Vitamin D is often called the sunshine vitamin, and the nickname is accurate: sunlight on skin is the main way most people make it. Yet deficiency remains common, especially through autumn and winter, when the sun is too weak for the skin to produce enough. Understanding where vitamin D comes from, what low levels can feel like, and how to supplement safely is one of the simplest pieces of preventive health knowledge worth having.
This article is general information only, not a substitute for professional medical advice. If you have a health condition, take regular medicine, or are worried about symptoms, speak to a clinician or pharmacist before starting a supplement.
What vitamin D actually does
Vitamin D helps regulate the amount of calcium and phosphate in the body, which keeps bones, teeth and muscles healthy. Without enough of it, bones cannot absorb enough minerals, and over time they become soft and weak. In children, severe deficiency causes rickets, a condition in which growing bones soften and bend. In adults, the equivalent condition is called osteomalacia, which tends to cause bone pain and tenderness, particularly in the legs, hips and lower back, and bones that break more easily.
Vitamin D also supports normal muscle function and plays a role in the immune system, which is one reason low levels are often linked with feeling run down.
Sunlight: the main source, with limits
The body makes vitamin D when ultraviolet B (UVB) light from the sun hits the skin. This only happens when the sun is high enough in the sky — roughly speaking, when your shadow is shorter than you are. In the UK, that window runs from about late March or early April to the end of September; between October and early March, the sun is not strong enough for the body to make vitamin D, however long you stay outside.
Skin tone matters. Melanin acts as a natural sun filter, so people with darker skin need more time in the sun to make the same amount of vitamin D as people with lighter skin, and are at higher risk of deficiency in less sunny climates. Do not burn in pursuit of vitamin D: short, regular midday exposure of the face, arms and hands during summer is the usual pattern, not prolonged sunbathing.
One reassuring fact: you cannot overdose on vitamin D from sunlight, because the skin limits how much it produces. Problems from excess vitamin D almost always come from supplements, not from the sun.
Food sources: helpful, but rarely enough alone
Relatively few foods contain significant amounts of vitamin D, which is why diet alone rarely covers the full need. The best natural sources are oily fish such as salmon, sardines, trout, herring and mackerel. Smaller amounts come from red meat and egg yolks, and from fortified foods such as some fat spreads and breakfast cereals. Liver is also a source, although it should be avoided during pregnancy.
In the UK, cows’ milk is generally not a good source, because it is not fortified in the way it is in some other countries. Oily fish a couple of times a week, plus eggs and fortified foods, makes a real contribution — but for most people it still falls short, which is why guidance pairs food with sunlight in summer and a supplement in winter.
How much do you actually need?
Guidance differs slightly between countries, but the practical picture is consistent. The NHS recommends that children from the age of one year and adults need 10 micrograms (400 IU) of vitamin D a day, and babies up to one year need 8.5 to 10 micrograms a day. The US National Institutes of Health sets the recommended dietary allowance slightly higher, at 15 micrograms (600 IU) a day for adults up to age 70, and 20 micrograms (800 IU) a day for adults over 70.
During autumn and winter, everyone should consider taking a daily supplement containing 10 micrograms of vitamin D. Some people should take one all year round: those who get little or no sunlight, those who cover their skin for cultural reasons, and people with dark skin. Babies and young children have their own year-round guidance, so parents should follow the advice given for their child’s age.
Signs that levels may be low
Deficiency often develops gradually, which makes it easy to miss. The most commonly reported signs include persistent tiredness despite enough sleep, muscle weakness or aches, bone discomfort, low mood, and catching coughs and colds more often than usual. None of these symptoms is specific to vitamin D — each has many possible causes — so they cannot be used to diagnose yourself. They are a reason to review your sunlight, diet and supplementation, and to seek advice if they persist.
Choosing a supplement safely
Vitamin D supplements come in two forms: D2 (ergocalciferol), which is plant-derived, and D3 (cholecalciferol), the form the skin makes from sunlight. Both raise blood levels, and D3 is generally considered the more effective of the two at maintaining levels over time. Either is acceptable for everyday use; vegans and vegetarians may prefer D2 or a lichen-derived D3.
Vitamin D is fat-soluble, so it is best absorbed when taken with food containing some fat, such as a meal or a glass of milk. A standard daily tablet of 10 micrograms is the usual choice for general use in the UK, and there is no benefit in taking a much larger dose “just in case”.
The safe upper limit for adults is 100 micrograms (4,000 IU) a day from all sources combined — food, drinks and supplements together. Taking more than this over a long period can raise calcium levels in the blood, causing nausea, vomiting, confusion, excessive thirst and urination, and, in severe cases, kidney damage and irregular heartbeat. Check the total across everything you take: multivitamins and fortified products add up, and high-dose products should only be used when a clinician has advised them, for example to correct a confirmed deficiency.
Tell a pharmacist about medicines you take: some, such as the weight-loss drug orlistat, reduce how much vitamin D the body absorbs, and some conditions, such as high blood calcium, make supplementation unsuitable without medical advice.
A simple seasonal routine
From late spring to early autumn, get short, regular midday sun exposure without burning, and eat vitamin D-rich foods a few times a week. From October to March, take a daily 10-microgram supplement and keep eating those foods. If you get little sun, have dark skin, or cover your skin, take the supplement all year. Persistent tiredness, bone pain, muscle weakness or frequent illness deserve medical advice rather than high doses.
The bottom line
Vitamin D is a small nutrient with an outsized role in bone, muscle and immune health, and its supply depends on season, skin and diet in ways that are easy to overlook. The fix is simple: sunlight in summer, food all year, and a modest daily supplement through the darker months, within recommended amounts and taken with food.
This article provides general information only and is not a substitute for professional medical advice, diagnosis or treatment. Always seek the advice of a qualified health professional with any questions about a medical condition or supplement.
