Everything you need to know about vitamin D

How do you know if you're getting enough vitamin D? Look at where we get vitamin D from and its important role in the body.

Vitamin D is one of the most common deficiencies in the UK, with around one in five people having low levels [1]. We may have heard of vitamin D, but what really does it do, and how do we know if we have enough?

What is vitamin D?

Vitamin D is a fat-soluble vitamin produced in the skin in response to sunlight. It is often referred to as the sunshine vitamin and is important for maintaining healthy teeth, muscles, and bones as it helps the body absorb calcium.

The vitamin also blocks the release of the parathyroid hormone and helps strengthen the immune system.

How do we get vitamin D?

Although the body produces vitamin D naturally when exposed to sunlight, it can also be absorbed through certain foods and supplements.

During summer, we can get enough vitamin D from being outdoors. However, we can’t get adequate amounts during winter, leading to vitamin D deficiency.

The current advice from Public Health England is that adults, and children over the age of one, need 10 micrograms of vitamin D a day for healthy bones and muscles [2].

People at higher risk of vitamin D deficiency, such as babies, pregnant women, the elderly, and people with dark skin, should consider taking a vitamin D supplement all year round.

You can find out more about vitamin D deficiency and people at risk in the guide to vitamin D deficiency.

What does vitamin D do?

Vitamin D is necessary for calcium absorption. If the body isn’t getting enough, there is a risk of developing bone disorders where the bones become weak and deformed (osteomalacia in adults, and rickets in children) or porous and brittle (osteopenia and osteoporosis).

Vitamin D deficiency can also increase the risk of the body getting infections, as it affects the immune system.

Studies have linked vitamin D deficiency to several diseases including [3]:

  • Breast cancer
  • Colon cancer Prostate cancer
  • Heart disease
  • Depression
  • Obesity

The two primary dietary forms of vitamin D are:

  1. Vitamin D3 (cholecalciferol) – found in some animal foods such as fatty fish and egg yolks.
  2. Vitamin D2 (ergocalciferol) – found in some plants, mushrooms, and yeasts.

When vitamin D enters the body, it enters in a raw form that needs to undergo two steps to become the active hormone that the body uses to absorb calcium.

How vitamin D becomes active:

  1. In the first step, the liver converts vitamin D into calcidiol 25 (OH) D – the stored form of the vitamin.
  2. The kidneys then convert vitamin D into calcitriol, or 1,25 (OH) 2D, which is the active form of the vitamin. The active form then binds to the vitamin D receptor, found in almost every cell of the body, and changes those cells.


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Vitamin D toxicity

Although rare, it is also possible to have too much vitamin D.

Vitamin D toxicity is usually caused by taking substantial doses of vitamin D supplements, rather than diet or sun exposure. The body regulates the amount of vitamin D produced by sun exposure, and fortified foods don’t generally contain large enough amounts of vitamin D to be dangerous.

The main consequence of toxicity is a build-up of calcium in your blood (hypercalcemia), which can cause several symptoms.

Symptoms of hypercalcemia include:

  • Nausea
  • Vomiting
  • Bone weakness
  • Frequent urination

Vitamin D toxicity might also progress to bone pain and kidney problems, such as calcium stones.

Treatment for vitamin D toxicity includes stopping vitamin D intake from food and supplements and restricting dietary calcium. Medications may also be required.

If you are taking vitamin D supplements, be sure to read the label to check you aren’t taking too much – 10 micrograms a day should be enough to maintain proper levels.

Vitamin D and Covid-19

A recent Medichecks study found that nearly two-thirds of Brits tested during lockdown restrictions had good levels of vitamin D, a 10% improvement on the previous year.

The study examined the findings of over 11,000 people from the period March to June, in both 2019 and 2020.

Optimum levels in lockdown could be attributed to spending more time outside due to better work-life balance, being furloughed, not being preoccupied with closed services such as gyms, bars, and restaurants, and those taking early advice from Public Health England on supplementing the vitamin.

In December 2020, Public Health England (PHE), the Scientific Advisory Committee on Nutrition (SACN) and the National Institute for Health and Care Excellence (NICE) conducted a review evaluating evidence from recent studies on vitamin D in relation to COVID-19.

The joint guidance concludes that there is little evidence on vitamin D and COVID-19. This highlights the need for further research and supports existing government advice that adults and children in the UK should take 10 micrograms a day between October and March, to optimise musculoskeletal health.

How can I ensure my vitamin D levels are optimal?

The number of people with vitamin D deficiency in the UK remains high. Winter is a good time to check your levels of vitamin D and consider taking a supplement if you are deficient.

While studies are still investigating the role of vitamin D in the prevention of chronic disease, it does have significant benefits to bone health and immune function.

Get tested

Our Vitamin D (25 OH) Blood Test is a simple finger-prick test done from the comfort of your own home. It tests your vitamin D levels to see whether you have low levels that could be contributing to symptoms like low energy and muscle aches and pains.

With Medichecks, you can also monitor your levels over time to see how they improve with supplementation.


  1. Crowe, F., Jolly, K., MacArthur, C., Manaseki-Holland, S., Gittoes, N., Hewison, M., Scragg, R. and Nirantharakumar, K., 2019. Trends in the incidence of testing for vitamin D deficiency in primary care in the UK: a retrospective analysis of The Health Improvement Network (THIN), 2005–2015. BMJ Open, 9(6), p.e028355.
  2. GOV.UK. 2021. Vitamin D supplementation during winter: PHE and NICE statement. [online] Available at: <https://www.gov.uk/government/publications/vitamin-d-supplementation-during-winter-phe-and-nice-statement> [Accessed 22 December 2021].
  3. Wang, H., Chen, W., Li, D., Yin, X., Zhang, X., Olsen, N. and Zheng, S., 2017. Vitamin D and Chronic Diseases. Aging and disease, 8(3), p.346.
  4. Nair S. Vitamin d deficiency and liver disease. Gastroenterol Hepatol (N Y). 2010 Aug;6(8):491-3. PMID: 20978551; PMCID: PMC2950664.
  5. Marangoni, F., Cetin, I., Verduci, E., Canzone, G., Giovannini, M., Scollo, P., Corsello, G. and Poli, A., 2016. Maternal Diet and Nutrient Requirements in Pregnancy and Breastfeeding. An Italian Consensus Document. Nutrients, 8(10), p.629.

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