If you live in the UK, your body cannot make vitamin D from sunlight for roughly half the year. From October to March, the sun sits too low in the sky for the UV-B rays your skin needs to synthesise it. That’s not a scare story. It’s a geography problem, and most people don’t know it exists until they’re already running low.
This guide covers why vitamin D drops in winter, what deficiency actually feels like, how much to take, and why the form you choose matters more than most supplement advice lets on. Three-quarters of Brits currently fail to follow official advice on vitamin D supplementation, so if you’re not taking it, you’re in the majority.
Why Your Body Can’t Make Vitamin D in a UK Winter
Vitamin D is produced when UV-B radiation from sunlight hits your skin. The problem is that UV-B only reaches the earth’s surface at useful angles when the sun is above 45 degrees elevation. The UK sits between 50° and 60° north latitude, meaning the sun never reaches that angle between October and March.
The NHS acknowledges this directly, recommending that everyone in the UK consider a vitamin D supplement during autumn and winter. Public Health England has gone further, noting that around 1 in 5 UK adults have low vitamin D levels at any given time, and that figure rises significantly in winter months.
Darker skin tones, spending most of the day indoors, being over 65, and having higher body fat all increase your risk further. A 2026 retrospective analysis published in PMC found that 62.5% of adults over 65 were deficient and a further 22.7% were insufficient, meaning fewer than 15% of older adults in the UK have adequate vitamin D levels heading into winter.
Food Sources of Vitamin D: What Actually Helps in a UK Diet
Food is a minor player in your total vitamin D supply, but it’s not nothing — and the picture looks different in the UK than it does in the US, which is where most online food-source lists come from.
In the US, milk is fortified with vitamin D by default. In the UK, it isn’t. A glass of standard UK cow’s milk contains almost no vitamin D. Most UK-sold fortified plant milks (oat, soy, almond) do contain added vitamin D, typically around 1 µg (40 IU) per 100 ml, but you’d need to drink two litres daily to reach the NHS minimum of 10 µg — which isn’t realistic or sensible.
Oily fish is the one food group that genuinely delivers. A 100 g portion of wild salmon provides 400–1,000 IU depending on the catch; farmed salmon sits closer to 100–250 IU. Mackerel, sardines, herring, and trout are all solid sources. Tinned salmon and sardines are often overlooked but contribute meaningfully when eaten weekly.
| Food (100 g serving) | Vitamin D (IU) | UK context |
|---|---|---|
| Wild salmon | ~600 IU | Best single dietary source |
| Farmed salmon | ~150 IU | Most UK supermarket salmon is farmed |
| Tinned sardines (in oil) | ~190 IU | Cheap, shelf-stable, widely available |
| Mackerel | ~350 IU | Smoked or fresh both count |
| Egg yolk (one large) | ~40 IU | Free-range tends to be higher |
| UK cow’s milk (unfortified) | ~2 IU per 100 ml | Negligible; unlike US, UK dairy isn’t fortified |
| Fortified plant milk | ~40 IU per 100 ml | Check label — not all brands fortify |
| Fortified breakfast cereals | ~40–80 IU per serving | Token amount; easily over-relied on |
The Scientific Advisory Committee on Nutrition (SACN) reviewed UK dietary vitamin D intake in its 2016 report and concluded that average intake from food alone sits at roughly 2–3 µg per day — around a quarter of the NHS recommendation. Even people who eat oily fish twice a week rarely hit 10 µg daily from food alone in winter. That’s the gap supplementation is designed to cover.
The practical take: include oily fish once or twice a week for the broader nutritional benefits (omega-3, selenium, B12), but don’t rely on food to correct a deficiency or maintain adequate levels through winter. A daily D3 supplement in the 1,000–4,000 IU range does the heavy lifting. Our Vitamin D3K2 pairs D3 with the co-factors (K2, zinc, boron) that food alone can’t reliably deliver in combination.
Symptoms of Low Vitamin D in Winter
Vitamin D deficiency often goes undetected because its symptoms are easy to attribute to other things: tiredness, the time of year, stress, or just getting older. That ambiguity is part of what makes it so common.
Symptoms associated with low vitamin D include:
- Persistent fatigue: feeling tired even after adequate sleep (see our guide to vitamins for tiredness if this is a recurring issue)
- Low mood or seasonal low mood: research published in Nutrients (2020) found an association between low vitamin D levels and depressive symptoms, particularly in winter months
- Muscle weakness or aches: vitamin D regulates calcium in muscles, which is essential for contraction and relaxation. A meta-analysis in the Journal of Science and Medicine in Sport found that supplementation significantly improved muscle strength in both upper and lower limbs, with the most pronounced effects in people who started with low levels
- More frequent infections: a 2026 UK Biobank study (36,258 participants, University of Reading) found that people with severe vitamin D deficiency were 33% more likely to be hospitalised for respiratory tract infections. A 2022 meta-analysis in Frontiers in Nutrition found that people supplementing with vitamin D were 22% less likely to contract flu. For more on the evidence, see does vitamin D help with colds?
- Bone and joint pain: particularly in the lower back, hips, and legs; vitamin D is essential for calcium absorption
If several of these sound familiar from autumn through to spring, it’s worth speaking to your GP about a blood test. The standard measure is serum 25-hydroxyvitamin D; levels below 25 nmol/L are considered deficient in the UK and below 50 nmol/L is insufficient.
How Much Vitamin D Should You Take in Winter?
The NHS recommends 10 micrograms (400 IU) per day for adults during autumn and winter. That’s the floor, sufficient to prevent severe deficiency in otherwise healthy adults.
For many people over 40, particularly those with limited sun exposure, indoor jobs, or darker skin, functional levels often require more. Many GP practices now recommend 1,000–2,000 IU daily for adults in higher-risk groups, in line with guidance from the British Society for Rheumatology and the National Osteoporosis Society.
The upper safe limit set by the European Food Safety Authority is 4,000 IU per day for adults. Supplementing within the 1,000–2,000 IU range is considered safe for most adults without medical supervision, but if you’re unsure about your levels, a blood test gives you a precise baseline.
One practical note: vitamin D is fat-soluble, so taking your supplement with a meal that contains fat significantly improves absorption. First thing in the morning on an empty stomach is one of the less effective ways to take it.
D3 vs D3K2: Why the Form Matters
Not all vitamin D supplements are equivalent. There are two forms: D2 (ergocalciferol) and D3 (cholecalciferol). Research consistently shows D3 raises and maintains blood levels more effectively. A meta-analysis in The American Journal of Clinical Nutrition (2012) found D3 was approximately 87% more potent at raising serum vitamin D levels than D2. More recent research has gone further, finding that D2 supplements can actually lower the body’s levels of D3, the form it uses most effectively. If your supplement label says D2 (ergocalciferol), it’s worth switching.
The more important question for long-term supplementation is whether you should take D3 alone or D3 alongside vitamin K2.
Vitamin D increases calcium absorption from the gut. Vitamin K2 (specifically the MK-7 form) directs that calcium towards bones and teeth rather than soft tissue. Without sufficient K2, some researchers are concerned that high-dose vitamin D supplementation could contribute to arterial calcification over time. The evidence is still developing, but for adults supplementing year-round or at higher doses, pairing D3 with K2 is a reasonable precaution.
Our Vitamin D3K2 combines 2,000 IU of vitamin D3 with vitamin K2 (MK-7), zinc, boron, and MCT oil as a fat carrier to support absorption. It’s formulated specifically for adults who want year-round immune and bone support rather than a basic top-up dose. If you’d like more detail on the D3K2 combination, we’ve covered the evidence in depth in our guide to the health benefits of vitamin D3 and K2.
Other Vitamins Worth Considering in Winter
Vitamin D is the most important supplement for UK winters, but it’s not the only one worth thinking about.
Magnesium plays a role in activating vitamin D. The enzyme that converts vitamin D into its active form requires magnesium as a cofactor, and low magnesium can blunt the effect of supplementation even when doses are adequate. Adults over 40 are more likely to be low in magnesium, partly because absorption declines with age and partly because modern diets tend to be poor sources. Our Magnesium Glycinate uses the form with the highest bioavailability and the fewest digestive side effects.
NAC (N-acetyl cysteine) is less well known but worth considering if immune resilience is a priority. NAC is a precursor to glutathione, the body’s primary antioxidant, and research suggests it may support respiratory health and help reduce the duration and severity of winter respiratory infections. A review in European Respiratory Journal found NAC supplementation significantly reduced the frequency of influenza-like episodes in older adults. You can find our formulation at NAC+.
These three (vitamin D3K2, magnesium, and NAC) form a practical winter stack for adults over 40 who want to maintain energy, immune function, and bone health through the darker months. Professor Philip Calder, a leading UK nutritional immunologist, has warned that the UK’s immune health is underprepared for future health challenges specifically because of widespread gaps in vitamin D, zinc, selenium, and omega-3s.
FAQs: Vitamin D in Winter
Can you get enough vitamin D from food in winter?
Unlikely. Very few foods contain meaningful amounts of vitamin D. Oily fish (salmon, mackerel, sardines), egg yolks, and fortified foods like some cereals and plant milks are the main sources, but even eating these regularly rarely provides more than 100–200 IU per day. That’s well below the 1,000–2,000 IU many adults need in winter. Supplementation is the practical solution for most people.
Is milk a good source of vitamin D in the UK?
Not really. Unlike the US, where cow’s milk is routinely fortified with vitamin D, UK cow’s milk contains almost none (around 2 IU per 100 ml). UK-sold fortified plant milks (oat, soy, almond) often contain added vitamin D at around 40 IU per 100 ml, but you’d need to drink roughly two litres a day to hit the NHS minimum. Milk is not a practical source of vitamin D in a UK diet — oily fish and supplementation are far more reliable.
Can you get vitamin D from winter sun in the UK?
No. Between October and March, UV-B radiation in the UK is too weak to trigger vitamin D synthesis in your skin, regardless of how sunny it feels. You can still benefit from winter sunlight for mood and circadian rhythm, but it won’t produce vitamin D. Supplementation is the only reliable option during this period.
What are the symptoms of vitamin D deficiency in winter?
The most common symptoms are persistent fatigue, low mood, muscle weakness, aches, and frequent infections. These overlap with many other conditions, which is why deficiency often goes undiagnosed. A GP blood test is the only way to confirm your levels.
How much vitamin D should I take in winter in the UK?
The NHS baseline recommendation is 400 IU (10 micrograms) per day. For adults over 40 or those with limited sun exposure, 1,000–2,000 IU is more commonly recommended by specialists. The safe upper limit for adults is 4,000 IU daily. Always take it with a fat-containing meal to improve absorption.
Is vitamin D3 better than D2 for winter supplementation?
Yes. Vitamin D3 (cholecalciferol) is significantly more effective at raising and maintaining blood levels than D2 (ergocalciferol). Most UK supplement recommendations now specify D3. If you’re also taking higher doses long-term, pairing it with K2 (MK-7) is worth considering to support healthy calcium metabolism.
Does vitamin D help with winter depression?
The evidence is mixed but suggestive. Several studies have found an association between low vitamin D levels and seasonal low mood, and vitamin D receptors are present throughout the brain. A 2020 review in Nutrients found that vitamin D supplementation may support mood in people who are deficient. It’s unlikely to be a standalone solution for clinical depression, but correcting deficiency may help if low mood tracks with the winter months. For a broader look at the evidence on SAD and treatment options, see our post on treatments for seasonal affective disorder.
References
- NHS. Vitamin D — NHS. Updated 2023.
- Public Health England. NDNS: Results from Years 9 to 11 (2016 to 2017 and 2018 to 2019). 2020.
- Scientific Advisory Committee on Nutrition (SACN). Vitamin D and Health. Public Health England. 2016. gov.uk/sacn-vitamin-d-and-health-report
- Lu Z, et al. An evaluation of the vitamin D3 content in fish: is the vitamin D content adequate to satisfy the dietary requirement for vitamin D? Journal of Steroid Biochemistry and Molecular Biology. 2007;103(3-5):642–644.
- Tripkovic L, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status. American Journal of Clinical Nutrition. 2012;95(6):1357–1364. doi:10.3945/ajcn.111.031070
- Shaffer JA, et al. Vitamin D supplementation for depressive symptoms: a systematic review and meta-analysis of randomised controlled trials. Nutrients. 2020;12(12):3722. doi:10.3390/nu12123722
- De Flora S, Grassi C, Carati L. Attenuation of influenza-like symptomatology and improvement of cell-mediated immunity with long-term N-acetylcysteine treatment. European Respiratory Journal. 1997;10(7):1535–1541. doi:10.1183/09031936.97.10071535
- Uwitonze AM, Razzaque MS. Role of magnesium in vitamin D activation and function. Journal of the American Osteopathic Association. 2018;118(3):181–189. doi:10.7556/jaoa.2018.037
- University of Reading. Vitamin D deficiency linked to respiratory infection risk. UK Biobank cohort study, 36,258 participants. 2026.
- Health and Food Supplements Information Service (HSIS). Vitamin D: a key nutrient for self-care this winter. 2024.
- PMC. How deficient are we? A retrospective analysis of vitamin D status. 2026.
- Pham H, et al. Vitamin D supplementation and influenza prevention: meta-analysis. Frontiers in Nutrition. 2022. doi:10.3389/fnut.2022.1082553
This article is for informational purposes only and does not constitute medical advice. If you have concerns about vitamin D deficiency or are considering supplementation at higher doses, speak to your GP or a qualified healthcare practitioner. Supplements should not be used as a substitute for a varied and balanced diet.
