Vitamin D — The Complete Guide

If you live in the UK, there’s a roughly one-in-five chance that your vitamin D level is too low right now — and that probability roughly doubles between October and March. Vitamin D is unusual among nutrients because we get most of it not from food but from sunlight on our skin, and at UK latitudes, the sun is too weak to make any from late autumn to early spring. This is why the NHS now formally recommends that everyone in the UK considers a vitamin D supplement during the winter months.

This guide covers what vitamin D actually does, how much you need, where to get it, who’s at higher risk of deficiency, and how to choose a sensible supplement if you decide to take one.

Quick disclaimer: Information here is for general guidance and is not medical advice. If you suspect a deficiency, take medication, are pregnant or breastfeeding, or have a health condition such as kidney or liver disease, please speak to your GP before starting a vitamin D supplement at any dose above 10 micrograms (400 IU) per day.

What vitamin D actually is

Despite the name, vitamin D is technically a hormone rather than a vitamin. Your skin makes it when UVB rays from the sun convert a cholesterol compound into vitamin D3 (cholecalciferol), which is then processed by your liver and kidneys into the active form that your body uses. The fact that we can make it ourselves — given enough sunlight — is why it slipped into the “vitamin” category historically rather than the “hormone” one.

Two forms appear on supplement labels:

• Vitamin D3 (cholecalciferol) — the form made by your skin and the form found in animal foods. This is the form most studies use and most experts recommend for supplementation.
• Vitamin D2 (ergocalciferol) — the form made by some plants and fungi exposed to UV. It’s vegan but generally less effective at raising blood vitamin D levels than D3.

If you’re choosing a supplement, D3 is the better default for most people. Vegan D3 is now available, made from lichen rather than lanolin (sheep wool oil).

What it does in the body

The European Food Safety Authority (EFSA) has authorised the following health claims for vitamin D:

• Vitamin D contributes to the normal function of the immune system
• Vitamin D contributes to the maintenance of normal bones and teeth
• Vitamin D contributes to the maintenance of normal muscle function
• Vitamin D contributes to the normal absorption and use of calcium and phosphorus
• Vitamin D contributes to normal blood calcium levels
• Vitamin D has a role in the process of cell division

That EFSA list is the regulatory standard for what we’re allowed to say vitamin D does. Beyond those authorised claims, there’s a wide and active research literature on possible roles in mood, autoimmune conditions, cardiovascular health, and infection risk. Some of that research is promising; some is mixed or has produced conflicting trial results. We summarise the state of evidence below for the most-asked-about areas.

Bone health (well-established)

Vitamin D’s most thoroughly proven role is helping your body absorb calcium from food. Without enough vitamin D, even a calcium-rich diet won’t deliver enough calcium to your bones — which over time can lead to softer bones (rickets in children, osteomalacia in adults) and increased fracture risk in older adults. This is the original reason vitamin D fortification was added to UK margarine in the 1940s.

Immune function (well-established for some outcomes, mixed for others)

Vitamin D receptors are present on most immune cells, and observational studies consistently find associations between low vitamin D status and increased rates of respiratory infections. Randomised controlled trials of vitamin D supplementation for reducing respiratory infections have produced mixed but generally modest positive results — a Cochrane review concluded vitamin D supplementation produces a small reduction in acute respiratory infection risk in deficient people. For people who already have adequate vitamin D, the additional benefit is less clear.

Muscle function and falls in older adults (reasonable evidence)

Several studies have linked low vitamin D status with reduced muscle strength and increased fall risk in older adults, and supplementation in deficient older people appears to reduce falls. This is why some falls-prevention programmes in the UK now include vitamin D supplementation as part of their advice.

Mood and depression (mixed evidence)

There’s a consistent observational association between low vitamin D status and depression, but randomised trials of vitamin D supplementation as a treatment for depression have produced mixed results. The current consensus is that correcting a deficiency may help mood in people who were deficient, but vitamin D is not a substitute for evidence-based treatments for clinical depression.

Cardiovascular health (mixed evidence)

Observational studies link low vitamin D to higher cardiovascular risk, but large trials of supplementation (such as VITAL in the US) have not consistently shown a reduction in heart attacks or strokes. The picture is unresolved.

COVID-19 and other infections (uncertain)

A large amount of pandemic-era research looked at vitamin D and COVID-19 outcomes. The honest summary: severe vitamin D deficiency appeared associated with worse outcomes, but supplementation results have been mixed, and the question of optimal dosing during acute illness remains unresolved.

If you take only one thing from this section: vitamin D is genuinely important for bone health and immune function, and correcting a deficiency is worthwhile. But it’s not the cure-all that some wellness content makes it out to be.

How much you need

The recommendations vary between countries because experts disagree on what counts as “enough.” We list the UK NHS figures and the US figures, which are the two most commonly cited:

Group	NHS RNI (UK)	US RDA
Adults 19-64	10 µg (400 IU)	15 µg (600 IU)
Adults 65+	10 µg (400 IU)	20 µg (800 IU)
Pregnant or breastfeeding	10 µg (400 IU)	15 µg (600 IU)
Children 1-18 years	10 µg (400 IU)	15 µg (600 IU)
Infants under 1 (formula-fed under 500ml/day)	8.5-10 µg	10 µg (400 IU)

The Tolerable Upper Intake Level for adults is 100 micrograms (4,000 IU) per day from supplements, according to both UK Scientific Advisory Committee on Nutrition (SACN) and EFSA. Going above this for sustained periods without medical supervision can lead to vitamin D toxicity (hypercalcaemia).

What blood levels mean

If you’ve had a vitamin D blood test, the result is reported as 25-hydroxyvitamin D in nmol/L (UK) or ng/mL (US). The Department of Health uses these categories:

• Below 25 nmol/L (10 ng/mL): deficient — supplementation generally recommended
• 25-50 nmol/L (10-20 ng/mL): insufficient — supplementation generally recommended in winter
• Above 50 nmol/L (20 ng/mL): adequate for most people

Some clinicians and researchers prefer levels above 75 nmol/L (30 ng/mL), but this is debated. The threshold for “optimal” depends on which outcome you’re optimising for (bone, immune, all-cause mortality), and the evidence isn’t settled.

Where to get it

From sunlight (when latitude and lifestyle permit)

In the summer in the UK, 10-15 minutes of midday sun on bare forearms a few times a week is generally enough for most people to make adequate vitamin D. Several caveats:

• The further north you are, the harder this is — in Scotland, sun-derived vitamin D production essentially stops from October to April
• People with darker skin need significantly more sun exposure to produce the same amount of vitamin D
• Sunscreen, clothing coverage, glass windows, and air pollution all reduce production
• Older adults’ skin makes vitamin D less efficiently
• People who work indoors, cover up for cultural or religious reasons, or who are housebound get little to none

From food

Vitamin D is naturally present in a relatively small number of foods. Approximate amounts per typical serving:

Food	Vitamin D (µg per serving)
Cooked oily fish (salmon, sardines, mackerel, herring) — 100g	10-25 µg
Cod liver oil — 1 tsp (5ml)	11-12 µg
Egg yolk — 1 large	1-2 µg
Fortified breakfast cereal (UK varies) — 30g	1-2 µg
Fortified plant milk — 250ml	1.5-2 µg
UV-exposed mushrooms — 100g	5-10 µg (variable)
Beef liver — 100g	1-2 µg

A reality check: hitting the 10 µg NHS recommendation from food alone is genuinely difficult unless you’re eating oily fish three or four times a week. Most UK adults eat oily fish less than once a week.

From supplementation

For most UK adults, supplementation is the easiest reliable source of vitamin D, especially from October to March. The NHS specifically advises that:

• Everyone in the UK should consider a 10 µg (400 IU) supplement during autumn and winter
• People at higher risk of deficiency should consider taking it year-round
• People with very limited sun exposure (housebound, cover up for cultural reasons, dark skin) should take it year-round

Who’s at higher risk of deficiency

Some groups are more likely to be deficient and should pay particular attention:

• People with darker skin living in the UK — melanin reduces vitamin D production from sunlight
• People who cover most of their skin for cultural or religious reasons
• Housebound or institutionalised people with limited outdoor time
• People aged 65+ — skin produces less efficiently
• Pregnant and breastfeeding women — needs are higher
• People with malabsorption conditions (Crohn’s, coeliac, gastric bypass) — absorb less of what they take in
• People taking certain medications (some anticonvulsants, glucocorticoids) — increased breakdown
• People with very low body weight or very high body weight — different mechanisms in each case

If you’re in any of these groups, it’s worth asking your GP for a vitamin D blood test, which is available on the NHS.

How to choose a supplement

If you’ve decided to supplement, the choices come down to:

Form

D3 over D2 for most people. D3 is more effective at raising and maintaining blood vitamin D levels. Vegan D3 made from lichen is widely available and works the same as animal-derived D3.

Dose

For general winter maintenance in a UK adult without known deficiency: 10-25 µg (400-1,000 IU) per day is sensible. The NHS minimum is 10 µg, but many experts consider 20-25 µg more reflective of current evidence for optimal status.

If you have a known deficiency, your GP may recommend higher doses (often 25-75 µg / 1,000-3,000 IU daily, sometimes higher loading doses short-term). Don’t self-prescribe high doses without guidance — vitamin D is fat-soluble and accumulates in the body, so toxicity is possible at sustained high doses.

Avoid the very-high-dose products (10,000+ IU) marketed online unless prescribed.

Quality markers

• Third-party tested. Look for brands that publish independent lab testing or hold Informed Sport / NSF certification.
• Pure form with sensible carrier. Vitamin D is fat-soluble, so capsules with an oil base (olive oil, MCT oil, coconut oil) absorb well. Tablet forms work too but should be taken with a fat-containing meal.
• No proprietary blends. The label should state exactly how much vitamin D is in each capsule/drop.
• Reasonable price. Vitamin D is one of the cheapest supplements to produce. Anything significantly more than £0.05-£0.10 per 1,000 IU daily dose is likely overpriced for what it is.
• D3 + K2 combinations are popular and have a plausible rationale (K2 helps direct calcium to bones rather than soft tissues), but evidence for needing the combination in most people is weaker than the marketing suggests. A plain D3 product from a reputable brand is fine for most.

Avoid

• “Liposomal vitamin D” at premium prices — the absorption advantage is poorly evidenced for fat-soluble vitamins, which are already well-absorbed when taken with a meal
• Combination products that hide vitamin D inside a 15-ingredient “immune booster” with vague proprietary amounts
• Mega-dose loading regimens (300,000+ IU monthly) unless prescribed for a specific reason
• Products without batch numbers and expiry dates clearly printed

When to take it

• With a meal containing fat — improves absorption
• Time of day doesn’t matter much. Some people report it affects sleep if taken late, but the evidence is anecdotal
• Daily is more consistent than weekly or monthly mega-doses for keeping levels steady, though both work

Signs and symptoms of deficiency

Vitamin D deficiency is often called “silent” because symptoms develop slowly. Possible signs include:

• Bone or muscle pain, particularly in the lower back, hips, or thighs
• Muscle weakness, especially noticeable when climbing stairs or rising from a chair
• Fatigue and low mood
• More frequent illnesses or infections
• Slow wound healing
• Hair thinning (sometimes)
• In children: developmental delays, dental issues, bowed legs in severe cases

These symptoms are non-specific and could have many other causes. The only way to know for certain is a 25-hydroxyvitamin D blood test, which your GP can arrange.

Interactions and cautions

Vitamin D can interact with several conditions and medications:

• Sarcoidosis, lymphoma, hyperparathyroidism, or kidney disease: vitamin D supplementation should only be done under medical supervision because of altered calcium handling
• Thiazide diuretics: increased risk of high blood calcium
• Digoxin: high vitamin D-induced calcium elevation can affect digoxin toxicity
• Statins (atorvastatin): vitamin D can affect statin metabolism in some studies
• Steroids (long-term): reduce vitamin D activation; people on long-term steroids may need higher intakes
• Weight-loss medications (orlistat) and bile acid sequestrants: reduce fat-soluble vitamin absorption

If you take any prescription medication or have a chronic condition, ask your GP or pharmacist before starting a high-dose vitamin D supplement.

Frequently asked questions

Should the whole family take vitamin D in winter?
NHS guidance is that everyone in the UK aged 1 and over should consider a 10 µg supplement during autumn and winter. Breastfed babies should be given vitamin D drops from birth (8.5-10 µg) unless they’re getting formula with sufficient added vitamin D.

Can I just spend more time in the sun?
In summer, yes — modest midday sun exposure on bare forearms a few times a week is generally enough for most people. In winter at UK latitudes, no — the UVB intensity from October to April is too weak to make significant amounts of vitamin D, even on a bright cloudless day.

Does sunbed exposure help?
Sunbeds do produce UVB and so will increase vitamin D, but the skin cancer risk from sunbed use is well-established and outweighs the benefit when supplements are a cheap and safe alternative.

Is too much vitamin D dangerous?
Yes, though deliberately overdosing is hard. Sustained intakes above 100 µg (4,000 IU) per day from supplements without monitoring can cause hypercalcaemia (high blood calcium), with symptoms including nausea, vomiting, weakness, frequent urination, and in severe cases kidney damage. Stick to the recommended range unless you’re being monitored.

Vitamin D3 vs D2 — does the source matter?
D3 (cholecalciferol) is generally more effective at raising blood levels and maintaining them. D2 (ergocalciferol) works but appears to be roughly 2-3 times less effective per unit. Vegan D3 from lichen is now widely available, so vegans don’t need to choose D2 by default.

Do I need vitamin K2 with my D3?
The theoretical rationale (K2 directs calcium to bones rather than arteries) is plausible, but the human evidence for needing K2 alongside D3 supplementation is weaker than supplement marketing suggests. A varied diet with green vegetables and fermented foods usually provides adequate K2. The combination products are fine to take but not clearly necessary for most people.

Does vitamin D help with hair loss?
There’s a consistent association between low vitamin D status and several hair loss conditions, including alopecia areata. Whether supplementation reverses hair loss in people who aren’t deficient is much less clear. If your hair is thinning, getting your vitamin D tested is reasonable; expecting D supplementation alone to regrow lost hair generally isn’t.

Can I take vitamin D with my other supplements?
Generally yes, especially with calcium, magnesium, and vitamin K. Take with a meal containing fat for best absorption.

Where to go from here

• If you suspect deficiency: ask your GP for a 25-hydroxyvitamin D blood test
• For winter maintenance: consider a 10-25 µg D3 supplement from October to March
• For year-round maintenance if you’re in a higher-risk group: same dose, year-round
• For diagnosed deficiency: follow your GP’s recommended dose

Related reading on Nutrition Encyclopedia

• Best foods for vitamin D — coming soon
• Calcium guide — coming soon
• Magnesium — the complete guide

Primary sources

• NHS — Vitamin D guidance
• Scientific Advisory Committee on Nutrition (SACN) — Vitamin D and Health report
• NIH Office of Dietary Supplements — Vitamin D fact sheet for health professionals
• European Food Safety Authority — authorised health claims register
• Cochrane reviews on vitamin D supplementation
• USDA FoodData Central — for food vitamin D content

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Information on this page is for general guidance and is not a substitute for medical advice. Speak to your GP if you suspect a vitamin D deficiency or before starting supplementation above 10 µg (400 IU) per day, especially if you take medication or have a health condition.

Last updated: May 2026