Vitamin D is an essential fat-soluble vitamin that your body makes when exposed to sunlight. It’s also found in a small number of foods. You need vitamin D for healthy bones, muscles, and a strong immune system. Low levels are linked to a range of conditions like osteoporosis, heart disease, diabetes, cancer, and depression. If you’re not getting enough it’s important to know what could be causing this and what you can do to increase your levels.

What is vitamin D?

Vitamin D is an essential fat-soluble vitamin — a type of nutrient.

What does vitamin D do?

Vitamin D helps your body absorb calcium in your gut and maintains your calcium and phosphate levels. These are needed for healthy bones, teeth, and muscles. It also supports your immune system — helping you fight off infections. And is thought to help reduce inflammation in your body.

Vitamin D has been linked to health in many other ways. For example, low levels are linked to a range of conditions like osteoporosis, heart disease, diabetes, cancer, and depression. Maintaining vitamin D levels between 75-100 nmol/L is linked to optimal health.

How to measure your vitamin D levels

You can measure your vitamin D levels at home with a finger-prick blood test or you can go to your GP. This test will measure your levels of 25-hydroxy vitamin D and the reference ranges are:

  • 50-100 nmol/L is normal — around 75 nmol/L is considered optimal
  • 30-50 nmol/L is insufficient
  • less than 30 nmol/L is deficient

How your body makes vitamin D

Your body can make vitamin D when your skin is exposed to direct sunlight — which is why it’s often called the sunshine vitamin. The energy from the sun converts a vitamin D precursor to vitamin D3. Then your liver and kidneys convert it to its active form, called 1,25-dihydroxyvitamin D3 or calcitriol. And it’s this form of vitamin D that your body can use.

Food sources of vitamin D

Vitamin D is found in foods like oily fish, liver, egg yolks, and some fortified foods. While dietary sources of vitamin D can help increase your levels, it’s difficult to get enough from food.

What causes vitamin D deficiency?

While your body is able to make all the vitamin D you need, it needs direct sunlight for this to happen. So when sunlight exposure is low, like during autumn and winter, you might not make enough. Other factors like smog and sunscreen can also impact how much vitamin D your body makes.

Because of this, vitamin D deficiency is quite common. In the UK, around 1 in 5 people have low levels. Worldwide, 1 billion people are thought to be vitamin D deficient.

Risk factors for vitamin D deficiency

There are a number of other factors that can increase your risk of developing vitamin D deficiency, including if you:

  • are vegan or vegetarian
  • have darker skin — for example, if you’re African, African-Caribbean, or South Asian
  • are elderly
  • stay indoors a lot
  • cover up most of your skin outdoors

Signs of vitamin D deficiency

The most common signs of vitamin D deficiency include:

  • getting sick often
  • feeling tired
  • aching bones and joints
  • weak bones — increasing your risk of osteoporosis
  • poor wound healing
  • weak muscles
  • depression

How to increase vitamin D levels

During spring and summer, extra sun exposure can help to boost your vitamin D levels. But if you're out for long periods in the sun, it's important to cover up or wear sun protection to protect yourself from sun damage and skin cancer.

During autumn and winter, Public Health England advises that everyone should consider taking a 10 mcg daily vitamin D supplement from October to March. And if you’re more at-risk, they recommend taking them year-round. The recommended doses for at-risk groups include:

  • 8.5-10 mcg daily for breastfed babies from birth to 1 year
  • 10 mcg daily for children aged 1-4 years
  • 10 mcg daily for at-risk adults — for example, if you’re elderly or have darker skin

As mentioned, there are some foods that can increase your levels. It’s unlikely you’ll get enough this way during autumn and winter, but it can help.

References

National Health Services (2017). Health: A-Z: Vitamins and minerals: Vitamin D. Retrieved 27 January 2020 from https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/.

National Institue of Health (2018). Office of dietary supplements. Vitamin D: Fact sheet for health professionals. Retrieved 27 January 2020 from https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/.

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Aisling Moran BSc (Hons)

Written by Aisling Moran BSc (Hons)

26th Apr 2021 • 4 min read