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Why is vitamin D so important?

Megan Widdows
Written by

Megan Widdows

Medically reviewed by

Fiona Moncrieff

10 min read
Last updated June 2024

Vitamin D is one of the only vitamins or minerals that a healthy, balanced diet alone can’t provide. This guide explores the research into why vitamin D is so important and what can happen when we don’t get enough.

Wondering about the role of vitamin D and coronavirus? Check out our other research-backed guide here.

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What is vitamin D?

Vitamin D is one of our essential vitamins. These are vitamins that are required for our bodies to be able to function correctly. Vitamin D is a fat-soluble nutrient that’s important for bone health, immune health, increased cognition, and general wellbeing.

We need vitamin D to help absorb calcium and phosphate from our diet, two essential minerals for healthy bones, teeth, and muscles. Vitamin D also has many other vital roles in the body, including our immune system, which helps us fight off infections. Different roles include helping to maintain our metabolism and nervous system.

In food sources and supplements, it comes in two different forms – D2 (ergocalciferol) and D3 (cholecalciferol). Studies suggest that the vitamin D3 form is more effective at raising and maintaining our vitamin D levels.

Key points:

  • Vitamin D is an essential vitamin with many vital functions in the body
  • In foods and supplements, there are two different forms – D2 and D3
  • Our body more readily absorbs vitamin D3, so it’s more effective at raising our vitamin D levels.

Sources of vitamin D

Dietary sources of vitamin D

Few foods naturally contain vitamin D. However, the best food sources of vitamin D are fatty fish such as salmon, fresh tuna, trout, mackerel, and fish liver oils. Other foods that are relatively rich in vitamin D include beef liver, cheese, and egg yolks, although these foods have much smaller amounts. The only plant-based source of vitamin D is mushrooms, and these only provide the less well absorbed D2 form.

Fortified foods are where most of us get vitamin D in our diet. Vitamin D is added to certain widely consumed foods to help us reach the recommended daily amounts. The most commonly fortified foods are dairy and plant-based milk, some kinds of margarine, and breakfast cereals.

Vitamin D and Sunlight

Since it can be hard to find sources of vitamin D from our diet, our main source is Sunlight. For this reason, it’s often referred to as the ‘sunshine vitamin’. When our skin gets exposed to the Sun’s ultraviolet rays, a chain of reactions is triggered, which means our body can produce several helpful chemicals, including vitamin D.

Vitamin D can then move around the body. Since it’s a fat-soluble vitamin, it’s stored within our fat cells until our body is ready to use it.

In the UK, the Sun’s rays aren’t strong enough to trigger this reaction all year round. We can typically only access high enough levels of ultraviolet rays between 11 am and 3 pm from April to September.

Key points:

  • There are few dietary sources of vitamin D, so our main source is Sunlight
  • In the UK, the Sun is not strong enough for us to produce our own vitamin
  • D all year round, so we have a higher risk of deficiency.

What do we need vitamin D for?

Bone health

Vitamin D’s main job is to build and maintain a healthy skeleton by helping us absorb calcium from our food. It also makes sure we can keep the calcium and phosphorus levels in our blood at just the right amount. It’s especially important to maintain healthy vitamin D levels during childhood as it’s also needed for our bones to grow.

Immune system

Vitamin D is also involved in our immune system, with research suggesting it plays a so-called ‘management’ role helping to keep our responses to infection in check.

Our immune cells, such as our white blood cells, are essential in fighting off foreign pathogens. They also produce chemicals and proteins that help them defeat the invaders.
However, these chemicals can become harmful in excess and cause damage to our cells and health in the long-term.

Vitamin D seems to manage this by preventing the immune cells from releasing too many of these chemicals, ensuring there’s enough to support the fight against the infection, but not too many where they could potentially cause damage to our body’s cells or tissues.

This link between immunity and vitamin D has led to lots of speculation over what role it may play in the coronavirus pandemic. The research currently suggests that a deficiency in vitamin D may lead to poorer outcomes for people infected with COVID-19.

Other roles

Many cells in our body contain vitamin D receptors. Vitamin D is, therefore, involved with many of our body’s functions. Other functions include lowering our internal levels of inflammation, controlling cell growth rate, and adjusting how our nerves systems respond to different stimuli.

There’s evidence to show that Vitamin D also helps our body to produce and break down glucose which is essential to maintaining healthy blood sugar levels. It’s also thought to play a role in regulating our sleep patterns ( ), mood, and risk of developing cancer.

Key points:

  • Vitamin D is involved in many of our bodily functions, including bone and muscle health and our immune system
  • It ensures we can maintain healthy calcium and phosphorous levels in our blood and absorb calcium from food
  • Vitamin D is also involved in controlling how we respond to infections and making sure our responses are relative to the threat.

Vitamin D deficiency

The NHS considers a blood concentration of vitamin D lower than 30nmol/L to be deficient and levels between 30-80nmol/L insufficient.

At least 1 in 5 adults in the UK are deficient in vitamin D. More people are likely to fall into the insufficient category.

The primary source of vitamin D is Sunlight. Rates of deficiency increase as we travel away from the equator, where the Sun’s rays are the strongest.

Other factors that increase the risk of developing vitamin D deficiency include:

old age

  • regular use of sunscreen
  • darker skin
  • spending little time outdoors
  • wearing clothes that cover all or most of the body

Many of us with vitamin D deficiency have no symptoms. Some people develop non-specific issues, such as tiredness and aches, that can be challenging to pinpoint.

With severe deficiency, symptoms can include:

  • Bone and muscle pain
  • Fractures
  • Rickets or osteomalacia (weakening of bones)
  • Weak muscles
  • Bowing of arms and legs
  • Osteoporosis

Often there’s a lack of symptoms, so vitamin D deficiency is usually diagnosed through a blood test.

Key points:

  • Vitamin D deficiency is prevalent in the UK, with at least 1 in 5 adults thought to be affected
  • Factors that increase your risk of developing vitamin D deficiency include old age, regular use of suncream, darker skin, spending little time outdoors, and wearing clothes covering all or most of the body.
  • Symptoms are typically only present in people with severe deficiency and include fractures and weakening bones and muscles.

Dangers of vitamin D deficiency

Vitamin D deficiency can have profound health implications. Low levels have been linked to an increased risk of developing metabolic syndrome, high blood pressure, type 2 diabetes, Alzheimer’s disease, autoimmune diseases, and certain cancers.

High blood pressure

High blood pressure adds stress to our blood vessels, increasing our heart disease risk. Studies have demonstrated that living further from the equator (where the Sun’s rays are the strongest) increases our risk of both vitamin D deficiency and high blood pressure. It’s now thought that there could be a link between these two factors.

Similarly, another study found that, across a range of people, higher levels of vitamin D in the blood meant that those participants were less likely to have high blood pressure. It found that for every 10ng/ml increase in vitamin D, there was a 12% reduced risk of developing high blood pressure.

Mental health conditions

Research suggests that vitamin D, alongside omega-3 fatty acids, may also play a role in producing serotonin in our brains. Serotonin is an important chemical that’s often called the ‘happy hormone’ as it contributes to our wellbeing and happiness.

Therefore, a deficiency in vitamin D may reduce our ability to produce serotonin, leaving us more vulnerable to mental health conditions.

Researchers recently suggested that optimising our vitamin D and omega-3 levels could help to prevent and possibly reduce the severity of mental illnesses.

This is a relatively new research area and needs more study before we can fully understand the impact of vitamin D deficiency.

Sleep disturbance

Another area of research suggests that vitamin D may play a role in how well we sleep. An extensive review on the topic found that, overall, the evidence suggests that there’s a link between vitamin D deficiency and sleep disorders. Sleep disorders include lack of sleep, reduced sleep quality, and ‘sleepiness’.

A recent clinical trial demonstrated that supplementing adults who have a sleep disorder with vitamin D significantly increased their sleep quality.


Sun exposure is often thought to increase our risk of developing cancer, especially skin cancers. Research suggests that this is not the case. Regular non-burning sun exposure has actually been shown to reduce our risk of developing the most dangerous form of skin cancer, malignant melanoma.

Additionally, a meta-analysis of studies from more than 100 different countries found that lower levels of sun exposure were linked to higher rates of other types of cancer, including bladder, breast, and pancreatic cancer.

Links were found in this study between low levels of sun exposure and ten different types of cancer. However, researchers concluded that the strongest relationship was between too little sun exposure and colorectal and breast cancer specifically. Further research is needed to fully understand the relationship between sun exposure and cancers other than skin cancer.

We know that sun exposure is directly linked to our vitamin D levels, so it’s thought that the increased risk of cancer could be in part due to vitamin D deficiency.

However, observational studies like these often leave a lot of unanswered questions. There are usually other factors that we can’t control, which may impact the participants’ outcomes. For example, it may be that sun exposure has other unknown benefits that affect cancer risks. It’s also possible that people who spend lots of time outdoors, getting more sun exposure, are healthier than those who don’t, perhaps by exercising more and living a less sedentary lifestyle.

Similarly, another study found that supplementing vitamin D to post-menopausal women didn’t reduce their overall risk of cancer, suggesting there could be other factors involved. More research is needed so that we can fully understand the role of vitamin D deficiency and cancer.

Key points:

  • Vitamin D deficiency is linked to an increased risk of developing a number of conditions and illnesses
  • Vitamin D deficiency increases your risk of developing high blood pressure, though this can be alleviated through regular Sun exposure
  • Deficiency may also mean you are more susceptible to sleep disorders and mental illness
  • The link between vitamin D deficiency and various cancers is unclear and needs further research.

Should I take a vitamin D supplement?

With vitamin D deficiency so common, and the current restrictions on outdoor activities, it’s more important than ever to make sure we are getting enough vitamin D. Most people need a vitamin D supplement to achieve this.

There is some debate around how much vitamin D we need. The NHS currently recommends everyone takes a daily dose of 400 IU (international units). However, recent research suggests that nearer 1,000-2,000 IU is necessary to maintain healthy levels of vitamin D. Doses of up to 5,000 IU might be required if you are deficient. There’s little evidence to show that even high-doses of up to 10,000 IU are toxic, but few people need more than 4,000 IU per day.

Studies suggest that vitamin D3 form of vitamin D is more effective than D2 for raising and maintaining serum levels of vitamin D. So, if you buy supplements, look for the D3 type. Vitamin D is widely available online and from most pharmacies. If you’re unsure if this is appropriate or how much you need, please ask your GP for medical advice.

Key points:

  • Everyone should take a vitamin D supplement, especially during the winter
  • The NHS recommends 400 IU per day, but research suggests this may not be enough to maintain healthy levels
  • Doses as high as 10,000 IU per day aren’t thought to be toxic, but few people need more than 4,000 IU.

Take home message

  • Vitamin D is an essential nutrient that’s difficult to get from our diet alone, making Sunlight our primary source
  • Deficiency is widespread in the UK, with at least 1 in 5 adults thought to be affected
  • There aren’t many symptoms, especially for a mild to moderate deficiency, so it’s common to be deficient without realising
  • It’s important to make sure we are getting enough vitamin D as a vitamin D deficiency can have serious health implications, including increasing our risk of high blood pressure, sleep disorders, and mental illness.
  • The NHS recommends everyone take a daily vitamin D supplement of 400 IU, especially during the winter months where the Sun isn’t strong enough for us to produce our own vitamin D
  • More recent research suggests that a daily dose of 1,000-2,000 IU is necessary to maintain healthy levels of vitamin D, or even higher if you’re deficient.
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