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Vitamin D nutrient guide: deficiency, foods and supplements

Dr Sarah Brewer explores the benefits of vitamin D, what happens if you're deficient, how to get vitamin D from your diet and what supplements are available.

What does vitamin D do?

Vitamin D is best known for its essential role in the absorption of calcium to maintain strong bones and teeth, but it has many other important actions, including supporting immunity. Vitamin D:

  • Contributes to the normal function of the immune system in adults and children
  • Helps with calcium absorption, contributes to the maintenance of bones and teeth
  • Contributes to normal muscle function
  • Is needed for growth and development of bones in children

Vitamin D deficiency in the UK

Although you can make vitamin D in your skin on exposure to sunlight, this is usually insufficient to meet your needs. In fact, you can only make vitamin D when the UV index is greater than 3 – as a rule, if your shadow is taller than you are, you're not producing enough vitamin D.

As a result, vitamin D deficiency becomes common during autumn and winter for people living in northern latitudes such as the UK. Even when the sun is shining, many people also fail to make enough vitamin D because they, sensibly, avoid the sun, cover up or use high-SPF sunscreens. This problem is compounded during times of social distancing, as more of us stay inside for the majority of the day.

Vitamin D foods

Because of this, it's important to get vitamin D from your diet. Vitamin D can be found in useful amounts in:

  • Oily fish
  • Liver products
  • Eggs
  • Butter
  • Food fortified with vitamin D

This can make it difficult for those following certain dietary regimes (particularly vegetarians and vegans) to get enough, or even those without particular dietary requirements during the colder months.

For this reason, the National Institute for Health and Care Excellence (NICE) recommends that all adults in the UK take a daily supplement containing 400IU (10mcg) of vitamin D from October to March.

Overhead view of vitamin D-rich foods

Vitamin D can be found in oily fish, liver, eggs and butter, so if you don't eat a lot of those foods, consider a supplement.

Vitamin D benefits and deficiency symptoms

Vitamin D supports many functions in the body, and deficiency can cause problems in anything from immunity to bones and muscles.


  • Needed for the production of natural defences in the body
  • Sufficient vitamin D levels help protect against colds, flu and pneumonia

Vitamin D helps to protect against a number of infections by increasing the production of proteins that are needed for the body's natural antimicrobial defences.1

Studies involving over 19,000 adults found that those with the lowest levels of vitamin D were 36% more likely to develop a common cold than those with high levels, for example.2

The results from 25 clinical trials involving around 11,000 patients from 14 countries show that taking vitamin D supplements reduced the risk of acute respiratory infections such as cold, influenza and pneumonia by 12%, with those who were initially lacking in vitamin D gaining the most benefit.

Because asthma is often triggered by a respiratory infection, taking vitamin D supplements can help to reduce the risk of a severe asthma attack that needs emergency treatment by as much as 61%.

In pregnant women, low vitamin D levels are also associated with bacterial vaginosis (BV), which increases the risk of miscarriage and premature labour. As a result, pregnant women are advised to take a vitamin D supplement.3

Vitamin D deficiency and coronavirus

It has been suggested that one reason why a disproportionate number of people from BAME communities have experienced serious COVID-19 infections may relate to vitamin D status. This is because a higher level of melanin pigment in the skin reduces the ability to synthesise vitamin D on exposure to sunlight.

NICE recently carried out a 'rapid review' of five published studies to see if vitamin D might offer benefits against the COVID-19 pandemic. Unfortunately, none of these studies were actual intervention trials of vitamin D supplements for preventing or treating COVID-19, which would have provided the strongest evidence.

Despite this, four of the studies did find an association, with those having a lower vitamin D status having an increased risk of subsequently developing COVID-19.

However, because four of the five studies did not adjust for other confounding factors such as obesity and underlying health conditions, NICE concluded that, although there are health benefits associated with vitamin D, there is insufficient evidence to confirm that vitamin D might prevent or treat COVID-19.


  • Needed for the absorption of calcium and phosphorous, essential for strengthening bones
  • Deficiency can lead to a lack of bone mineral, particularly in post-menopausal women

Vitamin D is needed for the absorption of calcium and phosphorus, which are required in both adults and children to strengthen bones.

Lack of vitamin D is therefore a risk factor for loss of bone mineral, especially in post-menopausal women, and is therefore also a risk factor for bone fractures resulting from osteoporosis.

The results from 12 studies involving over 42,000 adults show that vitamin D supplements reduced the risk of hip fractures by 9% and other non-vertebral fractures by 14%, with higher doses of over 10 mcg (400 IU) vitamin D per day reducing the risk of non-vertebral bone fractures by at least 20%.4


  • Has beneficial effects on muscle fibres
  • Can reduce the risk of falling due to muscle weakness

Vitamin D also helps to maintain muscles, which in turn helps to reduce the risk of falling due to muscle weakness.

The results from 10 studies involving older adults show that taking a daily vitamin D supplement can reduce falls by 14% compared with placebo.5 This is another reason why vitamin D helps to reduce the risk of bone fractures among men and women aged 60 and older.

Vitamin D3 versus vitamin D2

Vitamin D3 (cholecalciferol) is the form most important for human health, and is the best form to take as a supplement.

The plant form vitamin D2 (ergocalciferol) has a slightly different structure, which makes it less beneficial for maintaining our vitamin D levels.6

Supplements can be taken as tablets, capsules, gummies or in the form of an oral spray. Vegan vitamin D3 is available to ensure you're getting the most body-ready form of the vitamin.

As a rule, if your shadow is taller than you are, you're not producing enough vitamin D

How much vitamin D do you need per day?

The EU nutrient reference value (NRV) for vitamin D is just 5 mcg (200 IU) per day. This is widely regarded as too low for the UK climate, and NICE recommends that everyone takes 10 mcg per day during the cold months of the year, or during spring and summer too if you're spending a lot of time inside.

Your ability to make vitamin D in the skin falls as you grow older, and at least halves, or more than halves, between the ages of 20 to 80 years.

In one study, researchers found that people aged 62 to 80 years synthesised four times less natural vitamin D than those aged 20 to 30 years.7

It's therefore a good idea to take a supplement designed for your age. Healthspan's Medical Director Dr. Sarah Brewer recommends 25 mcg for adults aged up to 50 years, and 50 mcg per day thereafter.

Can you take too much vitamin D?

Excessive vitamin D intake can cause side effects due to disturbances in calcium metabolism, such as headache, loss of appetite, nausea, vomiting, diarrhoea or constipation, palpitations and fatigue.

An upper safe intake level of 100 mcg per day (4000 IU) is therefore recommended by the EU.8

Dr Sarah Brewer is Healthspan's Medical Director and holds degrees in Natural Sciences, Surgery and Medicine from the University of Cambridge. Having worked as a GP and hospital doctor, Dr Sarah now holds an MSc in Nutritional Medicine from the University of Surrey and specialises in nutrition. She is also an award-winning writer and author.

Find out more at Dr. Sarah Brewer's website, or read more about Healthspan's health experts.

Nothing beats a healthy, balanced diet to provide all the nutrients we need. But when this isn't possible, supplements can help. This article isn't intended to replace medical advice. Please consult your healthcare professional before trying supplements or herbal medicines.

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