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A snowy winter woodland. Vitamin D deficiency is more common in winter.

What causes low vitamin D levels?

Vitamin D, nicknamed the “sunshine vitamin” because it’s produced naturally in the body when you expose your skin to sunlight, is a vital nutrient that supports your immune system as well as healthy bones, muscles and teeth.¹ 

Because of this, many people assume it’s impossible or at least unlikely for a person to get a vitamin D deficiency – but in fact, it’s widespread in many parts of the world, and particularly in those regions where sunlight is rare or weakened.²

Unlike vitamin C deficiency, which is not very prevalent today, vitamin D deficiency affects millions of people around the world. Understanding the causes of low vitamin D levels can help you make the changes needed to avoid deficiency and support your health and wellbeing.

What could cause a vitamin D deficiency?

There is no single cause of low vitamin D levels. Instead, several factors may increase the risk of deficiency. These include dietary habits, lifestyle choices, age and certain health conditions. Below, we’ll explore the most common causes.

Not enough vitamin D in the diet

One reason some people experience low vitamin D levels is that their diet does not provide enough of it. Naturally, very few foods are rich in vitamin D. Fatty fish such as salmon, mackerel and sardines contain some vitamin D, as do eggs and fortified products like certain cereals and plant-based milks.³

For individuals following a strict vegan diet or those who avoid fortified foods, intake can be particularly low. Since diet alone rarely supplies sufficient vitamin D, many people rely on sunlight or supplements to maintain adequate levels.⁴

Lack of sun exposure

For most people, sunlight is the main way the body gets vitamin D. When UVB rays hit your skin, your body starts producing vitamin D. But not everyone gets enough sunlight to meet their needs, and there are several reasons why.

  • Seasonal changes: In the UK and other similar climates, sunlight is weaker during autumn and winter. This means your skin may not get enough UVB rays to make the vitamin D your body needs. Even a sunny day might not be enough if it’s short and cold.⁵
  • Skin tone: Melanin, the pigment that gives skin its color, can reduce vitamin D production. People with darker skin need more time in the sun to make the same amount of vitamin D as those with lighter skin.⁶
  • Lifestyle: How you spend your day makes a big difference. People who spend most of their time indoors, such as office workers or older adults in care homes, may not get enough sun on their skin, even in summer.⁷
  • Where you live: If you live far north or spend winters in northern countries, the sun’s rays may not be strong enough for vitamin D production. Even when on holiday in places like Scandinavia during winter, it can be tricky to get enough from sunlight alone.
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This explains why vitamin D supplements are frequently advised during the winter.⁸

Age

Getting older can affect your body’s ability to make vitamin D. As we age, the skin becomes less efficient at producing vitamin D when exposed to sunlight.⁹ On top of that, older adults often spend more time indoors, may have mobility issues, or face dietary limitations, all of which can make it harder to get enough vitamin D.

Low vitamin D levels in older adults can have real consequences. This nutrient is key for keeping bones strong and muscles working properly. A deficiency can increase the risk of falls and fractures, which are already more common in later life.¹⁰ It can also affect energy, balance, and even overall mobility, making everyday activities like walking, climbing stairs, or carrying groceries more challenging. By understanding these risks, older adults and their caregivers can take steps such as safe sun exposure, vitamin D-rich foods, or supplements (under medical guidance) to help maintain strength and reduce health risks.

Underlying health conditions

Certain medical conditions can interfere with the body’s ability to absorb or process vitamin D. These include:

  • Kidney disease, which affects the conversion of vitamin D into its active form.
  • Liver disease, which plays a role in metabolising vitamin D.
  • Digestive disorders such as Crohn’s disease, coeliac disease or cystic fibrosis, which impair nutrient absorption.¹¹

People taking certain medications, such as some anticonvulsants and glucocorticoids, may also experience reduced vitamin D levels due to changes in metabolism.¹²

Vitamin D deficiency is far more common than many people realise, particularly in regions with limited sunlight during parts of the year. Factors such as diet, lifestyle, age and health conditions all play a role in determining vitamin D levels. Being aware of these causes is the first step in taking action – whether through diet, safe sun exposure or supplementation – to support your health.

For those concerned about their intake, supplements can be an effective way to ensure adequate levels throughout the year.

References

  1. National Health Service (NHS). (2023). Vitamins and minerals – Vitamin D. Retrieved from https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/
  2. Scientific Advisory Committee on Nutrition (SACN). (2016). Vitamin D and Health. Public Health England.
  3. U.S. Department of Agriculture (USDA). (2022). FoodData Central. Retrieved from https://fdc.nal.usda.gov/
  4. National Institutes of Health (NIH). (2022). Vitamin D Fact Sheet for Health Professionals.
  5. Lanham-New, S. A., et al. (2020). Importance of vitamin D in the prevention and treatment of osteoporosis. Proceedings of the Nutrition Society, 79(2), 115–127.
  6. Holick, M. F. (2007). Vitamin D deficiency. New England Journal of Medicine, 357(3), 266–281.
  7. Martineau, A. R., & Forouhi, N. G. (2014). Vitamin D for health: A review of the evidence. BMJ, 348, g2035.
  8. NICE. (2022). Vitamin D: Supplement use in specific population groups. Retrieved from https://www.nice.org.uk/
  9. MacLaughlin, J., & Holick, M. F. (1985). Aging decreases the capacity of human skin to produce vitamin D3. Journal of Clinical Investigation, 76(4), 1536–1538.
  10. Bischoff-Ferrari, H. A., et al. (2005). Effect of Vitamin D on falls: A meta-analysis. JAMA, 293(18), 2257–2264.
  11. Deluca, H. F. (2004). Overview of general physiologic features and functions of vitamin D. American Journal of Clinical Nutrition, 80(6), 1689S–1696S.
  12. Mazess, R. B., & Barden, H. S. (1991). Bone density in chronic anticonvulsant therapy. American Journal of Clinical Nutrition, 53(5), 1181–1189.