Vitamin D: The Complete, Evidence Based Guide

• Vitamin D is unique among vitamins: the body can make it itself in the skin with the help of UVB sunlight, rather than having to get it entirely from food [1].
• According to EU-approved claims, it contributes to normal calcium absorption, normal bones, normal teeth, normal muscle function, and the normal functioning of the immune system [3][4].
• The maximum tolerable intake in the EU is 100 µg (4,000 IU) per day for adults, including pregnant and breastfeeding women [2].
• The only reliable way to determine your vitamin D status is with a 25-hydroxyvitamin D (25-OH-D) blood test, which a doctor will then assess [8].
• Deficiency is most common in winter, in northern latitudes, and among people who get little exposure to sunlight [8][9].

This article is for general information purposes only and does not replace medical advice. It is not intended to diagnose, treat, prevent, or cure any disease. If you suspect a deficiency, or before starting, stopping, or changing a dietary supplement, consult a doctor, pharmacist, or nutritionist – especially if you are pregnant, breastfeeding, taking medication, or have any existing medical conditions.

What Is Vitamin D and What Does It Do?

Despite its name, vitamin D behaves more like a hormone than a typical vitamin. It's synthesised in the skin from a cholesterol derivative when exposed to UVB radiation, then activated in the liver and kidneys into its usable form [1][8]. It exists in two dietary and supplement forms: vitamin D2 (ergocalciferol, from plant and fungal sources) and vitamin D3 (cholecalciferol, from animal sources or lichen derived vegan sources). The body can use both, though D3 is generally considered somewhat more effective at raising and maintaining blood levels [8].

Under the EU Nutrition and Health Claims Regulation, the following statements about vitamin D are scientifically substantiated and legally authorised for use on foods and supplements [3][4]:

• Contributes to normal absorption and utilisation of calcium and phosphorus
• Contributes to normal blood calcium levels
• Contributes to the maintenance of normal bones
• Contributes to the maintenance of normal teeth
• Contributes to normal muscle function
• Contributes to the normal function of the immune system
• Contributes to normal cell division
• Contributes to the reduction of the risk of falling associated with postural instability and muscle weakness, in men and women aged 60 and older [6]

These are the claims a reputable vitamin D article or product label should stick to, not broader statements linking vitamin D to preventing or treating specific diseases, which fall outside what's legally substantiated for a food supplement.

How Much Vitamin D Do You Need?

Requirements differ by age, body weight, skin tone, latitude, season, and how much the body already produces on its own. There is no single global number. Instead there are several reference frameworks, each set by a different body:

*Reference intake values vary slightly by country and organisation. For example, Germany's DGE sets 20 µg/day, or 800 IU, for the portion not produced by the body itself [7]. The Upper Intake Level, by contrast, is EU wide and represents the highest daily intake unlikely to pose a risk of adverse effects in the general population. It is not a target to aim for, but a ceiling not to exceed without medical supervision [2].

A blood test is the only way to know where you personally stand. A 25 OH D blood test measures your circulating vitamin D and lets a doctor advise whether supplementation is appropriate and, if so, at what dose [8]. This is especially relevant for infants, pregnant or breastfeeding women, older adults, and people with chronic conditions, whose needs can differ meaningfully from general population figures [8][9].

Signs That May Point to Low Vitamin D

Low vitamin D doesn't always cause noticeable symptoms. Many people with low levels feel fine. When symptoms do occur, commonly reported ones include:

• Fatigue or low energy
• Increased hair shedding
• More frequent colds or infections
• Muscle weakness or aches
• Bone or joint discomfort
• Low mood
• Difficulty concentrating

These symptoms are non specific. Each has many possible causes unrelated to vitamin D, so they're a reason to consult a doctor and consider testing, not a basis for self diagnosis [8][9].

Left untreated for a long period, deficiency can eventually impair bone mineralisation. In children this is linked to rickets; in adults, to osteomalacia, and over the longer term to reduced bone density [8][9]. These are medical conditions requiring diagnosis and management by a healthcare professional. This article does not diagnose or treat them.

Who Is Most at Risk of Low Vitamin D?

According to national and international health bodies, groups more likely to have low vitamin D status include [8][9]:

• People who spend little time outdoors
• People with darker skin tones, since melanin reduces UVB driven synthesis
• People who cover most of their skin when outside, for cultural, religious, or occupational reasons
• Older adults, since skin becomes less efficient at producing vitamin D with age, and kidney conversion also declines
• People living at higher latitudes, especially in autumn and winter
• People with obesity, since vitamin D is fat soluble and sequestered differently in adipose tissue [8]
• People with malabsorption conditions, such as Crohn's disease, celiac disease, cystic fibrosis, or those who have had gastric bypass surgery [8]
• People with chronic liver or kidney disease, since both organs are involved in activating vitamin D [8]
• Exclusively breastfed infants, unless supplemented, since human milk is typically low in vitamin D [8]

If any of these apply to you, it's worth raising vitamin D at your next medical check up.

Sunlight: How the Body Makes Vitamin D

For most people, sunlight, not diet, is the primary source of vitamin D [1][8]. In summer, many adults with lighter skin can generate a substantial share of their daily requirement from roughly 15 minutes of midday sun on the arms and face, though the exact amount varies with skin tone, latitude, season, cloud cover, and sunscreen use [8].

Between approximately October and March at latitudes north of about 35 to 40 degrees, the sun sits too low in the sky for UVB rays to reach the ground in sufficient intensity, regardless of time spent outside. This is a key reason deficiency is more common in winter in these regions [8]. Sunscreen, while reducing vitamin D synthesis, is still recommended by health authorities during extended sun exposure because of skin cancer risk. The ODS consumer fact sheet notes this trade off explicitly [9].

Food Sources of Vitamin D

Diet contributes a smaller share of vitamin D for most people, since relatively few foods naturally contain significant amounts [1][8]. Useful sources include:

• Oily fish, such as salmon, mackerel, herring, and sardines
• Egg yolks
• Oysters and some shellfish
• Vitamin D fortified foods, which in many countries include milk, some plant based milks, some cereals, and some margarines
• Certain mushrooms, particularly those exposed to UV light during growth

Because natural dietary sources are limited, many countries operate food fortification programmes specifically to help the population meet baseline vitamin D needs [8].

Vitamin D Supplements: Forms and What to Consider

Supplements are one option some people discuss with their doctor when sunlight and diet aren't enough to meet their needs, particularly over winter, or for the higher risk groups listed above. Common forms include:

• Capsules. Often chosen for reliable bioavailability; suitable for people with allergies to fillers used in other formats.
• Tablets. Convenient, shelf stable, easy to store.
• Drops and liquid. Allow precise, flexible dosing; convenient for travel; often preferred for infants, children, or people with difficulty swallowing.

Whatever the format, check the label for the actual IU or µg amount per serving, and discuss the appropriate amount for your situation with a healthcare professional rather than assuming a higher dose is automatically better.

Can You Take Too Much Vitamin D?

At typical, label recommended doses, side effects are uncommon. Vitamin D toxicity is rare and, based on the studies EFSA reviewed in setting the Upper Intake Level, is generally associated with sustained daily intakes far above the UL. The reviewed data point to hypercalcaemia risk emerging around 234 to 275 µg/day (roughly 9,000 to 11,000 IU/day) in adult studies, well beyond the 100 µg/day (4,000 IU) ceiling set for general safety [2]. This is not something that occurs from following standard label dosing.

Possible signs of excessive intake include:

• Nausea or vomiting
• Loss of appetite
• Abdominal pain
• Headache
• Muscle weakness
• Increased thirst and urination, related to disrupted calcium metabolism

If you experience these symptoms while taking a vitamin D supplement, stop taking it and contact a doctor. It's also worth keeping total intake, from food, sunlight, and any supplements combined, within officially recommended ranges unless a doctor advises otherwise based on a blood test [2][8].

Glossary

• 25 OH D (25 hydroxyvitamin D). The form of vitamin D measured in a blood test to assess someone's vitamin D status.
• IU (International Unit). A standardised unit for measuring vitamin activity. For vitamin D, 40 IU equals 1 µg.
• UL (Tolerable Upper Intake Level). The highest average daily intake unlikely to cause adverse health effects in the general population. Not a recommended target.
• Hypercalcaemia. Abnormally high calcium levels in the blood, the main risk associated with vitamin D overintake.
• Osteomalacia. Softening of the bones in adults due to impaired mineralisation, often linked to prolonged deficiency.
• Rickets. The childhood equivalent of osteomalacia, causing skeletal deformities during growth.

References

[1] National Institutes of Health, Office of Dietary Supplements. Vitamin D, Health Professional Fact Sheet. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/ 

[2] EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA). Scientific opinion on the tolerable upper intake level for vitamin D, including the derivation of a conversion factor for calcidiol monohydrate. EFSA Journal 2023;21(8):8145. https://efsa.onlinelibrary.wiley.com/doi/10.2903/j.efsa.2023.8145 (updates the original 2012 opinion: EFSA Journal 2012;10(7):2813, https://efsa.onlinelibrary.wiley.com/doi/10.2903/j.efsa.2012.2813)

[3] European Commission. Regulation (EC) No 1924/2006 of the European Parliament and of the Council on nutrition and health claims made on foods. https://food.ec.europa.eu/food-safety/labelling-and-nutrition/nutrition-and-health-claims_en 

[4] European Commission. Commission Regulation (EU) No 432/2012 establishing a list of permitted health claims made on foods. EU Register of Nutrition and Health Claims. https://food.ec.europa.eu/food-safety/labelling-and-nutrition/nutrition-and-health-claims/health-claims_en 

[5] EFSA NDA Panel. Scientific Opinion on the substantiation of a health claim related to vitamin D and contribution to the normal function of the immune system pursuant to Article 14 of Regulation (EC) No 1924/2006. EFSA Journal 2015;13(5):4096. https://efsa.onlinelibrary.wiley.com/doi/abs/10.2903/j.efsa.2015.4096 

[6] EFSA NDA Panel. Scientific Opinion on the substantiation of health claims related to vitamin D and normal function of the immune system and inflammatory response (ID 154, 159), maintenance of normal muscle function (ID 155) and maintenance of normal cardiovascular function (ID 159) pursuant to Article 13(1) of Regulation (EC) No 1924/2006. EFSA Journal 2010;8(2):1468. https://efsa.onlinelibrary.wiley.com/doi/10.2903/j.efsa.2010.1468 

[7] German Nutrition Society (Deutsche Gesellschaft fur Ernahrung, DGE). Reference values for vitamin D intake. https://www.dge.de/wissenschaft/referenzwerte/vitamin-d/ 

[8] National Institutes of Health, Office of Dietary Supplements. Vitamin D, Health Professional Fact Sheet (deficiency risk groups, absorption, sunlight, food sources sections). https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/ 

[9] National Institutes of Health, Office of Dietary Supplements. Vitamin D, Consumer Fact Sheet. https://ods.od.nih.gov/factsheets/VitaminD-Consumer/ 

Food supplements are not a substitute for a varied and balanced diet and a healthy lifestyle. This article is for general information only and does not constitute medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional before starting, stopping, or changing any supplement, particularly during pregnancy or breastfeeding, if taking medication, or if managing a health condition.