Vitamin D Deficiency Symptoms, Causes & How Much You Need

What Is Vitamin D?

Vitamin D is a fat-soluble vitamin that behaves more like a hormone than a typical nutrient. Your skin produces it when exposed to UVB sunlight, and it’s also found in a small number of foods and supplements. Unlike most vitamins, the body can manufacture its own supply — but only under the right conditions, which is exactly why deficiency is so widespread across the UK and Northern Europe.

There are two forms you’ll see on labels: D2 (ergocalciferol), usually plant- or fungus-derived, and D3 (cholecalciferol), the form made in human skin and generally considered more effective at raising blood levels.

Why Your Body Needs It

Vitamin D’s best-established role is in mineral metabolism and skeletal health. Under EU Regulation (EC) 1924/2006, the following functions are authorised health claims backed by EFSA’s scientific review:

Contributes to the normal absorption and utilisation of calcium and phosphorus
Contributes to normal blood calcium levels
Contributes to the maintenance of normal bones and normal teeth
Contributes to normal muscle function
Contributes to the normal function of the immune system
Has a role in the process of cell division

Because vitamin D also supports normal immune signalling, it’s one of the most commonly recommended nutrients heading into autumn and winter.

Signs of Vitamin D Deficiency

Mild deficiency is often silent, which is part of the problem. When symptoms do appear, the most frequently reported are:

Persistent tiredness or low energy that doesn’t improve with rest
Bone pain or general aching, particularly in the lower back and legs
Muscle weakness or cramps
Low mood, especially in winter months
Frequent colds or infections
In children, delayed growth; in adults, an increased risk of fractures and falls

These symptoms overlap with several other deficiencies (iron and B12 in particular), so they’re a signal to investigate further — via a GP blood test — rather than a diagnosis in themselves.

Who’s Most at Risk

Vitamin D deficiency isn’t a niche problem in the UK and Northern Europe — public health bodies estimate roughly 1 in 5 adults have low vitamin D status, and it rises sharply in winter when sunlight isn’t strong enough to trigger skin synthesis. You’re at higher risk if you:

Spend most of your time indoors (office workers, shift workers, older adults)
Have darker skin, which needs longer sun exposure to produce the same amount of vitamin D
Are pregnant or breastfeeding
Are over 65, since skin becomes less efficient at synthesising vitamin D with age
Cover most of your skin for cultural or religious reasons
Follow a vegan diet (most natural dietary sources are animal-derived)
Have a BMI over 30, malabsorption conditions (e.g. Crohn’s, coeliac disease), or take medications that interfere with vitamin D metabolism
Live in the northern half of Europe, where UVB sunlight is too weak to produce vitamin D for roughly half the year

This is why UK health authorities recommend that everyone consider a daily supplement during autumn and winter, and that at-risk groups take one year-round.

How Much Do You Need?

UK guidance (NHS/NICE) recommends 10 micrograms (400 IU) per day for adults and children over 1, including during pregnancy and breastfeeding. This is a baseline maintenance dose designed to prevent deficiency, not a therapeutic dose for someone already deficient — that requires medical guidance and blood testing.

Food Sources

Few foods naturally contain meaningful vitamin D, which is part of why supplementation is so widely recommended:

Oily fish (salmon, mackerel, sardines) — the richest natural source
Egg yolks
Red meat and liver
Fortified foods: some cereals, plant milks, and spreads in the UK/EU are fortified with vitamin D
UV-exposed mushrooms (a rare option for vegans)

Two to three portions of oily fish a week provides a meaningful contribution, but for most people in the UK/EU, diet and sunlight alone aren’t reliable enough to maintain sufficient levels year-round — which is why national guidance leans toward supplementation.

Vitamin D2 or D3: Which Is Better?

For most people, D3 (cholecalciferol) is the better choice. It’s the form your own skin produces from sunlight, and it raises and maintains blood levels more effectively than D2.

D2 (ergocalciferol) is derived from UV-exposed yeast or fungi. It does work — it was the standard form in prescription supplements for decades — but it’s less potent unit for unit, and it clears from the body faster, so blood levels tend to fall back sooner between doses.

The practical difference matters most if you’re correcting a low level rather than maintaining a normal one. For simple maintenance through a UK winter, either will do the job; D3 will do it with a smaller dose.

If you avoid animal products: most D3 is derived from lanolin (sheep’s wool), but D3 from lichen is widely available and is fully vegan. That’s usually a better option than defaulting to D2.

D3 over D2 where possible — it’s generally better absorbed and more effective at raising and maintaining blood levels
K2 pairing — some formulations combine D3 with vitamin K2, which supports how calcium is directed toward bone rather than soft tissue; this is a common combination in higher-strength products
Dose appropriate to your situation — 400 IU (10mcg) covers general maintenance; higher-strength products (1,000–2,000 IU) are common for people with limited sun exposure, but very high doses should be discussed with a healthcare professional
Third-party tested / recognisable manufacturer — look for established supplement brands with transparent sourcing

How Much Vitamin D Should You Take in Winter in the UK?

For most UK adults, 1000–2000 IU (25–50mcg) daily from October to March is a sensible range. The NHS minimum recommendation is 10mcg (400 IU) daily for everyone over the age of 1 — that’s a floor designed to prevent deficiency across the population, not a target for someone with limited sun exposure.

The reason winter is treated differently isn’t marketing. From roughly October to March, UVB sunlight in the UK is too weak for your skin to produce any vitamin D at all, regardless of how long you spend outdoors. Levels built up over summer decline steadily through autumn and bottom out in late winter. This is why UK guidance recommends that everyone consider supplementing during autumn and winter, and that higher-risk groups supplement year-round.

Who should sit at the upper end of that range:

Office workers and anyone indoors during daylight hours
People with darker skin, which needs considerably longer sun exposure to produce the same amount
Adults over 65
People with a BMI over 30, since vitamin D is sequestered in fat tissue
Anyone who covers most of their skin outdoors

The ceiling: EFSA sets the tolerable upper intake at 100mcg (4000 IU) daily for adults. Doses above that should only be taken on a healthcare professional’s advice, typically to correct a confirmed deficiency.

A 25-hydroxyvitamin D blood test through your GP removes the guesswork entirely — worth doing if you’ve been supplementing and still feel run down.

Our pick: Terranova Vitamin D3 2000 IU + K2 Complex — a higher-strength D3/K2 combination suited to people with limited sun exposure or those in the higher-risk groups above.

Not sure if you’re low on vitamin D — or something else?

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This page is for general educational purposes only and is not medical advice. If you’re concerned about a deficiency, speak with your GP — a simple blood test can confirm your vitamin D status.

Frequently Asked Questions

Can you take magnesium, zinc and vitamin D together?

Yes. There’s no meaningful interaction between these three at normal supplemental doses, and they’re often combined in a single product. Two practical notes: vitamin D is fat-soluble, so take it with a meal containing some fat; and if you also take iron, keep zinc at the opposite end of the day, since zinc and iron compete for absorption. Many people take magnesium in the evening, as it’s commonly used to support sleep and relaxation — see our magnesium guide and zinc guide.

Is it better to take vitamin D in the morning or at night?

Either works, as long as you take it with food containing fat. Morning is the more common choice, partly because it’s easier to remember alongside breakfast. Some people report that vitamin D taken late in the evening disrupts their sleep, though this isn’t consistently shown in research — if you notice it, move the dose earlier.

How long does it take for vitamin D levels to rise?

With daily supplementation at 1000–2000 IU, blood levels typically rise over several weeks to a few months. Large single “bolus” doses raise levels faster but have performed less well than daily dosing in trials of respiratory infection outcomes.

Do I still need vitamin D if I take a multivitamin?

Check the label. Most multivitamins contain 400 IU — enough to meet the NHS minimum, but possibly low if you’re in one of the higher-risk groups above. Add up your total across all products before adding a standalone supplement.

Can you take too much vitamin D?

Yes, but it’s uncommon at sensible doses. Toxicity is generally associated with very high intakes (well above 4000 IU daily) sustained over long periods, and causes raised blood calcium, nausea and kidney problems. At 1000–2000 IU daily the risk is essentially nil.

Sources & Further Reading

NHS — Vitamin D
NIH Office of Dietary Supplements — Vitamin D Fact Sheet
EFSA — Health claims on vitamin D
Linus Pauling Institute — Vitamin D Micronutrient Information
Examine.com — Vitamin D

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