Vitamin D (25 OH)

July 21, 2020

What is vitamin D?

Vitamin D is needed by the body to facilitate the uptake of calcium from the diet. It is also needed for the regulation of phosphate from food, too. Therefore, it has a key role in maintaining bone health as well as the proper functioning of our immune system, muscle function, energy levels and helps to reduce inflammation.[1]

Our 2024 Vitamin D Statistics found that 61% of UK adults had vitamin D levels below the optimal range.

What role does vitamin D play in the body?

Vitamin D is needed by the body to regulate the amount of calcium and phosphate in the body. Both nutrients are essential for bone, tooth and muscle health, without it deformities such as rickets and osteomalacia can occur.

There is two forms of vitamin D, 25 hydroxyvitamin D and 1,25 dihydroxyvitamin D. The most common form is 25 hydroxyvitamin D and it is the most useful indicator of vitamin D status. 25 hydroxyvitamin D stays in the blood for longer and at higher concentrations.

There are two sources of vitamin D available to humans:

  1. It is produced in the skin following exposure to sunlight
  2. Taken in through the diet via food and supplements

How does vitamin D affect my wellbeing?

Low levels of vitamin D can indicate deficiency. In children, vitamin D deficiency can cause rickets. Deficiency of vitamin D prevents dietary calcium and phosphorus from being absorbed efficiently. Therefore, the deficiency can result in rickets which causes bone pain, poor growth as well as soft and weak bones which leads to bone deformities.[2] Rickets was almost eradicated in the western world in the 20th century as foods like margarine and breakfast cereals were fortified with vitamin D, but more recently it has made a comeback.

In adults, vitamin D deficiency can lead to osteomalacia. The main symptoms are:

  • Bone pain
  • Muscle pain
  • Muscle cramps
  • Fractures
  • Gait disorders

Increased risk of falling particularly in the elderly[3]

Vitamin D deficiency can also cause the following symptoms:

  • Fatigue
  • Bone pain
  • Recurrent infections
  • Hair loss
  • Muscle pain
  • Depression

Vitamin D toxicity usually occurs when too much of the vitamin is supplemented. Some of the symptoms include:

  • Nausea
  • Anorexia
  • Constipation
  • Headaches
  • High blood pressure
  • Kidney stones
  • Calcium toxicity[4]

How can I improve my result?

Although vitamin D can be ingested through the diet, most of our requirements come from our exposure to the sun. Research shows that in the UK 8.4% of UK white people aged 19-64 have vitamin D deficiency during the summer, but this rises to 39.3% during the winter months. The risks of skin cancer from sunlight exposure and UV radiation can prevent people from exposing their skin to the sun. However, you don’t need to sunbathe to acquire vitamin D from the sun, instead 5-30 minutes of exposure at midday twice per week should be enough. Studies have found that sunscreen with an SPF of 15 or above can, in fact, decrease the production of vitamin D by the skin by 99%.[5]

Sunlight exposure in the UK can be limited, particularly during the winter months.[6] Therefore, most of the UK population are at risk of vitamin D deficiency. This can be prevented through supplementation. Public Health England recommends that all adults and children over the age of one should consider taking a daily supplement of vitamin D. This may be solely a vitamin D supplement or a daily supplement but should contain at least 10mcg of vitamin D.[7]

You will not be able to source all the vitamin D your body requires from vitamin D alone. However, some good sources include:

  • fortified foods – milk, yoghurt, butter, cheese, orange juice, bread, breakfast cereal
  • oily fish – mackerel, salmon, sardines
  • egg yolk

Research shows that exercise has a positive association with vitamin D levels. Therefore, vitamin D could influence physical performance. The study showed that this was true across a broad spectrum of ages and could be beneficial in future healthcare.[8]

Physical activity is an important factor in keeping our bones healthy. It helps to keep our bones strong and reduces the chance of falling over.

Some good examples include:

  • walking
  • resistance band exercises
  • gardening
  • weightlifting
  • using cross training machines

Perhaps surprisingly people who already have osteoporosis should still partake in regular exercise to help strengthen their bones and reduce the risk of fracture. You should avoid high-impact exercise but if you are otherwise fit and healthy you should continue with physical activities.

- Health scores calculated



[1] The Open University. (2017). Vitamin D. Nutrition: Vitamins and Minerals. The Open University.

[2] National Health Service. (2018). Rickets and Osteomalacia. Available at:

[3] Rader, C, P., Corsten, N and Rolf, O. (2015). Osteomalazie und Vitamin-D-Hypovitaminose. Der Orthopäde: 44(9), pp 695-702.

[4] Maji, D. (2012). Vitamin D Toxicity. Indian J Endocrinol Metab: 16(2), pp 295-296.

[5] American Osteopathic Association. (2017). Widespread vitamin D deficiency likely due to sunscreen use, increase of chronic diseases, review finds. ScienceDaily. Retrieved June 13, 2018 from

[6] Pearce, S, H, S and Cheetham, T, D. (2010). Diagnosis and Management of Vitamin D Deficiency. BMJ: 340.

[7] National Health Service. (2016). The New Guidelines on Vitamin D – What You Need to Know. Available at:

[8] Kaul, A., Gläser, S., Hannemann, A., Schäper, C., Nauck, M., Felix, S, B., Bollmann, T., Ewert, R and Friedrich, N. (2015). Vitamin D is Associated with Cardiopulmonary Exercise Capacity: Results of Two Independent Cohorts of Healthy Adults. British Journal of Nutrition.

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This information has been medically reviewed by Dr Nicky Keay

Nicola has extensive clinical and research experience in the fields of endocrinology and sport and exercise medicine. Nicky is a member of the Royal College of Physicians, Honorary Fellow in the Department of Sport and Exercise Sciences at Durham University and former Research Fellow at St. Thomas' Hospital.

Dr Nicky Keay

Dr Nicky Keay

BA, MA (Cantab), MB, BChir, MRCP.