Vitamin D is often described as the “sunshine vitamin”, but its role is much more important than that nickname suggests.
Your body needs it to help regulate calcium and phosphate, which are important for healthy bones, teeth and muscles. A severe lack of this vitamin can lead to bone problems such as rickets in children and osteomalacia in adults.
The difficulty is that vitamin D is unusual.
Your body can make it when your skin is exposed to sunlight, but only under the right conditions. Relatively few foods naturally contain much of it, and some people are more likely than others to have low levels.
That is why understanding how it works is more useful than simply hearing that “you need more sun”.
What Is Vitamin D?
It’s a fat-soluble vitamin that helps your body regulate calcium and phosphate.
Those minerals are important for healthy bones and teeth, and this vitamin also supports normal muscle function.
It comes from two main sources:
- Production in the skin after exposure to sunlight
- Food and dietary supplements
The amount produced by sunlight varies considerably by season, time of day, skin type, location, clothing, and how much skin is exposed.
Why Is Vitamin D Important?
The best-established role of this vitamin is in bone and mineral health.
It helps the body regulate calcium and phosphate. Without enough of it, calcium absorption can be affected, and bone health can suffer. Severe deficiency in children can cause rickets, while severe deficiency in adults can cause osteomalacia, which can cause bone pain and muscle weakness.
It also plays a role in normal muscle and immune function, but that should not be confused with the much broader claims sometimes made by supplement marketing.
There is not enough evidence to say that taking extra vitamin D will automatically prevent a wide range of unrelated diseases.
The sensible approach is to meet your nutritional needs rather than assume more is always better.
How Does Your Body Make Vitamin D From Sunlight?
Your skin can make it when it is exposed to ultraviolet B (UVB) radiation from sunlight.
In the UK, most people can make enough of this vitamin from sunlight during the sunnier months, broadly from late March or early April until the end of September. During autumn and winter, sunlight in the UK is not strong enough for most people to make sufficient vitamin D this way.
That seasonal difference is one reason UK guidance recommends considering a supplement during autumn and winter.
The amount you make is not simply determined by how long you sit in the sun.
Skin type, age, clothing, cloud cover, time of day and geographical location all influence how much UVB reaches your skin.
Does That Mean You Should Get More Sun?
Not necessarily.
Sunlight is important for its production, but more ultraviolet exposure also increases the risk of skin damage and skin cancer.
The goal is not to spend long periods deliberately exposing yourself to the sun without protection.
The NHS advises people to avoid sunburn and excessive sun exposure. Vitamin D should therefore be considered alongside sensible sun-safety measures, not as a reason to seek prolonged UV exposure.
In other words:
Do not trade one health problem for another.
How Much Vitamin D Do You Need?
UK guidance says adults and children aged 1 year and over need 10 micrograms (µg) of vitamin D a day. This is equivalent to 400 International Units (IU).
Babies under 1 year have different requirements.
The 10 microgram figure is a useful reference intake. It does not mean that everyone has exactly the same biological requirement, and it is not a recommendation to take very high doses.
Your needs can also differ in particular medical circumstances.
Which Foods Contain Vitamin D?
This vitamin is relatively unusual because naturally rich food sources are limited.
Useful sources include:
- oily fish such as salmon, sardines, mackerel and trout
- egg yolks
- red meat
- liver
- fortified breakfast cereals
- some fortified fat spreads
- some fortified plant-based products
The NHS notes that liver is a source of vitamin D but should be avoided during pregnancy.
In the UK, ordinary cow’s milk is generally not fortified with it, unlike milk in some other countries. That is an easy detail to miss when reading information from international websites.
Oily Fish Is One of the Best Food Sources
Oily fish is particularly useful because it can provide vitamin D alongside other nutrients, including long-chain omega-3 fatty acids.
Examples include:
- salmon
- sardines
- mackerel
- trout
- herring
A balanced diet can therefore support vitamin D intake without needing a supplement to replace nutritious foods.
Even so, food alone can be difficult to rely on for meeting your needs, particularly during the UK winter.
What About Mushrooms?
Some mushrooms contain it, and mushrooms exposed to ultraviolet light can contain more.
However, the amount varies considerably depending on the mushroom type and how it has been grown or treated.
They can therefore contribute to a diet, but it is better not to assume that ordinary mushrooms provide large amounts of it.
Should You Take a Vitamin D Supplement?
UK guidance recommends that adults and children over 4 years should consider taking a daily supplement containing 10 micrograms of vitamin D during autumn and winter because it is difficult to obtain enough from food alone during that period.
Some people are advised to take it year-round because they are less likely to get enough from sunlight.
The key point is that supplementation fills a likely gap.
It is not an invitation to take large doses because you assume that more must produce more health benefits.
Who Is More at Risk of Vitamin D Deficiency?
Some people are less likely to make enough vitamin D from sunlight.
According to NHS guidance, groups who should consider year-round supplementation include people who:
- are not often outdoors, such as people who are housebound or living in a care home
- usually wear clothing that covers most of their skin when outdoors
- have darker skin
Older adults are also at higher risk, partly because their skin becomes less efficient at producing it and many older people spend less time outdoors.
Vitamin D During Pregnancy and Breastfeeding
Pregnancy does not remove the need for vitamin D.
NHS guidance recommends 10 micrograms of vitamin D every day between early October and late March during pregnancy because the body cannot make enough from sunlight during this time.
Pregnant women should also be careful with vitamin A, and liver is not recommended during pregnancy.
Pregnancy is one reason nutritional advice should be based on the person’s circumstances rather than a generic “healthy diet” checklist.
Vitamin D and Babies
Babies have different nutritional needs from adults.
The NHS recommends supplementation for babies from birth to 1 year in specified circumstances, including breastfed babies, while babies who consume more than 500ml of infant formula a day generally do not need an additional supplement because formula is already fortified.
Parents should follow current NHS advice for infants rather than applying adult supplement recommendations to babies.
What Happens If You Do Not Get Enough Vitamin D?
A severe deficiency can affect bones and muscles.
In children, deficiency can cause rickets, which can affect bone development.
In adults, severe deficiency can lead to osteomalacia, which may cause bone pain and muscle weakness.
However, common symptoms such as tiredness or feeling generally unwell do not prove a deficiency.
Those symptoms have many possible causes.
That is why taking a large supplement simply because you feel tired is not a reliable way to diagnose or treat the problem.
Do You Need a Vitamin D Blood Test?
Not everyone needs one.
Testing is generally considered when a clinician suspects deficiency or when a particular medical circumstance makes testing appropriate.
Routine testing for the entire population is generally not recommended.
A healthcare professional can decide whether a blood test is appropriate based on your symptoms, risk factors and medical history.
Can You Take Too Much Vitamin D?
Yes.
It is fat-soluble, which means excessive supplementation can allow levels to build up.
Very high intakes can cause abnormally high calcium levels and may lead to symptoms such as nausea, vomiting, weakness, confusion, excessive thirst, frequent urination and kidney problems. Excessive toxicity usually comes from taking too many supplements rather than from sunlight.
That is why you should not treat high-dose vitamin D products as ordinary wellness products.
Take a supplement at an appropriate dose unless a healthcare professional has prescribed something different to treat a deficiency.
Is More Vitamin D Better?
No.
This is one of the most important messages to get right.
If someone is deficient, correcting the deficiency can be important.
But once intake is adequate, there is no reason to assume that repeatedly increasing the dose will continue to produce benefits.
Large doses can cause harm.
The NIH notes that the upper limits for daily vitamin D intake are set to reduce the risk of toxicity, although healthcare professionals may deliberately use higher doses for a limited period when treating a confirmed deficiency.
Does Vitamin D Prevent Colds?
You may see vitamin D marketed as a way to prevent colds, flu or other infections.
Research into it and respiratory infections is ongoing, but this should not be presented as a simple guarantee.
The NHS specifically states that there is not enough evidence to support taking vitamin D solely to prevent or treat COVID-19.
More generally, the safest message is that vitamin D supports normal body function, while supplementation should be based on nutritional needs and established guidance.
Does Vitamin D Help With Weight Loss?
It is not a weight-loss supplement.
Some studies have found associations between vitamin D status and body weight, but that does not mean taking extra causes weight loss.
Good evidence does not support using it as a shortcut for weight management.
For healthy weight management, overall diet, physical activity, sleep and sustainable habits matter much more.
What About Vitamin D and Bone Health?
This is one of the strongest reasons vitamin D matters.
It helps regulate calcium and phosphate, which are needed for healthy bones.
Getting enough vitamin D is therefore part of maintaining bone health.
But strong bones also require other factors, including adequate calcium intake, appropriate physical activity and, in some circumstances, medical treatment.
It is one part of the picture, not the whole picture.
For more information about calcium-rich foods, see Get Your Calcium From Natural Sources.
Is Vitamin D Important as You Get Older?
Yes.
Older adults can be more vulnerable to low vitamin D because their skin produces less vitamin D from sunlight, they may spend less time outdoors, and dietary intake may not always provide enough.
Maintaining adequate vitamin D supports bone and muscle health as we age.
However, older adults may also have medical conditions or take medications that affect their nutritional requirements.
That is another reason high-dose supplementation shouldn’t be started casually.
What About People With Darker Skin?
People with darker skin can make vitamin D from sunlight, but they generally need longer exposure to produce the same amount as people with lighter skin.
The NHS identifies people with darker skin as a group more likely to be at risk of vitamin D deficiency and recommends considering year-round supplementation of 10 micrograms a day.
This is a useful example of why vitamin D advice cannot be reduced to “just get outside”.
A Practical Vitamin D Strategy
For most adults in the UK, a sensible approach is straightforward.
Eat a varied diet that includes vitamin D sources such as oily fish, eggs and fortified foods.
Get sensible outdoor daylight exposure when appropriate, while avoiding sunburn and excessive UV exposure.
Consider a 10 microgram vitamin D supplement during autumn and winter.
If you are in a higher-risk group, consider whether year-round supplementation is appropriate according to current NHS guidance.
And do not take high doses unless there is a specific medical reason.
That is much more useful than obsessing over vitamin D as a miracle nutrient.
The Bigger Picture
Vitamin D is important.
Your body needs it for normal calcium and phosphate regulation, and it plays an important part in maintaining healthy bones, teeth and muscles.
But it is also a nutrient where internet advice can quickly become exaggerated.
You do not need to chase huge doses.
You do not need to deliberately spend hours in strong sunlight.
And you do not need to treat every episode of tiredness as proof of deficiency.
For most people in the UK, the practical approach is to eat a varied diet, get sensible outdoor exposure during the brighter months, and consider the recommended vitamin supplement during autumn and winter.
People at higher risk may need supplementation year-round.
That is not particularly glamorous advice.
It is simply useful advice.
For a broader explanation of how vitamins and minerals fit into a balanced diet, see Nutrition Terms Explained: RDA, DRI, EAR, AI and UL.
You can also read Special Nutritional Needs: When Your Diet May Need Extra Attention for situations where nutritional requirements need more careful planning.
Medical Disclaimer
This information is provided for general educational purposes only. It is not intended to diagnose, treat or prevent any medical condition and should not be considered a substitute for advice from a qualified healthcare professional.
People with symptoms, diagnosed deficiencies, medical conditions, pregnancy-related nutritional questions or concerns about supplements should seek appropriate individual advice from a doctor, pharmacist or registered dietitian.
Do not take high-dose supplements unless a healthcare professional specifically advises it.