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

What causes vitamin D deficiency?

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

Your body can 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, which is linked to depression.

You can check your vitamin D levels as part of a vitamins blood test. And you can prevent or treat a deficiency with our premium vitamin D supplements.

You can also get vitamin D from food, but most people don't get enough this way. Good dietary sources of vitamin D include:

  • oily fish
  • red meat
  • liver
  • egg yolks
  • fortified food  — like margarine and some breakfast cereals

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

  • don’t get enough vitamin D from your food — particularly in a vegan diet 
  • have darker skin
  • are over the age of 65
  • are obese
  • have a digestive disorder, like coeliac disease or Crohn’s disease
  • reduce your exposure to the sun, stay indoors often, or cover your skin when outdoors
  • are taking some types of medication, like antiepileptic drugs or steroids

In the UK, 40% of people aged 19-64 years old are estimated to be vitamin D deficient during the winter months.

Find out about the link between vitamin D and COVID-19.

Vitamin D deficiency symptoms

Most people with vitamin D deficiency don’t show any symptoms. When symptoms do occur, they can be vague and might include:

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

You can treat a deficiency with our vegan vitamin D supplements.

How to measure your vitamin D levels

You can measure your levels at home with a vitamins blood test

This test will measure your levels of 25-hydroxy vitamin D, and the reference ranges are:

  • 50-200 nmol/L is normal — around 75 nmol/L is considered optimal
  • 30-50 nmol/L is insufficient
  • less than 25 nmol/L is deficient

Certain inflammatory diseases might make it challenging to interpret your vitamin D levels. These include:

  • sarcoidosis — a rare condition that causes small patches of red and swollen tissue
  • a condition that affects your parathyroid glands — these regulate calcium levels in your body

In these cases, it's best to work with your doctor and get specialist input.

Find out about the link between vitamin D and muscle recovery.

How to prevent 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 essential 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 (400 IU) daily vitamin D supplement from October to March to prevent low vitamin D levels. And if you’re more at risk of having low levels, they recommend taking them year-round. 

The recommended doses include:

  • 8.5-10 mcg (340 to 400 IU) daily for breastfed babies from birth to age 1
  • 10 mcg (400 IU) daily for children from the age of 1
  • 10 mcg (400IU) daily for all adults, including pregnant and breastfeeding women 

If your levels are already low, taking up to 25 mcg (1,000 IU) a day is suitable for all adults.

Remember that some medical conditions mean that you should take lower doses. It’s best to follow the advice of your GP in these cases. 

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Craig, W. J. (2009). Health effects of vegan diets. The American journal of clinical nutrition, 89(5), 1627S-1633S.

National Health Services (2017). Health: A-Z: Vitamins and minerals: Vitamin D. Retrieved 4 November 2021 from

National Institute of Health (2018). Office of dietary supplements. Vitamin D: Fact sheet for health professionals. Retrieved 4 November 2021 from

Public Health England (2020). Statement from PHE and NICE on vitamin D supplementation during winter. Accessed 4 November 2021 from

Royal Osteoporosis Society (2019). Vitamin D and bone health: a practical clinical guideline for patient management. Accessed 4 November 2021 from

Dawson-Hughes, B. Vitamin D deficiency in adults: Definition, clinical manifestations, and treatment. In Mulder, JE (Ed.), UpToDate. Accessed 4 November 2021 from