Vitamin D is an essential fat-soluble vitamin that’s needed for healthy bones, muscles, and a strong immune system. Low levels of vitamin D are linked to a range of conditions like osteoporosis, heart disease, diabetes, cancer, and depression. If you lack vitamin D it’s important to know what could be causing this and what you can do to increase your levels.


What causes low vitamin D levels?

The main cause of low vitamin D levels is a lack of exposure to sunlight.

Your body is able to make all the vitamin D you need. But this can only happen when your skin is exposed to sunlight. So during autumn and winter, when sunlight exposure is at an all-time low, it’s really common for your vitamin D levels to drop — putting you at risk of developing a deficiency.

You can also get vitamin D from food. Good dietary sources of vitamin D include oily fish, liver, egg yolks, and fortified food but most people don't get enough this way.

Apart from lack of sunshine, there are other probable causes of low vitamin D. You might also be at an increased risk of developing vitamin D deficiency if you:

  • are vegan or vegetarian
  • have darker skin
  • are elderly
  • always wear sunscreen
  • stay indoors a lot
  • cover up most of your skin outdoors

In the UK, 40% of people are estimated to be vitamin D deficient during winter.

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

If you have sarcoidosis (an inflammatory disease) or a condition that affects your parathyroid glands (these regulate calcium in your body), it can make it difficult to interpret your vitamin D results. So in this case, it's best to work with your doctor and get specialist input.

Vitamin D finger-prick blood test kit Thriva

Signs and symptoms of vitamin D deficiency

The most common signs of low vitamin D 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 treat vitamin D deficiency

During spring and summer, extra sun exposure can help to boost your vitamin D levels. 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

If you'd like to find out your levels of vitamin D and whether you need a supplement, test your vitamin D levels.



National Health Services (2017). Health: A-Z: Vitamins and minerals: Vitamin D. Retrieved 25 February 2019 from

National Institue of Health (2018). Office of dietary supplements. Vitamin D: Fact sheet for health professionals. Retrieved 9 January 2019 from

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Aisling Moran BSc (Hons)

Written by Aisling Moran BSc (Hons)

10th Aug 2021 • 3 min read