Vitamin D Guide: Benefits, Deficiency & Supplements
Vitamin D is a fat‑soluble vitamin that helps regulate calcium and phosphate levels in the body. It supports healthy bones, teeth and muscles and also plays a role in immune function. In the UK, many people struggle to get enough vitamin D because sunlight is the main source, and our northern latitude means the sun is not strong enough during autumn and winter[1]. This article, written for readers in London and across the UK, explains what vitamin D does, how much you need, who is most at risk of deficiency and how to obtain it through sunlight, food and supplements. Internal links point to dedicated pages covering each subtopic in greater detail.
What is Vitamin D?
Vitamin D refers to a group of related compounds. The two most important forms for human health are vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). Both forms can be converted by the liver and kidneys into the active hormone calcitriol, but vitamin D3 is generally better absorbed and more effective at raising blood vitamin D levels[2]. This guide focuses mainly on vitamin D3 because it is the form used in most supplements and is naturally produced in the skin when sunlight interacts with a cholesterol precursor.
Types of Vitamin D
- Vitamin D3 (cholecalciferol) – Produced in human skin when exposed to ultraviolet B (UV‑B) rays from sunlight. It’s found in animal‑based foods and most supplements. Research suggests vitamin D3 is the most effective form for raising and maintaining blood vitamin D levels[2].
- Vitamin D2 (ergocalciferol) – Produced by plants and fungi. It’s used to fortify some foods and is available in vegetarian and vegan supplements. Vitamin D2 is still beneficial, but it tends to raise blood levels less efficiently than vitamin D3[2].
For a detailed comparison of these two forms, see our supporting article “Vitamin D3 vs D2: Which Form Is Better?”.
Health Benefits of Vitamin D
Supports Bone Health and Calcium Absorption
Vitamin D enables the body to absorb calcium and phosphorus from the diet. Without sufficient vitamin D, only 10–15 % of dietary calcium may be absorbed. In children, severe deficiency causes rickets, a condition where bones become soft and deformed. In adults, long‑term deficiency can lead to osteomalacia, causing bone pain and muscle weakness[3]. Adequate vitamin D supports peak bone mass and reduces the risk of fractures in older adults by helping maintain bone density.
Immune System Support
Research suggests vitamin D receptors are present on many immune cells, and the hormone form of vitamin D modulates immune responses. It may help the body defend against respiratory infections and regulate inflammation. However, definitive evidence for vitamin D preventing or treating infections like COVID‑19 is lacking. The NHS states that there is currently not enough evidence to support taking vitamin D solely to prevent or treat COVID‑19[4]. Nevertheless, maintaining adequate levels is important for overall immune function and bone health.
Muscle and Mood Benefits
Vitamin D deficiency is linked to muscle weakness and aches. Adults with deficiency often report tiredness, general aches and bone pain, particularly in the ribs, hips and legs[5]. Some studies associate low vitamin D with mood disorders, including depression and seasonal affective disorder, although causal relationships are still being investigated.
Deficiency Symptoms & Causes
How Common Is Vitamin D Deficiency?
The UK National Diet and Nutrition Survey (NDNS) found evidence of low vitamin D status in 16 % of adults aged 19–64, 13 % of adults aged 65 years and over, 19 % of children aged 11–18 and 2 % of children aged 4–10[6]. Another patient‑information review reports that roughly 1 in 5 adults and 1 in 5 children in the UK have low vitamin D levels[7]. Low blood levels are more common in the winter and spring because of reduced sun exposure[7].
Symptoms in Adults
Many adults with vitamin D deficiency have no specific symptoms, but common signs include fatigue, low energy and general aches or pains[8]. Severe deficiency may cause muscle weakness, making it hard to climb stairs or rise from a chair, and bone pain in the ribs, hips or shins[9]. In extreme cases, low calcium levels due to deficiency can lead to muscle spasms, seizures and heart problems[10].
Symptoms in Children
Children with vitamin D deficiency may develop soft or weak bones, leading to bowed legs, delayed growth, delayed tooth eruption and irritability[11]. In very severe cases, they may experience muscle spasms or seizures due to low calcium[12]. Rickets, although rare, remains more common among children of South Asian, African or African‑Caribbean heritage in the UK[6].
Causes and Risk Factors
Deficiency arises when the body either requires more vitamin D or cannot produce enough. Risk factors include:
- Limited sun exposure – People who stay indoors, live in care homes or hospital wards, or cover most of their skin for cultural reasons may not make enough vitamin D[13].
- Darker skin – Individuals with African, African‑Caribbean or South Asian backgrounds produce less vitamin D from sunlight and are advised to take supplements year‑round[14].
- Age – Older adults produce less vitamin D in the skin and are more likely to be housebound[15].
- Pregnancy and breastfeeding – Women need more vitamin D during pregnancy and breastfeeding, and deficiency is more likely with closely spaced pregnancies[16].
- Medical conditions and medications – People with Crohn’s disease, coeliac disease or kidney and liver disorders may not absorb vitamin D well[17]. Certain medications (e.g. antiepileptic drugs and some HIV medicines) increase deficiency risk[18].
- Dietary factors – Strict vegan diets and diets low in oily fish, fortified foods or eggs provide little vitamin D[19].
Vitamin D3 vs D2: What’s the Difference?
Vitamin D3 and D2 are both converted into the active hormone but differ in their origin and effectiveness:
| Form | Source & Characteristics | Notes |
| Vitamin D3 (Cholecalciferol) | Produced in human skin via sunlight; found in oily fish, liver and egg yolks; widely used in supplements | More efficient at raising and maintaining blood vitamin D levels[2] |
| Vitamin D2 (Ergocalciferol) | Produced by plants and fungi; used to fortify some foods and available in vegan supplements | Suitable for vegetarians/vegans but may not increase blood levels as effectively as D3[2] |
For more detail, see our article “Vitamin D3 vs D2: Which Form Is Better?”.
How Much Vitamin D Do You Need?
Official UK Recommendations
The Scientific Advisory Committee on Nutrition (SACN) recommends a Reference Nutrient Intake (RNI) of 10 µg (micrograms) per day for adults and children aged four years and over. Government advice is that everyone should consider taking a daily supplement containing 10 µg (400 IU) of vitamin D during autumn and winter[20]. Babies up to age 1 need 8.5-10 µg per day[21].
People with little or no sunlight exposure – such as those who are housebound, live in care homes, wear clothing that covers most of their skin or have dark skin – should take a 10 µg supplement throughout the year[22]. Pregnant and breastfeeding women are included in this recommendation[23].
Evidence on Intake and Deficiency Prevalence
The NDNS survey found that mean vitamin D intakes from food sources were far below the RNI in all age groups, around a fifth to a quarter of the RNI in children and a quarter to a third in adults[24]. When supplements were included, intakes increased but remained below the RNI for many adults[25]. Only 17 % of adults aged 19-64 and 34 % of adults aged 65-74 reported taking supplements[26].
Top Sources: Sunlight, Food and Supplements
Sunlight Exposure
From late March/early April to September, most people can make sufficient vitamin D from sunlight[27]. The UK’s northern latitude means sunlight is too weak during autumn and winter for the skin to produce vitamin D. To obtain vitamin D safely from sunlight:
- Expose your face and forearms or hands daily for short periods during spring and summer.
- Avoid sunburn by seeking shade between 11 a.m. and 3 p.m., covering up and using sunscreen (SPF 30 or higher)[28].
- People with darker skin need longer exposure to produce adequate vitamin D.
Food Sources
Vitamin D is naturally present in few foods, so diet alone rarely provides enough. Good sources include oily fish (salmon, sardines, trout, herring), red meat, liver, egg yolks and fortified foods like breakfast cereals and fat spreads[29]. UK cow’s milk is typically not fortified[30]. Some mushrooms grown under UV light also contain vitamin D[31].
Supplements
Because it’s difficult to get enough vitamin D from food alone, supplements are recommended for most people during autumn and winter and for at‑risk groups year‑round[32]. Vitamin D3 (cholecalciferol) supplements are the most effective[2]. You can purchase vitamin D supplements in various forms (tablets, capsules, liquids) from pharmacies such as MedCare Health Clinic, supermarkets and health stores.
For those who prefer plant‑based products, vitamin D2 supplements are suitable. Look for products providing 10 µg of vitamin D per day, unless directed otherwise by a healthcare professional. Our FAQ page on vitamin D supplements answers common questions about timing, dosage and safety.
Vitamin D and Your Immune System
While vitamin D’s primary role is bone and calcium metabolism, it also influences immune function. Vitamin D receptors on immune cells help modulate innate and adaptive immune responses. Epidemiological studies suggest that adequate vitamin D status may reduce the risk of respiratory infections, but clinical trials provide mixed results. Public health agencies caution that more research is needed and people should not rely on vitamin D supplements to prevent or treat COVID‑19[4]. Nevertheless, maintaining sufficient levels is important for overall health.
Who Needs Supplements in the UK?
The following groups should take a daily supplement containing 10 µg vitamin D year‑round[22][33]:
- People with little or no sunlight exposure – Frail or housebound individuals, those in care homes and anyone who covers most of their skin when outdoors.
- People with darker skin – Including African, African‑Caribbean and South Asian backgrounds[14].
- Pregnant and breastfeeding women – Vitamin D supports maternal and infant bone health[16].
- Children aged 1–4 years – Should receive a daily 10 µg supplement[34].
- Babies up to 1 year – Need 8.5–10 µg per day unless consuming more than 500 ml of formula[35].
Clinically extremely vulnerable adults (e.g., people shielding due to COVID‑19 or living in care homes) were offered free 10 µg vitamin D supplements during the winter[36]. If you belong to any high‑risk group or are unsure about your vitamin D status, consult a GP or pharmacist.
FAQs
When is the best time to take a vitamin D supplement?
Vitamin D is fat‑soluble, so take it with a meal containing some fat for better absorption. Many people take it with breakfast. Consistency matters more than timing; aim to take your supplement at the same time each day.
How much is too much?
The NHS advises that taking 10 µg (400 IU) daily is safe and meets the needs of most people[37]. High doses prescribed by a doctor may be used to treat deficiency but should not be taken long‑term without medical supervision. Excessive intake can cause a dangerous build‑up of calcium in the body (hypercalcaemia), leading to kidney stones and heart problems[38]. Never exceed the recommended amount unless instructed by a healthcare professional.
What if I miss a dose?
If you forget to take your supplement, take it as soon as you remember unless it’s almost time for your next dose. Do not double up. Vitamin D is fat‑soluble, and your body stores it, so missing an occasional dose is unlikely to cause deficiency.
Can I get enough vitamin D from food alone?
Diet alone rarely provides the RNI of 10 µg per day[39]. Oily fish, red meat, egg yolks and fortified foods contribute but usually fall short. That is why public health authorities recommend supplements during autumn and winter and year‑round for people with limited sun exposure[32].
Do sunscreen and staying indoors cause deficiency?
The regular use of sunscreen protects against skin cancer and does not appear to cause vitamin D deficiency in real life[40]. However, staying indoors or covering up most of your skin will reduce vitamin D synthesis, so people who avoid sun exposure should take supplements year‑round[33].
Are vitamin D supplements suitable for vegetarians and vegans?
Yes. Vitamin D2 supplements are plant‑derived and suitable for vegetarians and vegans[2]. Some vitamin D3 supplements use lichen or algal sources, making them vegan‑friendly. Always check the label and choose products providing 10 µg daily.
Internal Linking and Next Steps
This comprehensive guide introduces key aspects of vitamin D. To explore specific topics in more depth, read our supporting articles:
- What is Vitamin D and Why It’s Essential – Basic overview of vitamin D and its role in the body.
- Symptoms and Risks of Vitamin D Deficiency -Detailed discussion of signs, health risks and when to get tested.
- Vitamin D3 vs D2: Which Is Better? – Compare structure, absorption and sources.
- How Much Vitamin D Do You Need Daily? -Age‑specific dosage recommendations and special considerations.
- Top Sources of Vitamin D (Food, Sun, Supplements) – Practical tips for obtaining vitamin D safely and effectively.
- Vitamin D for Bone Health and Immunity – Explore how vitamin D supports bones and immune function.
- Who is Most at Risk for Vitamin D Deficiency in the UK? – Identify high‑risk groups and tailored solutions.
- FAQs About Vitamin D Supplements – Answers to common questions about supplements, dosing and interactions.
By following the official NHS and Public Health England recommendations, consulting your GP or pharmacist and considering supplements from trusted pharmacies like MedCare Health Clinic, you can maintain healthy vitamin D levels year‑round.
References
[1] [3] [4] [20] [21] [22] [23] [27] [29] [30] [32] [34] [35] Vitamin D – NHS
https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/
[2] [28] [31] [37] Vitamin D – BDA
https://www.bda.uk.com/resource/vitamin-d.html
[5] [7] [8] [9] [10] [11] [12] [15] [16] [17] [18] [19] [40] Vitamin D Deficiency: Causes, Symptoms, and Treatment
https://patient.info/bones-joints-muscles/osteoporosis-leaflet/vitamin-d-deficiency
[6] [24] [25] [26] [39] NDNS: results from years 9 to 11 (combined) – statistical summary – GOV.UK
[13] [14] [33] Vitamin D deficiency: migrant health guide – GOV.UK
https://www.gov.uk/guidance/vitamin-d-deficiency-migrant-health-guide
[36] Vitamin D and clinically extremely vulnerable (CEV) guidance – GOV.UK
[38] About colecalciferol – NHS
https://www.nhs.uk/medicines/colecalciferol/about-colecalciferol/