Vitamin D: the definitive guide

Vitamin D: the definitive guide

Author Miriam Ferrer, PhDLast updated 30th September 2019

  • Ingredients & Nutrition

Vitamin D is an essential micronutrient that helps you to absorb the calcium you need for healthy bones and plays a role in a whole variety of functions in your body. Yet although it is produced naturally by the skin in response to sunlight, one in five white British people in the UK and a far higher proportion of those from darker-skinned ethnic groups are deficient.

In this article we explain why this micronutrient is so important for your health and discover the best ways you can ensure that your body gets the vitamin D that it needs.

  • What is vitamin D
  • What does vitamin D do?
  • How to get Vitamin D
  • Why is vitamin D deficiency so common?
  • Who is at risk of vitamin D deficiency?
  • How can I increase my vitamin D intake?
  • Should I take a vitamin D supplement?
  • The best vitamin D supplements – what to look out for
  • Summary

What is vitamin D

Vitamins are micronutrients that are essential for life. In fact, we know that organisms that have remained unchanged for 500 million years produce vitamin D when exposed to sunlight1.

As with other vitamins, it was first identified by scientists who were searching for the cause of diseases like beriberi, rickets and scurvy.

These diseases were afflicting people who ate an adequate diet of macronutrients (such as carbohydrates and fats) but seemed to be lacking some unknown nutrient. Two teams of researchers independently discovered the first of the new micronutrients, vitamin A, between 1912 and 1914.

The existence of vitamin D was first suggested during the 1920s by Edward Mellanby, a lecturer at King's College for Women in London, who was researching the causes of rickets, a disease that had become increasingly common in the UK. Rickets causes children to have weakened bones and leads to distinctive symptoms including bowed legs, curvature of the spine and a susceptibility to bone fractures.

During the industrial revolution, people left the countryside for overcrowded cities with narrow streets, where the sun was often hidden behind clouds of smog and pollution. As early as 1822 a Polish physician had observed that children in such conditions in Warsaw were prone to developing rickets, whereas those who lived in the surrounding countryside were unaffected.

A hundred years later, researchers in England and Austria found that they could cure rickets using artificial UV light2. This helped to establish the connection between vitamin D and sunlight.

At the same time Mellanby proved that there was a dietary connection when he found that he could induce rickets in dogs by restricting their diet to porridge and keeping them indoors, and could then cure it by giving them cod liver oil.

In 1932, vitamin D was first isolated and described by a team led by F.A. Askew. Following these discoveries, cases of rickets became less and less frequent as supplements became more common and companies began to fortify foods like milk with the vitamin.

What does vitamin D do?

Today, thanks to further research, we know that vitamin D acts as a hormone in the body. It is responsible for regulating calcium and phosphate in the bloodstream to promote healthy bones3.

It is also vitally important in a whole range of other processes, playing a part in everything from disease prevention to the growth of tissue4,5.

In fact, vitamin D is so vital that every tissue in the body – from the brain and heart to the immune system – has Vitamin D receptors. Multiple studies have even linked low vitamin D levels with depression 6,7.

Symptoms of vitamin D deficiency may include:

  • Frequent illnesses and/or infections
  • Bone/back pain
  • Bone loss
  • Depression
  • Tiredness and fatigue

Children who don’t get enough of this essential vitamin are at risk of developing rickets, while adults can fall victim to osteomalacia, a condition that can cause softened bones and even bone pain.

How to get Vitamin D?

Your body generates roughly 90% of the vitamin D it needs from sunlight. Ideally, less than 10% will come from dietary sources1.

The amount produced by your body varies based on your age and skin colour 8. It is also affected by the season, time of day, and where you are in the world; in the UK the sun is generally not strong enough for your skin to synthesise needed levels of vitamin D from early April to the end of September. This makes dietary sources important in the winter months.

Most people can generate enough vitamin D during sunny months by regularly spending short periods out of doors with face, arms and legs uncovered. The best time of day for this is 9.If you have darker skin or the sunlight is weaker it is best to increase your exposure.

It has long been thought that wearing sun cream will reduce your body’s ability to produce vitamin D. However, sun cream is also extremely useful for protecting your skin and reducing the likelihood that you will develop skin cancer. Recent research suggests that sun cream might not significantly interfere with vitamin D production 10.

It is also possible that a tanning bed which generates UV radiation can increase vitamin D levels11. As with natural sunlight, there is a risk of developing skin problems from careless use.

Dietary sources of vitamin D include:

  • Oily fish like salmon, sardines, herring and even tinned tuna
  • Egg yolks
  • Mushrooms
  • Whole milk and yoghurt
  • Foods that have been fortified with added vitamin D (e.g. some breakfast cereals)
  • Nutritional supplements

Why is vitamin D deficiency so common?

According to UK government figures12, 21% of White British people are deficient. The percentage is higher for other ethnicities, with more than half of black people and nearly three quarters of Asian people affected. Deficiency has become common around the world as people adopt indoor lifestyles - even very sunny countries like Brazil have seen an increase 13.

There are several reasons for this:

  • From October to March the sun isn’t strong enough for the skin to produce Vitamin D in the UK.
  • Many people in industrialised countries are also spending a large proportion of their time indoors, away from direct sun exposure.
  • The ability to generate vitamin D declines with age, meaning that the over 50s are an at-risk group.
  • Topping up on vitamin D from your diet is difficult because our skin supplies such a large proportion of what we need, so dietary sources can’t compensate. It also only naturally occurs in relatively small amounts, in a few foods such as oily fish.

Who is at risk of vitamin D deficiency?

Anyone who doesn’t get enough time in the summer sun or eat the right foods can suffer from vitamin D deficiency. Nevertheless, it is more common among the following at-risk groups:

  • People who don’t spend a lot of time in direct sunlight. This includes those who lead indoor-oriented lifestyles, such as older people who find it difficult to get out and about, and people who choose to cover their skin when outdoors.
  • Older adults. The ability to create vitamin D from sunlight exposure decreases as with age.
  • People with dark skin (African, Afro-Caribbean and South Asian descent). Dark skin has a higher level of the pigment melanin in the epidermal layer, which means more protection from UV radiation but slower vitamin D production.
  • Obese people, as subcutaneous fat sequesters more of the vitamin and alters its release into circulation.
  • If you have a condition such as Crohn’s disease or cystic fibrosis you may struggle to absorb some fats. These conditions are associated with low vitamin D, although whether this is a cause or an effect is unknown14

If you are in an at-risk group, the Department of Health recommends that you take 10 micrograms (µg) of vitamin D per day as a supplement throughout the year15.

How can I increase my vitamin D intake?

Since most of the vitamin D your body needs is produced naturally in the skin, your best option is to spend more time outdoors, especially during the summer months.

Short periods outdoors with your face, arms and legs uncovered, especially between 11am and 3pm, should give your body enough UV to create sufficient amounts of vitamin D. You may need more sun exposure if you are part of a group that is at risk of deficiency, but ensure you have the right protection against overexposure if you have sensitive skin or are at risk of skin cancer.

You can also increase your consumption of foods containing vitamin D such as oily fish and egg yolks. Unfortunately, there are few other good dietary sources.

The other option is to take a vitamin D supplement either in the winter months or throughout the year, as recommended by the UK government.

Should I take a vitamin D supplement?

The short answer is probably ‘yes’.

Even if you live an active, outdoor lifestyle, your body still needs a little help to get enough Vitamin D, especially in the British winter months; that’s why the NHS now recommends that everyone takes a daily supplement between October and early March and that people in at-risk groups continue to take it throughout the year16.

A daily vitamin D tablet containing 10 micrograms (µg) should be enough for most people. Adults should not take more than 100 µg per day in supplement form, while children from 1-10 years old should not exceed 50 µg and infants should not be given more than 25 µg.

It is best to ensure that you are taking a high-quality product that allows for easy absorption of vitamin D, as a supplement will only be effective if your body can take in enough of the active ingredient.

The best vitamin D supplements - what to look out for

When choosing a vitamin D supplement, you should make sure that it is vitamin D3 rather than vitamin D2.

D3 is the form of vitamin D that is produced naturally by your skin in sunlight. Taking D3 is believed to be far more effective at raising levels in your bloodstream16.

That said, both forms do help to meet your body’s needs for vitamin D. D3 has historically been produced from animal sources, so if you are vegetarian or vegan you should be careful to check that your supplement is derived from lichen and therefore suitable for your dietary needs.

Another important factor is that vitamin D is fat soluble, so your body will absorb it more effectively when it is taken with oil. A vitamin D tablet that contains sunflower oil, for example, may increase absorption.


Vitamin D is a vital micronutrient that is necessary for life. It helps to keep bones strong and plays an important role in everything from the immune system to tissue formation.

Most of us can get the vitamin D we need from sunlight during warmer months, but it is difficult to get enough from diet in the winter months. For that reason the NHS recommends taking a supplement from October to early March, and people from at-risk groups should take one all year round.


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