Vitamin D 101: Deficiency, Benefits and How Much You Actually Need

Updated July 2026

Vitamin D gets talked about constantly, but a lot of what's out there overstates what it actually does. This is a full rundown of what vitamin D genuinely does for your body, why deficiency is so common in the UK, what to watch for, and how much you actually need. If you're after the full breakdown of food sources, including exact amounts, vegan options and the difference between D2 and D3, we've covered that in detail in our dedicated vitamin D food sources guide, so this one focuses on the rest.

What are the benefits of vitamin D?

Vitamin D contributes to the normal absorption and use of calcium and phosphorus, to normal blood calcium levels, to the maintenance of normal bones, muscle function and teeth, to the normal function of the immune system, and to the normal process of cell division. Those are the genuinely established, authorised functions of the vitamin, and they're worth taking seriously on their own without needing to be dressed up as anything more.

In practice, that means vitamin D is doing quiet, structural work: helping your gut pull calcium out of food, helping that calcium actually get used to build and maintain bone, keeping your muscles functioning properly, and supporting your immune system's normal day-to-day operation. It's not a cure for anything, and we'd rather be upfront about that than oversell it.

What causes vitamin D deficiency?

Mostly, not making or getting enough of it. Your skin produces vitamin D when UVB rays hit it directly, and in the UK, the sun isn't strong enough to do that between roughly October and early March, which is why deficiency is so much more common here than in sunnier countries. Beyond season and latitude, a few other things commonly drain or limit your vitamin D levels:

• Spending most of your time indoors, or covering most of your skin for cultural or religious reasons
• Having darker skin, since melanin absorbs UV radiation and reduces how much reaches the layers of skin that produce vitamin D
• Getting older, as skin becomes less efficient at producing vitamin D from sunlight with age
• Malabsorption conditions such as coeliac disease, Crohn's disease, cystic fibrosis, or having had gastric bypass surgery, all of which can interfere with absorbing fat-soluble vitamins like D
• Certain medications, including some anti-epileptic drugs and long-term steroids, which can affect how your body processes vitamin D
• Kidney or liver conditions, since both organs are involved in converting vitamin D into the active form your body actually uses

What are the signs of low vitamin D?

Fatigue, general aches, bone or back pain, and muscle weakness are the main ones. Over the long term, low vitamin D also raises your risk of fractures and falls, and in severe, prolonged cases it can lead to rickets in children (softened, weakened bones) or osteomalacia in adults, a similar softening of the bone structure. If several of these sound familiar and they've been sticking around, it's worth asking your GP about a blood test rather than trying to self-diagnose from a symptom list.

This is genuinely common in the UK. Recent UK Biobank data found around half of adults have lower-than-optimal vitamin D levels, rising to as many as two-thirds during winter and spring, and rates are notably higher again in South Asian and Black communities. If any of that applies to you, it's a solid reason to keep an eye on your intake year-round rather than only thinking about it in winter.

Does vitamin D affect your mood or sleep?

Possibly, and it's a genuinely active area of research, but it's not settled enough to make any firm promise. Some studies have found a link between low vitamin D levels and low mood, and a systematic review and meta-analysis looking at vitamin D supplementation and sleep found associations between vitamin D deficiency and sleep difficulties, shorter sleep duration and more night-time waking. That's worth knowing about, but association isn't the same as proof of cause and effect, and "vitamin D" doesn't currently carry an approved claim relating to mood or sleep in the UK or EU. If you feel noticeably better once your levels come back up, that's a reasonable thing to notice, just not something to expect on a fixed timeline.

How much vitamin D do you need, and can you take too much?

The NHS recommends 10 micrograms (400 IU) a day for adults and children over 4, and suggests everyone consider a daily supplement over autumn and winter, when UK sunlight isn't strong enough for your skin to make any at all. Some groups are advised to take that same amount all year round: anyone housebound or rarely outdoors, anyone who covers most of their skin, and people with darker skin, including those of African, Afro-Caribbean or South Asian family background.

Yes, you can take too much, though it's almost always a supplement issue rather than something that happens from food or sunlight. The NHS advises against taking more than 100 micrograms (4,000 IU) a day for adults, since sustained high doses can lead to a build-up of calcium in the blood, known as hypercalcaemia. Symptoms can include nausea, vomiting, weakness, unusual thirst, needing to urinate more than usual, and confusion, and in serious, prolonged cases it can affect the kidneys. Stick to the dose stated on your product's label unless a doctor has advised otherwise.

What foods contain vitamin D?

The short version: oily fish and cod liver oil are the strongest natural sources, followed by fortified foods like some milks, plant milks and orange juice, specifically UV-treated mushrooms, and egg yolks in smaller amounts. Ordinary supermarket mushrooms grown in the dark contain very little, so it's the UV-treated ones you want to look for on the label. For the full amounts per serving, which foods are and aren't vegan-friendly, and the difference between vitamin D2 and D3 supplements, our food sources guide covers all of it in detail.

Quick questions, quick answers

What are the main functions of vitamin D?
Helping your body absorb and use calcium and phosphorus, maintaining normal blood calcium levels, supporting normal bones, muscle function and teeth, keeping the immune system functioning normally, and supporting normal cell division.

Is it fine to take vitamin D every day?
Yes, for most people a daily 10 microgram dose is exactly what's recommended, particularly through autumn and winter. Some groups are advised to take it daily all year round.

Is vitamin D deficiency actually common in the UK?
Yes, very. Recent data suggests around half of UK adults have lower-than-optimal levels, rising to as many as two-thirds over winter and spring, with notably higher rates in South Asian and Black communities.

What's the difference between vitamin D2 and D3?
D3 (cholecalciferol) raises and sustains blood vitamin D levels more effectively than D2 (ergocalciferol). D3 has traditionally come from animal sources like fish oil or lanolin, though a vegan D3 made from lichen is now available too. We cover this in full in our food sources guide.

None of this is medical advice. If you're concerned about your vitamin D levels, or you've had several of the symptoms above for a while, your GP can arrange a simple blood test to check where you actually stand.

Looking to top up your levels through the colder months? Have a look at our immune health range.

Got more questions about vitamins or general wellbeing? Have a browse of the rest of our Health Insiders blog, or send us a message on Instagram.