Vitamin D3, or cholecalciferol, is the type of vitamin D your skin makes from sunlight, and it's the type you'll find in most UK supplements.¹ The NHS advises adults and children over 1 to consider taking 10 micrograms (400 IU) a day through autumn and winter, and not to go above 100 micrograms (4,000 IU) a day unless a doctor tells you otherwise.¹

Vitamin D3 at a glance

What is vitamin D3?

Vitamin D is really a family name rather than a single thing, and two members of that family matter for people. D2, properly called ergocalciferol, comes from plants and fungi. D3, or cholecalciferol, is the one your skin makes when sunlight lands on it, and it's also the one in animal foods and in most UK supplements.^1,3 Chemically the two are nearly identical, differing only in a small part of their structure.³

Both will raise the level of vitamin D in your blood, but research has consistently found that D3 raises it further and holds it there for longer, which is why you'll see D3 on almost every UK label.³ Neither form does anything useful until your body switches it on, and that happens in two steps. Your liver converts it into 25-hydroxyvitamin D, then your kidneys turn that into the active form your body actually uses.³ It's the middle one, 25-hydroxyvitamin D, that a blood test measures, which is why your result doesn't look like the number on the back of the bottle.³

What does vitamin D do in the body?

According to the NHS, vitamin D helps control how much calcium and phosphate are in your body, and those keep your bones, teeth, and muscles healthy.¹ Vitamin D is often paired with vitamin K, which also contributes to the maintenance of normal bones.²

Food supplements should not be used as a substitute for a varied, balanced diet and healthy lifestyle.

Where the evidence stops

Vitamin D has been tested against a long list of conditions over the past twenty years, and it's worth knowing that most of that research hasn't landed where people assume it has.

When the US expert committee responsible for setting vitamin D intake levels went through the evidence, it found the picture either too thin or too contradictory to draw any conclusion about health outcomes beyond bone health, and a separate review covering nearly 250 studies arrived in the same place.³

In practice that means trials haven't shown vitamin D supplements to reduce the risk of cancer, lower the risk of heart disease, prevent or treat depression, prevent multiple sclerosis, stop prediabetes becoming type 2 diabetes, or help with weight loss.³ In several of those areas the pattern has been the same: observational studies spotted a link, and the trials that followed couldn't confirm it.³ None of which makes vitamin D unimportant. It just means the reason to get enough of it is the one the research actually supports, which is your bones, teeth, and muscles.

Why so many people in the UK run low in winter

Your body makes vitamin D when sunlight hits your skin, and from roughly late March or early April through to the end of September, most people can make everything they need that way.¹

Between October and early March, though, we simply don't make enough of it from sunlight in the UK.¹

That one catches people out, because it has nothing to do with how bright the day looks or how long you stay out in it. The sun sits too low in the sky over a British winter for the right kind of light to reach you at all. Food won't quietly fill the gap either, since even a well-balanced diet is unlikely to contain enough vitamin D to meet your needs.⁵

Read more about the signs of low vitamin D.

Getting vitamin D from sunlight

You don't need to sunbathe to get it. From late March or early April to the end of September, short breaks in the sun are enough for most people, though how much you need depends on your skin tone and on the time of day and year.^1,4

The important part is to cover up or protect your skin before it goes red or starts to burn, because that's the point at which you're raising your risk of skin damage and skin cancer rather than topping up your vitamin D.¹ The British Dietetic Association suggests staying in the shade between 11am and 3pm, covering up with suitable clothing and sunglasses, and using at least factor 30 sunscreen.⁵ One thing you don't have to worry about is that you can't overdose on vitamin D from sunlight.¹

Do sunbeds give you vitamin D?

They aren't a sensible route to it, and the advice from UK dietitians is to avoid sunbeds and tanning beds altogether.⁵

Foods high in vitamin D

Sunshine, not food, is your best source of vitamin D, and only a small number of foods contain it at all, usually in small amounts.^1,5 The ones worth knowing about are:^1,5

• Oily fish, such as salmon, sardines, trout, herring, or mackerel

• Red meat

• Egg yolks

• Liver, though avoid liver if you're pregnant

• Some mushrooms

• Fortified foods, such as some fat spreads, breakfast cereals, and unsweetened soya drinks

Cows' milk is a common assumption that doesn't hold in the UK, because it isn't fortified here the way it is in some other countries.¹ 

How much vitamin D do you need?

Children from the age of 1 and adults need 10 micrograms a day, and that includes women who are pregnant or breastfeeding.¹ Everyone should consider taking a daily supplement containing that amount through autumn and winter.¹

Some people should keep taking it all year round. The Department of Health and Social Care advises this if you aren't often outdoors, perhaps because you're frail or housebound, if you live somewhere like a care home, if you usually wear clothes that cover most of your skin outdoors, or if you have dark skin, for example an African, African-Caribbean, or south Asian background.¹

The BDA extends that list a little further, to people over 65, people with a high BMI, and people with conditions that affect how they absorb or handle vitamin D, such as Crohn's disease, coeliac disease, or severe liver or chronic kidney disease.⁵

Whatever group you're in, don't take more than 100 micrograms (4,000 IU) a day, as it could be harmful. That applies to adults, including pregnant and breastfeeding women and older people, and to children aged 11 to 17.¹ Some people have medical conditions that mean even that's too much for them, so if you're in any doubt, ask your doctor, and if your doctor has already told you to take a different amount, follow their advice over anything you read here.¹

Vitamin D in IU and micrograms

Labels use two different units, which makes packs annoyingly hard to compare until you know that 1 microgram is the same as 40 IU.¹

When should you take vitamin D?

There's no NHS advice on the best time of day, so the honest answer is that it matters less than people think.

What the NHS does say is to swallow the tablets whole with water and take them with your main meal of the day.⁶ There's a reason for the meal, and it’s that vitamin D dissolves in fat, so having some fat in your gut helps you absorb it, although you'll absorb some of it either way.³ Beyond that, taking it at the same time each day is mostly about remembering to take it at all, which is the thing that actually makes the difference over a winter.

Can you take it at night? Yes. No time of day has been shown to work better than another.

Can you take too much vitamin D?

You can, although it almost always comes from supplements rather than from food or sunlight.^1,3

Taking too much over a long period causes calcium to build up in your body, which can weaken your bones and damage your kidneys and your heart.¹ For most people 10 micrograms a day is plenty, the limit for adults is 100 micrograms (4,000 IU) a day, and much larger amounts are unnecessary as well as potentially harmful.^1,5

If you're already taking a vitamin D supplement, check whether you're also taking cod liver oil or a multivitamin that contains vitamin D, because those amounts stack up without you noticing.⁵

If you take other medicines

Vitamin D can interact with a few, including orlistat, statins, steroids, and thiazide diuretics.³ The BDA also suggests checking first if you've been told your calcium is high, or if you take medicines for liver or kidney disease, cancer, epilepsy, fungal infections, or heart disease.⁵

If you take anything regularly, it's worth a two minute conversation with a pharmacist before you start.

Numan supplements

Take a look at the Numan supplement range. Check with a clinician first if you have any medical condition. Food supplements should not be used as a substitute for a varied, balanced diet and healthy lifestyle.

Common questions

Is vitamin D3 the same as vitamin D?

Vitamin D3 is one type of vitamin D. The term "vitamin D" covers both D2 and D3, and D3 is the type your skin makes from sunlight.^1,3

What happens if I take vitamin D3 every day?

Taking 10 micrograms (400 IU) a day is in line with NHS advice.¹ Going above 100 micrograms (4,000 IU) a day over a long period can be harmful, unless a doctor has told you to.¹

Do I need vitamin D in summer?

From late March or early April to the end of September, most people make all they need from sunlight.¹ If you're in one of the higher-risk groups above, the NHS advises 10 micrograms a day all year.¹

Can I take vitamin D at night?

Yes. There's no NHS advice on a best time of day, so taking it with a meal at whatever time you'll remember is the practical answer.⁶

Is 4,000 IU of vitamin D3 safe?

4,000 IU (100 micrograms) is the most adults should take in a day unless a doctor advises more, and it shouldn't be combined with another vitamin D supplement.^1,5

Is vitamin D3 vegan?

Usually not, because most vitamin D3 is made from lanolin, which comes from sheep's wool. Vitamin D2 is the plant-based alternative, and the BDA lists it as suitable for vegetarians and vegans.⁵ It’s worth confirming with whoever you buy your supplements from if you have diet or lifestyle requirements. 

How do I know if my level is low?

A blood test is the only way to know, and what it measures is 25-hydroxyvitamin D, the stored form.³ Numan's Nutritional Health Blood Test includes vitamin D, and you can see the rest in the Numan blood test range.

References

1. National Health Service. Vitamins and minerals: vitamin D [Internet]. London: NHS; 2020 [page last reviewed 2020 Aug 3; cited 2026 Nov 10]. Available from: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/

2. Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register [Internet]. London: DHSC [cited 2026 Nov 10]. Available from: https://www.gov.uk/government/publications/great-britain-nutrition-and-health-claims-nhc-register

3. National Institutes of Health, Office of Dietary Supplements. Vitamin D: fact sheet for health professionals [Internet]. Bethesda (MD): NIH ODS; 2025 [updated 2025 Jun 27; cited 2026 Nov 10]. Available from: https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/

4. Cancer Research UK. Sun and vitamin D [Internet]. London: Cancer Research UK; 2024 [last reviewed 2024 May 15; cited 2026 Nov 10]. Available from: https://www.cancerresearchuk.org/about-cancer/causes-of-cancer/sun-uv-and-cancer/sun-and-vitamin-d

5. British Dietetic Association. Vitamin D: food fact sheet [Internet]. Birmingham: BDA; 2024 [cited 2026 Nov 10]. Available from: https://www.bda.uk.com/resource/vitamin-d.html

6. National Health Service. Colecalciferol: how and when to take it [Internet]. London: NHS; 2023 [page last reviewed 2023 Jan 10; cited 2026 Nov 10]. Available from: https://www.nhs.uk/medicines/colecalciferol/how-and-when-to-take-colecalciferol/