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Simon Parrott – Vitacuity Ltd
From October to March, the UK sun is too low in the sky to produce the specific ultraviolet rays (UVB) needed to make vitamin D in your skin. This creates a five-to-six-month period each year where sunlight, no matter how bright or how long you spend outside, cannot help your body produce vitamin D. Research shows nearly half of UK adults have insufficient vitamin D levels, and this problem is worse for people with darker skin, those who are overweight, and older adults.
Your body cannot store enough vitamin D from summer sun to last through winter, despite what many people hope. The solution is straightforward: take a vitamin D3 supplement from October through March. Vitamin D3 (cholecalciferol) works better than vitamin D2 for maintaining blood levels over winter. A daily dose of 1,000 to 2,000 IU is widely considered safe and effective, though the UK government’s official recommendation of 400 IU is lower. This becomes even more important if you eat little oily fish, red meat, or eggs, since the UK has no mandatory fortification of food with vitamin D.
Testing before you supplement is optional since deficiency is already so common and supplementation at these doses carries essentially no risk. The practical approach is to simply take a vitamin D3 supplement through winter and beyond if your diet lacks natural sources.
Verdict: UK residents cannot make meaningful vitamin D from winter sunlight and should supplement with vitamin D3 daily from October to March, particularly if they have darker skin, are overweight, or eat little oily fish.
Here’s something most people in the UK simply don’t know: from October through to March, the sun is essentially useless for making vitamin D in your skin. Not *less useful*. Not *a bit weaker*. Completely, biologically useless, regardless of whether the sky is blue, the weather feels mild, or you’re standing outside at lunchtime. You could spend every clear winter’s day on a park bench and your vitamin D levels would still quietly decline, month by month, until spring. So if you’ve been telling yourself that you “get outside enough” to stay topped up, this post might change your mind. And if you’re aged 40-65, the stakes are higher than you might think.
Vitacuity has analysed over 1.77 million research papers and selected the most relevant studies on this topic. Here’s what the science actually shows.
Vitamin D is unusual. It’s technically classified as a vitamin, an essential nutrient we must obtain from outside the body, but it behaves more like a hormone. When UVB radiation from sunlight hits your skin, a chemical reaction converts a cholesterol-like compound into cholecalciferol (vitamin D3), which your liver then converts into the active, measurable form known as 25-hydroxyvitamin D, or 25(OH)D [8].
The critical word in all of that is UVB. Not all sunlight is created equal. The sun emits a spectrum of ultraviolet light, and only the UVB wavelengths, specifically those between 290 and 315 nanometres, are capable of triggering vitamin D synthesis in the skin [8]. UVA rays, which are present year-round, cause tanning and skin damage but do nothing for vitamin D.
Here’s the problem for anyone living in the UK: UVB radiation at these vitamin D-producing wavelengths only reaches the Earth’s surface when the sun is high enough in the sky, typically when it’s above 45 degrees on the horizon. At UK latitudes (roughly 50–58°N), the sun simply never climbs high enough between October and March for meaningful UVB to penetrate the atmosphere. The UV rays hit the atmosphere at too shallow an angle, scatter, and are absorbed before they reach you. The sun may look bright. The sky may be blue. But the UVB that makes vitamin D is simply not there [6].
This isn’t a fringe scientific claim. It’s a well-established atmospheric physics phenomenon, confirmed by modelling across European latitudes and corroborated by the patterns of vitamin D levels measured in real populations across the continent.
A 2016 study modelling UVB availability across nine European countries, ranging from 35°N to 69°N, found that the duration of what researchers call the “vitamin D winter”, the period in which UVB levels are too low for meaningful skin synthesis, ranged from zero months at the southern tip of Europe (35°N) to a full eight months at 69°N in northern Norway [6].
The UK sits at approximately 50–58°N. That puts us squarely in the zone where vitamin D winter runs from roughly October through to March, around five to six months of the year [6].
The same study compared population-level vitamin D blood levels in Ireland, Iceland, and Norway. Irish adults showed dramatic seasonal swings in their 25(OH)D concentrations, high in summer, plummeting in winter, closely mirroring the UVB availability data. Norwegians and Icelanders, despite living at even higher latitudes with even less UVB, showed *less* seasonal variation. The researchers attributed this to the significantly higher dietary vitamin D intake in those Nordic populations, primarily through fish consumption and fortified foods [6].
The takeaway? Diet and supplementation can effectively compensate for what the sun cannot provide. The UK, with neither the dietary traditions nor the mandatory fortification policies of its Nordic neighbours, is particularly exposed.
Evidence grade: Strong, Multiple population studies and validated UVB modelling across European latitudes, with consistent findings across different countries and datasets [6].
Very low, and the problem is worse than official figures suggest.
A 2024 cross-sectional study using UK Biobank data from 438,978 participants found that median 25(OH)D levels varied significantly by ethnicity: 47.9 nmol/L in White participants, 30.6 nmol/L in Black participants, and just 25.4 nmol/L in Asian participants [1]. All three figures sit at or below what even the UK’s conservative official threshold currently considers adequate, and a 2026 position statement from the multidisciplinary group ACT NOW Vitamin D argues the official threshold itself is set too low.
That 2026 paper makes a pointed case: the UK currently defines vitamin D deficiency as levels below 25 nmol/L, a threshold set by the Scientific Advisory Committee on Nutrition (SACN) in 2016. But National Diet and Nutrition Survey data show rising rates of severe deficiency, and the ACT NOW group argues that raising the threshold to 50 nmol/L, bringing the UK into line with international and clinical practice frameworks, would more accurately reflect the real-world scale of the problem [2].
If that reclassification happened, the proportion of the UK population officially classed as vitamin D deficient or insufficient would increase dramatically.
A separate 2025 UK Biobank study of 63,759 participants found that approximately 10% had frank vitamin D deficiency (below 25 nmol/L) and a further 38.9% were at risk of inadequacy, with levels between 25 and 49 nmol/L [4]. In other words, nearly half of participants in this large cohort had levels that would be considered insufficient under international clinical standards.
Evidence grade: Strong, Large UK Biobank cohort studies with hundreds of thousands of participants. Findings are highly consistent [1][2][4].
The 2025 UK Biobank dietary analysis found that season was one of the most powerful predictors of vitamin D deficiency in the dataset. Specifically, the odds of being vitamin D deficient in winter compared to autumn were 3.76-fold higher [4]. To put that in context: being obese, one of the strongest known biological risk factors for vitamin D deficiency, carried an odds ratio of 1.96. Season was almost twice as powerful a predictor as obesity [4].
This is striking. It means that simply being in the UK in winter, regardless of your diet, your weight, your lifestyle or your intentions, roughly quadruples your statistical risk of being deficient.
The study also found that darker skin tone was associated with a 3.11-fold greater likelihood of deficiency compared to fair skin, and that each additional hour of outdoor time reduced risk by only 4% [4]. Outdoor time helped, but modestly, and only during seasons where UVB was actually available. In winter, more time outside makes essentially no difference to vitamin D synthesis.
Evidence grade: Strong, 63,759 participants, UK Biobank cohort, with robust statistical modelling [4].
This is the question many people unconsciously rely on, the hope that a good summer somehow “banks” enough vitamin D to carry them through the dark months. The research is sobering on this point.
A 2025 review article specifically examining this question concluded clearly: skin synthesis alone, as the sole source of cholecalciferol, “does not guarantee the maintenance even of a minimal recommended serum calcidiol level throughout the year” [5]. While the body does store some vitamin D in adipose (fat) tissue and muscle, and these reserves do release it slowly into the bloodstream during winter, the storage is insufficient to bridge a five-to-six-month gap without supplementation [5].
The large seasonal swings in Irish adults observed in the European UVB modelling study support this: even in a relatively sunny European country, blood levels dropped significantly across winter months despite whatever the summer had provided [6].
The practical implication: summer sun is valuable and worthwhile, but it is not a substitute for winter supplementation.
Evidence grade: Promising, The storage capacity argument is well-reasoned and supported by multiple lines of evidence, but individual storage capacity varies considerably and more direct controlled trial data on this specific question would strengthen the picture [5][6].
If you’re going to supplement, this matters more than most people realise.
A 2013 double-blind randomised controlled trial assigned 95 healthy adults to either 1,000 IU daily of vitamin D3 (cholecalciferol), 1,000 IU daily of vitamin D2 (ergocalciferol), or placebo for 25 weeks over winter, the exact period when sunlight cannot maintain vitamin D levels [10].
The results were unambiguous. After 25 weeks, participants taking vitamin D3 maintained their total serum 25(OH)D concentrations essentially unchanged from baseline. Participants taking vitamin D2 *and* those taking placebo both showed significant declines. More surprisingly, vitamin D2 supplementation actually appeared to suppress 25(OH)D3 levels, producing a 9 nmol/L greater decline in the D3 metabolite compared to placebo. Overall, total 25(OH)D concentrations were 21 nmol/L lower in the D2 group than the D3 group by the end of the study [10].
This finding has been recognised at a policy level. A 2022 multidisciplinary expert forum on UK vitamin D strategy explicitly noted that “vitamin D3 appears more effective than vitamin D2 for raising serum 25-hydroxyvitamin D concentration”, and stated this has implications for which form should be used in any future food fortification strategy [14].
The current UK government recommendation does not distinguish between D2 and D3 [9]. The research suggests it probably should.
Evidence grade: Promising, The 2013 RCT was well-designed but had 95 participants and was conducted at 46°S latitude (New Zealand), so direct UK applicability has some caveats. The consistent direction of findings across studies and expert consensus strengthen confidence [10][14].
The research identifies several groups for whom the risk is compounded beyond the standard seasonal effect:
Non-white ethnic backgrounds. The 2024 UK Biobank study (438,978 participants) found that UVB was the strongest predictor of vitamin D status in White participants, but that for Asian and Black individuals, baseline levels were already substantially lower, and deficiency was more prevalent regardless of season [1][7]. Darker skin contains more melanin, which acts as a natural UV filter, beneficial for UV protection, but meaning longer sun exposure is needed for equivalent vitamin D synthesis. In a UK winter, that extra time outside is simply irrelevant: no useful UVB is reaching the skin at all.
People with obesity. The 2025 Biobank study found that obesity was associated with nearly double the odds of deficiency (OR=1.96), and abdominal obesity with 1.83-fold increased odds [4]. Vitamin D is fat-soluble and can become sequestered in adipose tissue, reducing its bioavailability in the bloodstream.
Older adults. Skin efficiency at synthesising vitamin D declines with age. This is well-documented in the broader literature and acknowledged in expert discussions on UK vitamin D strategy [8][14].
Those with lower dietary intake. The 2026 ACT NOW paper highlighted that National Diet and Nutrition Survey data show significant declines in vitamin D dietary intake in recent years, particularly as consumption of meat, fish and seafood has decreased [2]. Few foods naturally contain meaningful amounts of vitamin D, and the UK has no mandatory fortification programme [3][12].
Evidence grade: Strong for ethnic variation and obesity, Large cohort data. Promising for age-related synthesis decline, well-accepted mechanistically but specific magnitude data not detailed in these papers [1][2][4][7].
The vitamin D story, while compelling, has genuine gaps that honest science requires us to acknowledge.
The threshold debate is unresolved. The UK officially sets deficiency at below 25 nmol/L. Many international bodies and the ACT NOW group argue for 50 nmol/L. This matters enormously for how we interpret prevalence data and who we advise to supplement. The answer has real policy implications, and the science has not yet settled [2][11].
Optimal supplementation doses remain contested. There is genuine disagreement about how much vitamin D most people need, and this is made harder by the fact that individual responses to the same dose vary substantially, influenced by body weight, gut absorption, baseline levels, and genetic variation in vitamin D metabolism [5][9]. The UK government recommends 400 IU daily for the general adult population, but some researchers and clinicians argue this is insufficient for many people, particularly those starting from a low baseline [9][14].
Non-skeletal benefits are still being established. Vitamin D has been associated with immune function, mood, cardiovascular health, and cognitive outcomes, but the ACT NOW paper and the UK Nutrition Research Partnership workshop both acknowledge that for non-skeletal outcomes, causal evidence remains limited or unclear [8][15]. Associations in observational data are promising, but trials haven’t consistently replicated them. This doesn’t mean the associations are wrong, but we shouldn’t overstate what’s proven.
Food fortification modelling is incomplete. There’s active debate about whether the UK should introduce mandatory vitamin D fortification of staple foods. Research is ongoing into which foods work best as fortification vehicles and what real-world costs would look like [3][12][14]. This may change the landscape considerably in coming years.
The D2 vs D3 question needs more UK-specific data. The key trial supporting D3 superiority was conducted in New Zealand. More UK-based trials with diverse ethnic populations would help clarify optimal form and dose [10].
Let’s be direct about what the research tells you to do, practically, sensibly, and without unnecessary hedging.
From October to March, you cannot meaningfully top up your vitamin D from UK sunlight. Full stop. This isn’t a matter of going outside more often or finding a sunny spot at lunchtime. The UVB wavelengths required simply don’t reach the surface at UK latitudes during these months [6]. Summer sun is wonderful and worth maximising, but it won’t store enough to carry you through winter [5].
Supplement with vitamin D3, not D2. The evidence clearly favours D3 (cholecalciferol) for maintaining blood levels over winter. The gap in the trial, 21 nmol/L better outcomes in the D3 group versus D2 at the same dose, is clinically meaningful [10]. Check your current supplement’s label.
What dose? Vitamin D is fat-soluble, which means unlike vitamin C or the B vitamins, it does accumulate, but toxicity genuinely requires sustained mega-doses far beyond what’s in a standard supplement. The UK government recommends 400 IU daily, but this is considered conservative by many researchers and clinical practitioners, particularly for people starting from a low baseline [9][14]. A dose of 1,000–2,000 IU daily is where much of the research sits and is widely considered safe for general adult use. If you’re in a higher-risk group, non-white ethnicity, overweight, older, rarely outdoors, there’s a reasonable case for the upper end of that range.
You could get tested before supplementing. But let’s think practically: testing requires a GP referral or a private blood test (typically £30–60), and the UK Biobank data shows that nearly half the population is insufficient or deficient anyway [4]. The risk of supplementing at 1,000–2,000 IU daily is extremely low. The risk of spending winter deficient, for your bones, your immune system, and your overall health, is well-documented. A sensible, well-informed person would simply supplement through winter and get tested if they want confirmation or have specific health concerns.
If you eat little or no oily fish, red meat, or eggs, your dietary intake of vitamin D is likely negligible, and the absence of mandatory food fortification in the UK means you have no food-based safety net [3][12]. This makes supplementation even more straightforward a decision.
If you have darker skin and live in the UK, please take this seriously. The data from nearly 440,000 UK Biobank participants shows that Asian and Black individuals have median vitamin D levels that are already at or below deficiency thresholds, before winter even begins [1]. A standard supplement is a low-cost, high-value intervention for this group in particular.
The practical habit is simple: buy a vitamin D3 supplement, take it daily from October through to March as a minimum, and consider year-round supplementation if your diet is low in natural sources or you’re in a higher-risk group. It costs pennies a day and the weight of evidence is firmly on the side of doing it.
[1] Ambient ultraviolet-B radiation, supplements and other factors interact to impact vitamin D status differently depending on ethnicity: A cross-sectional study (2024). DOI: 10.1016/j.clnu.2024.04.006 | https://pubmed.ncbi.nlm.nih.gov/38663052/
[2] Vitamin D in the UK: an urgent call to redefine the threshold for deficiency (2026). DOI: 10.3389/fendo.2026.1870408 | https://pubmed.ncbi.nlm.nih.gov/42453499/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC13364595/
[3] Is it time to routinely fortify food or drink with vitamin D in the UK? (2024). DOI: 10.1111/nbu.12697 | https://pubmed.ncbi.nlm.nih.gov/39034614/
[4] Dietary and lifestyle determinants of vitamin D status in the UK Biobank Cohort study for predictive modeling (2025). DOI: 10.1016/j.jnutbio.2025.109919 | https://pubmed.ncbi.nlm.nih.gov/40221106/
[5] Vitamin D-is the sun enough for us? (2025). DOI: 10.1186/s12986-025-00962-9 | https://pubmed.ncbi.nlm.nih.gov/40597369/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12220666/
[6] Seasonal Changes in Vitamin D-Effective UVB Availability in Europe and Associations with Population Serum 25-Hydroxyvitamin D (2016). https://pubmed.ncbi.nlm.nih.gov/27589793/
[7] Ambient ultraviolet-B radiation, supplements and other factors interact to impact vitamin D status differently depending on ethnicity: A cross-sectional study (2024). DOI: 10.1016/j.clnu.2024.04.006 | https://pubmed.ncbi.nlm.nih.gov/38663052/
[8] UK Nutrition Research Partnership ‘Hot Topic’ workshop: Vitamin D-A multi-disciplinary approach to (1) elucidate its role in human health and (2) develop strategies to improve vitamin D status in the UK population (2022). DOI: 10.1111/nbu.12557 | https://pubmed.ncbi.nlm.nih.gov/36045095/
[9] Discrepancies between current guidance from NICE on the treatment of vitamin D deficiency and the recommended daily amounts [RDAs] for its prevention in the UK (2022). DOI: 10.1080/17446651.2022.2067143 | https://pubmed.ncbi.nlm.nih.gov/35450494/
[10] Long-term vitamin D3 supplementation is more effective than vitamin D2 in maintaining serum 25-hydroxyvitamin D status over the winter months (2013). https://pubmed.ncbi.nlm.nih.gov/23168298/
[11] Vitamin D in the UK: an urgent call to redefine the threshold for deficiency (2026). DOI: 10.3389/fendo.2026.1870408 | https://pubmed.ncbi.nlm.nih.gov/42453499/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC13364595/
[12] Is it time to routinely fortify food or drink with vitamin D in the UK? (2024). DOI: 10.1111/nbu.12697 | https://pubmed.ncbi.nlm.nih.gov/39034614/
[13] Vitamin D, the natural way (2021). DOI: 10.1016/j.clnesp.2020.12.001 | https://pubmed.ncbi.nlm.nih.gov/33487250/
[14] Implementation strategies for improving vitamin D status and increasing vitamin D intake in the UK: current controversies and future perspectives: proceedings of the 2nd Rank Prize Funds Forum on vitamin D (2022). DOI: 10.1017/S0007114521002555 | https://pubmed.ncbi.nlm.nih.gov/34284830/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8376911/
[15] Vitamin D (2021). DOI: 10.7861/clinmed.Let.21.1.7 | https://pubmed.ncbi.nlm.nih.gov/33479100/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7850192/
This article is for informational purposes only and does not constitute medical advice. Food supplements should not be used as a substitute for a varied and balanced diet and healthy lifestyle. If you are pregnant, breastfeeding, taking medication or have a medical condition, consult your doctor before taking any supplement. These statements have not been evaluated by the Food and Drug Administration (FDA) or the Medicines and Healthcare products Regulatory Agency (MHRA). This product is not intended to diagnose, treat, cure, or prevent any disease.
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