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.
Choosing a Vitamin D Supplement
A few practical things to look for on the label:
- 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
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?
A single symptom rarely points to one nutrient. Our free 2-minute AI analysis looks at your lifestyle, diet, and sun exposure together and builds a personalised 30-day wellness plan.
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.
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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