We need vitamin D to help the body absorb calcium and phosphate from our diet. These minerals are important for healthy bones, teeth and muscles.
A lack of vitamin D can cause bones to become soft and weak, which can lead to bone deformities.
In children, for example, a lack of vitamin D can lead to rickets. In adults, it can lead to osteomalacia, which causes bone pain and tenderness.
Some studies have linked a lack of vitamin D to low mood and fatigue, and suggest that optimum vitamin D levels help to boost your immune system.
How do we get vitamin D?
Our bodies create vitamin D from direct sunlight on our skin when we're outdoors. From about late March/early April to the end of September, most of us should be able to get all the vitamin D we need from sunlight.
We also get some vitamin D from a small number of foods, including oily fish such as salmon, mackerel, herring and sardines, as well as red meat and eggs.
How long should we spend in the sun?
Most people can make enough vitamin D from being out in the sun daily for short periods with their forearms, hands or lower legs uncovered and without sunscreen from late March or early April to the end of September, especially from 11am to 3pm.
But you should be careful not to burn in the sun, so take care to cover up or protect your skin with sunscreen before your skin starts to turn red or burn.
People with dark skin, such as those of African, African-Caribbean or south Asian origin, will need to spend longer in the sun to produce the same amount of vitamin D as someone with lighter skin.
How long it takes for your skin to go red or burn varies from person to person. Cancer Research UK has tips to help you protect your skin in the sun.
Your body can't make vitamin D if you're sitting indoors by a sunny window because ultraviolet B (UVB) rays (the ones your body needs to make vitamin D) can't get through the glass.
In the UK, sunlight doesn't contain enough UVB radiation in winter (October to early March) for our skin to be able to make vitamin D.
During these months, we rely on getting our vitamin D from food sources (including fortified foods) and supplements.
Who should take vitamin D supplements?
Some groups of the population are at greater risk of not getting enough vitamin D, and might benefit from taking daily supplements. These include:
- all babies from birth to 1 year old (including breastfed babies and formula-fed babies who have less than 500ml a day of infant formula)
- all children aged 1 to 4 years old
- people who aren't often exposed to the sun (for example, people who are frail or housebound, or are in an institution such as a care home, or if they usually wear clothes that cover up most of their skin when outdoors)
For the rest of the population, everyone over the age of 5 years (including pregnant and breastfeeding women) is advised to consider taking a daily supplement containing 10 micrograms (μg) of vitamin D from October to February.
Find out who should take vitamin D supplements and how much to take
Can you have too much vitamin D?
If you choose to take vitamin D supplements, 10μg a day will be enough for most people.
People who take supplements are advised not to take more than 100μg of vitamin D a day, as it could be harmful (100 micrograms is equal to 0.1 milligrams).
Some people have medical conditions that mean they may not be able to take as much vitamin D safely.
If in doubt, you should talk to your doctor. If your doctor has recommended you take a different amount of vitamin D, you should follow their advice.