the same sun. The CDC found the highest deficiency rates among non-Hispanic Black Americans. Ageing also reduces how much the skin produces, the CDC notes.
What official guidance says
This is the part where it matters most to describe guidance rather than prescribe. The UK government’s advice, as published by the NHS, is that everyone should consider taking a daily vitamin D supplement during the autumn and winter. People at high risk, all children aged one to four, and all babies — unless they have more than 500ml of infant formula a day, which is already fortified — should consider one all year. The word “consider” is deliberate: this is population guidance, not a diagnosis or a prescription. If a doctor has told you to take a different amount, follow their advice, the NHS says. This article states no dosage figure. The reasoning is simple: in autumn and winter, the sun is not strong enough for the body to make vitamin D, and it is difficult to get enough from food alone.
What happens when levels run low
A lack of vitamin D can lead to bone deformities such as rickets in children, and bone pain caused by a condition called osteomalacia in adults, the NHS says. The CDC adds that deficiency in adults can lead to bone loss and osteoporosis. Those are serious conditions, and they develop over time rather than overnight. So vitamin D is maintenance, not rescue: steady habits over time, not quick fixes. Persistent bone pain or muscle weakness is a reason to speak to a clinician.
What vitamin D cannot do
One claim deserves a direct answer, because it circulated widely during the pandemic. There have been reports about vitamin D reducing the risk of coronavirus (COVID-19), but there is currently not enough evidence to support taking vitamin D solely to prevent or treat COVID-19, the NHS says. It is worth attention for bones and muscles, not as a shield against infections. If you want help telling common winter illnesses apart, our guide to flu, cold, COVID or RSV explains the differences. The same caution applies to taking too much. Taking too much over a long period can cause too much calcium to build up in the body, weakening bones and damaging kidneys and heart, the NHS warns. More is not better, and high-dose supplements are not a shortcut.
Everyday habits that help
The most useful habits are ordinary ones. In the brighter months, spend time outdoors in daylight — a walk, a lunch break outside, gardening, or any reason to be out in the sun on your skin. The NHS sets no safe number of minutes in the sun, because burning damages skin. Put vitamin D foods into your regular meals. Oily fish, eggs, and fortified cereals or fat spreads are the everyday sources the NHS names. If you eat little fish or animal products, check fortified foods where you live and discuss your diet with a clinician. In winter, sunlight no longer carries the load — food and, if you choose, a supplement are what remain.
When to speak to a clinician
Speak to your doctor or pharmacist if you have lasting bone pain or muscle weakness, are in a higher-risk group, or take medicines that could affect vitamin D. A clinician can arrange a blood test if needed and advise on what is right for you. Our explainer on whether pneumonia is contagious covers another common winter health question in the same plain style.
Common questions
Can I get all the vitamin D I need from food? For most people, no — the NHS says it is difficult to get enough from food alone.
Does a sunny winter day make vitamin D? In the UK, no — between October and early March the sun is not strong enough, however bright the day looks.
Do I need vitamin D in summer? From late March to the end of September, most people can make all they need from sunlight, the NHS says. Higher-risk groups are advised to consider a supplement all year.
Does vitamin D prevent colds or COVID-19? There is not enough evidence to support taking it solely to prevent or treat COVID-19, the NHS says.
The bottom line
Vitamin D keeps bones, teeth and muscles healthy by helping the body regulate calcium and phosphate. Sunlight is the main source, food is a useful but limited one, and winter is when the supply runs thinnest — in the UK, from October to early March. People who spend little time outdoors, live in care homes, cover most of their skin, or have dark skin are more likely to run low. Official guidance is that everyone should consider a supplement in autumn and winter, and higher-risk groups all year. The decision about what is right for you belongs with your clinician.
This article is for general information only and is not a substitute for medical advice. If you are worried about your vitamin D level, contact a healthcare provider or pharmacist.
Sources verified 14 Nov 2026: NHS, Vitamin D — what vitamin D does, sunlight months, food sources, who is at risk, autumn and winter guidance, too much vitamin D, and the COVID-19 evidence note. CDC, Vitamin D factsheet — natural food sources, fortified foods in the United States, deficiency and race and ethnicity, bone health and muscle strength. CDC, Vitamin D sources and functions — sunlight and skin production, factors affecting production, rickets and osteomalacia. Left out: every dosage figure — checked but deliberately not stated, as this article describes guidance and does not prescribe.