Why do we need vitamin D?

Vitamin D supports your muscle health, immune function, mental health, and healthy bones and teeth. So how do you get it and what happens if you don’t get enough?

Vitamin D is actually a hormone that your body produces in response to sunlight rather than a vitamin. You can also get vitamin D from food, although only around 10% of your vitamin D will be absorbed this way.  

There are several forms of vitamin D. Vitamin D3, for example, is the form we get from sun exposure and animal products, whereas vitamin D2 is in plant-based and fortified foods. Vitamin 25-OH is the active form of vitamin D which circulates in the body, which is why it’s the most accurate indicator of your vitamin D levels. 

In this blog, we’ll go through exactly why we need vitamin D and what happens if you’re not getting enough of the sunshine vitamin. If you’ve already taken a vitamin D test and are looking to increase your vitamin D levels, head over to our top tips to increase your levels.  

Why do we need vitamin D?

We need vitamin D to maintain healthy teeth, muscles, and bones. It helps our bodies absorb other nutrients – mainly increasing the amount of calcium and phosphate the gut can absorb from food into the bloodstream. At the same time, it prevents calcium from being lost from the kidneys, which is why a vitamin D deficiency can impact your bone health.  

A lack of vitamin D has also been linked to a weakened immune system, low mood, and some lifestyle-related diseases like diabetes. It is part of a feedback loop with calcium, phosphate, and calcitriol [1]. If there’s a drop in the level of calcium in your bloodstream, the parathyroid glands will start to produce parathyroid hormone, which increases the enzyme that produces active vitamin D.  

How do we get vitamin D?

Our main source of vitamin D usually comes from exposing our skin to sunlight, which is why vitamin D is known as the sunshine vitamin. This process becomes more difficult when sunlight is limited in winter, or if you spend most of your time indoors or covered up. Your secondary source of vitamin D comes from certain foods and supplements.  

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. So, the UK government advises everyone to take a daily vitamin D supplement during autumn and winter. If you have a higher risk of vitamin D deficiency, you should consider taking a vitamin D supplement all year round. 

     Why do we need vitamin D? Exercise outside

People who are at a higher risk of vitamin D deficiency include: 

  • Babies 
  • Pregnant women 
  • The elderly 
  • People with dark skin 

You can read more about sources of vitamin D in our blog on how to increase your vitamin D levels.  

What happens if I don't get enough vitamin D?

Vitamin D is necessary for your body to absorb calcium. If you aren’t getting enough, you are at greater risk of disorders where your 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 infections as it affects the immune system. 

Studies have linked vitamin D deficiency to several diseases, including [2]: 

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

Seven signs of a vitamin D deficiency

The symptoms of low vitamin D can be subtle, so you may not notice that you are deficient.  

Seven common signs of a vitamin D deficiency include: 

  • Brain fog 
  • Hair loss 
  • Irritability 
  • Joint and bone pain 
  • Low mood 
  • Tiredness 
  • Weight gain 


Long-term, a vitamin D deficiency can affect: 

  • Bone health - without vitamin D, the body will only absorb 10-15% of calcium and 60% of phosphate from food. Calcium phosphate triggers the parathyroid hormone (PTH), which keeps bones strong and healthy. If your body has low vitamin D, PTH will increase bone breakdown to release more calcium and phosphate, causing a decrease in bone mineral density (BMD). This can lead to osteopenia and osteoporosis, placing a person at a higher risk of fractures. A severe lack of calcium and phosphate in the bones can lead to weakness and bone deformity (rickets in children and osteocalcin in adults). 
  • Immune system – low vitamin D is associated with increased autoimmunity and susceptibility to infection. When you get a viral infection, such as a common cold, vitamin D helps to create T cells. The T cells kill the infected host cells and help to fight the infection. One study suggests supplementing with vitamin D has significant benefits for the immune system [3]. 

Research is ongoing, but low vitamin D levels are associated with increased risk of many chronic diseases, including: 

  • Autism in children [1] 
  • Cardiovascular disease 
  • Cancer 
  • Depression 
  • Diabetes 
  • Obesity 
  • Pregnancy complications 
  • Severe asthma 


Vitamin D could play a role in the prevention and treatment of conditions including: 

  • Diabetes [5] 
  • Glucose intolerance [6] 
  • Multiple sclerosis [7] 


How is vitamin D deficiency diagnosed?

Diagnosing vitamin D deficiency usually starts with a blood test to measure how much 25-hydroxy-cholecalciferol (25 (OH) D) is in your blood. Testing for 25 (OH) D is the most accurate way to detect how much vitamin D is available in your blood because it is the primary circulating form of vitamin D. However, your results may not be reliable with this test if you have chronic kidney disease (in which case, you should visit your GP).  

Approximately 20% of the population in the UK have a vitamin D deficiency and almost one billion people worldwide have low levels of the sunshine vitamin.


Six causes of low vitamin D

1. Not spending enough time outdoors

There’s a reason we all love to talk about the weather in the UK and that’s because it’s fairly unpredictable (unless you’re predicting a chance of rain). The UK winter sunlight doesn’t provide enough ultraviolet B (UVB) for the body to create all the vitamin D it needs. 

One of the most common causes of vitamin D deficiency in the UK is inadequate exposure to sunlight. Most people can make enough vitamin D from being out in the sun for short periods, with uncovered forearms or lower legs, from April to the end of September [8], but this can all change during autumn and winter.  

     Why do we need vitamin D? Exposure to sun

2. Being over 65 or under six months old 

People over 65 have increased rates of vitamin D deficiency [9]. Scientists think that high levels of vitamin D deficiency in this age group could be caused by the body’s decreasing ability to absorb, synthesise, and convert vitamin D into its active form with age. Breastfed babies under six months are also at a higher risk of vitamin D deficiency as human milk may not solely provide enough vitamin D [10], which is why a supplement or drops are recommended for the first six months of breastfeeding. 

3. Having a darker skin tone 

People with darker skin, including people of African, African-Caribbean, or South Asian origin, have higher levels of melanin (a type of pigment that gives your skin, hair, and eyes their colour). This lowers the skin’s ability to make vitamin D in response to sunlight. As a result, people with darker skin may need to spend longer in the sun to produce the same amount of vitamin D as people with lighter skin. More time in the sun is not always the healthiest option though. Up to nine in ten cases of melanoma skin cancer could be prevented by enjoying the sun safely. Cancer Research UK explains how to protect your skin in the sun. 

4. Certain medical conditions 

With coeliac and Crohn’s disease, damage to the lining of the small intestine can affect the ability of the small intestine to absorb nutrients from food, including vitamin D. In kidney disease, the kidneys are less able to convert vitamin D into the active form that the body uses, resulting in low levels in the body. The liver works alongside the kidneys to convert vitamin D, which is why liver disease can also affect the absorption and synthesis of vitamin D [11]. Finally, some medications (such as anti-epileptic drugs and weight-loss drugs like Orlistat) can affect how the body processes vitamin D. 

5. Lifestyle factors  

Factors like your weight, whether you smoke, and if you are pregnant can increase your risk of a vitamin D deficiency. If you are obese, then you may be at an increased risk of a vitamin D deficiency. Scientists believe that vitamin D may become trapped inside fat tissue, making it less able to circulate in the blood throughout the body [12]. Research shows that smoking can affect lung function and lessen the levels of vitamin D in the body [13]. As the risk of a deficiency increases during pregnancy, the NHS recommends taking a daily vitamin D supplement [14] to ensure that you are getting your recommended daily intake. 

6. A lack of vitamin D in your diet 

A diet lacking in foods that contain vitamin D (like oily fish, egg yolks, red meat, and fortified cereals) can contribute to low vitamin D levels. While you cannot rely solely on food to meet your vitamin D requirements, it can help to top up your levels.  

     Why do we need vitamin D? Vitamin D diet

What happens if I get too much vitamin D?

Although rare, it is also possible to have too much vitamin D. Vitamin D toxicity is usually caused by taking too many vitamin D supplements over a long period rather than from your 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 vitamin D toxicity is a build-up of calcium in your blood (hypercalcemia), which can cause several symptoms. Vitamin D toxicity might also progress to bone pain and kidney problems like calcium stones. 

Symptoms of hypercalcemia include: 

  • Nausea 
  • Vomiting 
  • Bone weakness 
  • Frequent urination 

Treatment for vitamin D toxicity includes stopping vitamin D intake from food and supplements and restricting dietary calcium. In some cases, you may also need medication. If you are taking vitamin D supplements, always read the label to check you aren’t taking too much – 10 micrograms a day should be enough to maintain optimum levels. 

How can I make sure I get the right amount of vitamin D? 

The number of people with vitamin D deficiency in the UK remains high. You are more likely to become deficient during autumn and winter when there is less natural sunlight. Therefore, winter is a good time to check your levels of vitamin D and consider taking a supplement if you are deficient. Head over to our other blog for more ways on how to increase your vitamin D levels


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Vitamin D blood testing in the UK

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



  1. Vitamin D (2021) You and Your Hormones. Available at: https://www.yourhormones.info/hormones/vitamin-d/ (Accessed: 28 December 2023).
  2. Wang, H., Chen, W., Li, D., Yin, X., Zhang, X., Olsen, N. and Zheng, S.G. (2017). Vitamin D and Chronic DiseasesAging and disease, [online] 8(3), p.346. doi:https://doi.org/10.14336/ad.2016.1021.
  3. Aranow C. Vitamin D and the immune system. J Investig Med. 2011 Aug;59(6):881-6. doi: 10.2310/JIM.0b013e31821b8755. PMID: 21527855; PMCID: PMC3166406.
  4. Aljabri KS, Bokhari SA, Khan MJ. Glycemic changes after vitamin D supplementation in patients with type 1 diabetes mellitus and vitamin D deficiency. Ann Saudi Med. 2010 Nov-Dec;30(6):454-8. doi: 10.4103/0256-4947.72265. PMID: 21060157; PMCID: PMC2994161.
  5. H P, O C, DA U, O G, Ng D. The impact of Vitamin D Replacement on Glucose Metabolism. Pak J Med Sci. 2013 Nov;29(6):1311-4. doi: 10.12669/pjms.296.3891. PMID: 24550943; PMCID: PMC3905396.
  6. Sintzel MB, Rametta M, Reder AT. Vitamin D and Multiple Sclerosis: A Comprehensive Review. Neurol Ther. 2018 Jun;7(1):59-85. doi: 10.1007/s40120-017-0086-4. Epub 2017 Dec 14. PMID: 29243029; PMCID: PMC5990512.
  7. NHS (2020). Vitamin D - Vitamins and Minerals. [online] NHS. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/.
  8. Boucher BJ. The problems of vitamin d insufficiency in older people. Aging Dis. 2012 Aug;3(4):313-29. Epub 2012 Jun 6. PMID: 23185713; PMCID: PMC3501367.
  9. Balasubramanian S. Vitamin D deficiency in breastfed infants & the need for routine vitamin D supplementation. Indian J Med Res. 2011 Mar;133(3):250-2. PMID: 21441676; PMCID: PMC3103147.
  10. Marangoni F, Cetin I, Verduci E, Canzone G, Giovannini M, Scollo P, Corsello G, Poli A. Maternal Diet and Nutrient Requirements in Pregnancy and Breastfeeding. An Italian Consensus Document. Nutrients. 2016 Oct 14;8(10):629. doi: 10.3390/nu8100629. PMID: 27754423; PMCID: PMC5084016.
  11. Vimaleswaran KS, Berry DJ, Lu C, Tikkanen E, Pilz S, Hiraki LT, et al. (2013) Causal Relationship between Obesity and Vitamin D Status: Bi-Directional Mendelian Randomization Analysis of Multiple Cohorts. PLoS Med 10(2): e1001383. https://doi.org/10.1371/journal.pmed.1001383.
  12. Lange NE, Sparrow D, Vokonas P, Litonjua AA. Vitamin D deficiency, smoking, and lung function in the Normative Aging Study. Am J Respir Crit Care Med. 2012 Oct 1;186(7):616-21. doi: 10.1164/rccm.201110-1868OC. Epub 2012 Jul 19. PMID: 22822023; PMCID: PMC3480523.
  13. NHS (2020). Vitamins, minerals and supplements in pregnancy. [online] nhs.uk. Available at: https://www.nhs.uk/pregnancy/keeping-well/vitamins-supplements-and-nutrition/.

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Vitamin D (25 OH) Blood Test

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