What is Vitamin D and Why It’s Essential for Health
Vitamin D is often called the “sunshine vitamin” because your body makes it when your skin is exposed to sunlight. It’s a fat‑soluble nutrient that plays several vital roles, from building strong bones and teeth to supporting your immune system. Despite its importance, a large proportion of people in the UK do not get enough vitamin D, particularly during autumn and winter when daylight hours are short and UV‑B rays are weak[1]. Understanding what vitamin D does, how much you need and how to obtain it will help you stay healthy and avoid deficiency.
What Is Vitamin D?
Vitamin D is not a single substance but a family of related secosteroids that act more like hormones than traditional vitamins. The two main forms relevant to human health are:
- Vitamin D3 (cholecalciferol) – Produced in the skin when exposed to UV‑B radiation from sunlight and found in animal‑based foods such as oily fish, liver and egg yolks.
- Vitamin D2 (ergocalciferol) – Made by plants and fungi and used to fortify some foods. It is also available in vegetarian and vegan supplements.
Both forms are converted by your liver into 25‑hydroxyvitamin D (25‑OH D), the storage form measured by blood tests. Your kidneys then convert 25‑OH D into 1,25‑dihydroxyvitamin D (calcitriol), the active hormone that interacts with vitamin D receptors throughout the body to regulate calcium and phosphate balance, gene expression and cell growth. Vitamin D3 is generally more effective at raising and maintaining blood vitamin D levels than vitamin D2[2].
Why the Name “Vitamin” May Be Misleading
Although we call it a vitamin, vitamin D functions like a hormone. A true vitamin must be obtained entirely from the diet because our bodies cannot synthesise it. Vitamin D is unique because your skin can make it in the presence of sunlight. The ability to produce vitamin D through sunlight may have evolved in humans living near the equator. In northern latitudes like the UK, sunlight intensity in autumn and winter is insufficient to synthesise enough vitamin D[1], so we must rely on food and supplements.
How Vitamin D Works in Your Body
Absorbing Calcium and Phosphorus
The most well‑known function of vitamin D is helping your intestines absorb calcium and phosphorus from food. These minerals are essential for building and maintaining strong bones and teeth. Without sufficient vitamin D, only about 10-15 % of dietary calcium is absorbed. Adequate vitamin D levels ensure that children develop strong bones and prevent conditions like rickets; in adults, vitamin D helps maintain bone density and reduces the risk of osteomalacia (soft bones), osteoporosis and fractures[3].
Supporting Muscle Function
Muscle cells have vitamin D receptors that influence muscle contraction and performance. Deficiency can lead to muscle weakness and aches, making it difficult to climb stairs or rise from a chair[4]. Adequate vitamin D helps maintain muscle strength and may improve physical performance in older adults.
Modulating the Immune System
Vitamin D receptors are present on many immune cells, and the hormone form of vitamin D is involved in regulating both innate and adaptive immune responses. Research suggests that adequate vitamin D status may reduce the risk of respiratory infections and inflammation. However, public health agencies emphasize that there is not enough evidence to recommend vitamin D solely for preventing or treating infections like COVID‑19[5]. Nevertheless, maintaining sufficient vitamin D supports overall immune health and helps your body respond effectively to pathogens.
Influencing Mood and Brain Health
Emerging research links low vitamin D levels to mood disorders such as depression and seasonal affective disorder. It may play a role in serotonin production and the regulation of circadian rhythms. People living in northern countries often experience mood changes in winter, which may partly relate to decreased sunlight and lower vitamin D synthesis. While vitamin D supplementation is not a substitute for professional mental health care, maintaining adequate levels may support emotional well‑being.
Why Vitamin D Is Essential
Preventing Rickets and Osteomalacia
Vitamin D’s primary role in bone health makes it essential during growth and throughout adulthood. In children, severe deficiency causes rickets, characterised by soft and weak bones that may bend or bow. The NHS notes that rickets remains more common among children of South Asian, African or African‑Caribbean heritage in the UK[6]. In adults, deficiency leads to osteomalacia, causing bone pain and muscle weakness[3]. Ensuring sufficient vitamin D protects bone integrity and reduces the risk of fractures.
Supporting Lifelong Bone Health
Peak bone mass is typically reached in early adulthood, after which bone density gradually declines. Adequate vitamin D and calcium intake during childhood, adolescence and early adulthood help you build stronger bones that are more resilient later in life. Vitamin D also supports calcium and phosphorus balance, preventing the body from leaching calcium from bones to maintain blood levels.
Aiding Normal Immune Function
A healthy immune system defends against infections and helps wound healing. Vitamin D enhances the ability of immune cells to recognise pathogens and modulates inflammatory responses. Although research on vitamin D and specific diseases (including autoimmune conditions) is ongoing, maintaining adequate levels helps your immune system function optimally.
Potential Roles in Chronic Disease Prevention
Low vitamin D status has been associated with higher risks of certain chronic conditions, including cardiovascular disease, diabetes, multiple sclerosis and some cancers. These associations do not prove causation, and further research is needed. However, maintaining sufficient vitamin D is considered part of an overall healthy lifestyle and may contribute to better long‑term health.
How Much Vitamin D Do You Need?
UK Government Recommendations
The Scientific Advisory Committee on Nutrition (SACN) sets a Reference Nutrient Intake (RNI) of 10 micrograms (µg) of vitamin D per day for adults and children aged 4 years and over. Government advice is that everyone should consider taking a daily supplement containing 10 µg (400 international units) during autumn and winter[7].
- Children under 1 year: 8.5–10 µg per day[8].
- Pregnant and breastfeeding women, and people at risk of deficiency: 10 µg per day year‑round[9].
In the UK, diet alone rarely provides enough vitamin D. The National Diet and Nutrition Survey found that mean vitamin D intakes from food sources were well below the RNI in all age groups[10], and only 17 % of adults aged 19–64 reported taking a supplement[11].
Safe Upper Limits
For most people, taking 10 µg (400 IU) per day is sufficient. The NHS cautions that taking excessive amounts of vitamin D for a prolonged period can lead to hypercalcaemia (high blood calcium), which may cause kidney stones and heart problems[12]. High‑dose supplements should only be taken under medical supervision.
Who Needs Vitamin D the Most?
People with Limited Sun Exposure
Those who spend little time outdoors, live in care homes or hospitals, or wear clothing that covers most of their skin may not produce enough vitamin D. In the UK, from October to early March, sunlight is too weak for the skin to synthesise vitamin D[1]. People in these groups should take supplements year‑round[9].
People with Darker Skin
Individuals with African, African‑Caribbean or South Asian heritage produce less vitamin D from sunlight due to higher melanin content in their skin. The UK Office for Health Improvement and Disparities recommends that people with dark skin take a 10 µg supplement throughout the year[13].
Pregnant and Breastfeeding Women
Vitamin D is essential during pregnancy and lactation for both maternal health and the developing baby. Deficiency in mothers increases the risk of bone problems in infants. Pregnant and breastfeeding women in the UK are advised to take 10 µg per day[14].
Infants and Young Children
Babies who are exclusively breastfed should receive a daily vitamin D supplement of 8.5–10 µg from birth until they start receiving at least 500 ml of formula per day[15]. Children aged 1-4 years should take 10 µg per day[16]. These recommendations help prevent rickets and support healthy growth.
Elderly Adults
Older adults produce less vitamin D in their skin and are often less exposed to sunlight. They are at higher risk of deficiency and associated bone loss. Taking a daily 10 µg supplement supports bone strength and reduces fall risk.
Sources of Vitamin D
Sunlight Exposure
The most efficient way to obtain vitamin D is through safe sun exposure. During spring and summer in London, spending short periods outdoors with forearms and lower legs uncovered, without sunscreen, can help generate vitamin D. Public health agencies recommend protecting your skin from sunburn by seeking shade during peak hours (11 a.m. – 3 p.m.), wearing protective clothing and using sunscreen[17]. People with darker skin need longer exposure to produce the same amount of vitamin D.
Dietary Sources
Few foods naturally contain vitamin D. Good sources include:
- Oily fish – Salmon, mackerel, herring, sardines and trout[18].
- Meat and liver – Red meat and liver provide small amounts.
- Egg yolks – A moderate source.
- Fortified foods – Some breakfast cereals, fat spreads and plant milks are fortified with vitamin D[19]. Check product labels.
Because UK cow’s milk is usually not fortified[20], dietary intake often falls short of the RNI.
Supplements
Most people will need a supplement, especially during autumn and winter. Supplements are widely available in pharmacies such as MedCare Health Clinic, supermarkets and health stores. Look for products providing 10 µg (400 IU) of vitamin D3 (cholecalciferol) per dose. For vegetarians or vegans, vitamin D2 supplements are suitable. Some vitamin D3 products are made from algae or lichen and are vegan‑friendly.
Our FAQ page on vitamin D supplements discusses timing, interactions and dosing. Remember that high‑dose supplements should only be used under medical supervision.
Tips for Maintaining Adequate Vitamin D
- Take a daily 10 µg supplement from October to March; year‑round if you have little sun exposure or other risk factors.
- Get sensible sun exposure in spring and summer, but avoid burning – short periods with forearms uncovered are enough.
- Include vitamin D‑rich foods such as oily fish and fortified products in your diet.
- Check medication interactions – Some drugs can affect vitamin D levels[21]. Speak with your pharmacist if you take long‑term medication.
- Test your levels if concerned – If you have symptoms of deficiency or belong to a high‑risk group, your GP may arrange a blood test.
Internal Links and Further Reading
For more information on vitamin D and related topics, explore our other articles:
👉 Vitamin D Guide: Benefits, Deficiency & Supplements
- Symptoms and Risks of Vitamin D Deficiency – Detailed information on signs, health risks and when to get tested.
- Vitamin D3 vs D2: Which Form Is Better? – Learn about differences in structure, absorption and dietary sources.
- How Much Vitamin D Do You Need Daily? – Age‑specific dosage recommendations and special considerations.
- Best Sources of Vitamin D: Food, Sun & Supplements – Practical advice on obtaining vitamin D safely and effectively.
- Who’s at Risk of Vitamin D Deficiency in the UK? – Identify high‑risk groups and tailored solutions.
- FAQs About Vitamin D Supplements – Answers to common questions about timing, dosage and safety.
By understanding what vitamin D is, why it’s essential and how to obtain it, you can make informed choices to protect your bone health, immune function and overall well‑being. If you’re unsure about your vitamin D status or supplement needs, speak to your GP or the pharmacists at MedCare Health Clinic.
References
[1] [3] [5] [7] [8] [9] [14] [15] [16] [18] [19] [20] Vitamin D – NHS
https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/
https://www.bda.uk.com/resource/vitamin-d.html
[4] [21] Vitamin D Deficiency: Causes, Symptoms, and Treatment
https://patient.info/bones-joints-muscles/osteoporosis-leaflet/vitamin-d-deficiency
[6] [10] [11] NDNS: results from years 9 to 11 (combined) – statistical summary – GOV.UK
[12] About colecalciferol – NHS
https://www.nhs.uk/medicines/colecalciferol/about-colecalciferol/
[13] Vitamin D deficiency: migrant health guide – GOV.UK
https://www.gov.uk/guidance/vitamin-d-deficiency-migrant-health-guide