Seasonal Vitamin D Testing: When to Check, What Your Level Means, and Why Winter Matters

Between October and March in the UK, the sun sits too low in the sky for your skin to produce any meaningful vitamin D. That's roughly half the year where your body relies entirely on stored vitamin D and dietary intake to maintain adequate levels. And the evidence is clear that for most people, stored vitamin D and dietary sources aren't enough.

The UK Scientific Advisory Committee on Nutrition (SACN) reviewed the evidence in 2016 and concluded that a significant proportion of the UK population has inadequate vitamin D levels, particularly during autumn and winter. Public Health England now recommends that all adults consider taking a daily supplement of 10 micrograms (400 IU) of vitamin D during winter months, and that people at higher risk (those who spend limited time outdoors, cover their skin, or have darker skin) should consider supplementing year-round [SACN, 2016].

That's sensible population-level advice. But it raises a question: how do you know whether 400 IU is enough for you, or whether you need more? The answer is a blood test.

Why vitamin D levels follow a seasonal cycle

Your skin produces vitamin D when exposed to UVB radiation from sunlight. In the UK, at latitudes above 51 degrees north (London is 51.5), UVB intensity is only sufficient for vitamin D synthesis between approximately April and September, and only during the middle of the day. Outside that window, the UVB wavelength doesn't reach the Earth's surface at a high enough angle to trigger production.

Your body does store vitamin D in fat tissue, and these stores can help bridge the gap through autumn. But by late winter and early spring, most people's stores have been depleted. Research from the NDNS found that around 1 in 5 UK adults has a serum vitamin D level below 25 nmol/L (the threshold for deficiency) by the end of winter, and the proportion rises sharply in certain groups: people with darker skin, people who work indoors, older adults, and people who cover their skin for cultural or personal reasons.

What your vitamin D level means

Vitamin D is measured as 25-hydroxyvitamin D (25(OH)D) in your blood. The UK thresholds are:

• Below 25 nmol/L: Deficient. At this level, bone health is at risk and symptoms like fatigue, muscle weakness, and bone pain are common. Supplementation is recommended

  
  

• 25 to 50 nmol/L: Insufficient. You may not have overt symptoms, but you're below the level associated with optimal health. Many experts consider this the range where supplementation is worthwhile

  
  

• 50 to 75 nmol/L: Adequate. This is the range most clinical guidelines consider sufficient for general health

  
  

• Above 75 nmol/L: Optimal. Some researchers and clinicians advocate for this level, particularly for musculoskeletal health and immune function, though this remains a topic of ongoing debate

  
  

• Above 220 nmol/L: Potentially toxic. This is rare and almost exclusively caused by excessive supplementation, not sun exposure or diet

The best time to test

There are two strategically useful times to check your vitamin D. The first is in autumn (September or October), just as the UVB window closes. This tells you what level you're entering winter with and helps you decide whether to supplement, and at what dose. The second is in late winter (February or March), when your stores are at their lowest. This shows you how well you've maintained levels through the dark months.

If you've never had your vitamin D tested, a single late-winter test gives you the most useful baseline because it captures your lowest point. If that level is adequate, you can be reasonably confident the rest of the year is fine. If it's low, you know supplementation is necessary, and a retest after 8 to 12 weeks of supplementation confirms the dose is working.

Beyond bones: what else vitamin D affects

Vitamin D's best-established role is in calcium absorption and bone health. Severe deficiency causes rickets in children and osteomalacia (soft bones) in adults. But vitamin D receptors are found in virtually every tissue in the body, and research has linked low levels to a growing list of conditions:

• Immune function. Vitamin D modulates both innate and adaptive immunity. Low levels have been associated with increased susceptibility to respiratory infections. A large meta-analysis found that supplementation reduced the risk of acute respiratory tract infections, with the strongest benefit in those who were deficient at baseline [Martineau AR, et al., BMJ, 2017]

  
  

• Muscle function. Low vitamin D is associated with muscle weakness, particularly in older adults. For athletes, deficiency has been linked to impaired recovery and greater post-exercise inflammation

  
  

• Mood. Vitamin D receptors in the brain are concentrated in areas involved in mood regulation. While the evidence for a direct causal link between low vitamin D and depression is still evolving, supplementation has shown modest benefits in some trials

  
  

• The magnesium connection. Your body needs magnesium to convert vitamin D into its active form. If both are low, supplementing vitamin D alone may not fully correct the deficiency. This is one reason why testing both together gives a more complete picture

Supplementation: how much and what type

The PHE recommendation of 400 IU (10 micrograms) daily is a baseline for the general population. For someone who is genuinely deficient (below 25 nmol/L), this dose is often not enough to correct the deficit. Loading regimens of 1,000 to 4,000 IU daily or higher are commonly used, depending on the degree of deficiency. Vitamin D3 (cholecalciferol) is the preferred form because it's more effective at raising blood levels than D2 (ergocalciferol).

The advantage of testing is that it takes the guesswork out of dosing. A level of 38 nmol/L needs a different approach from a level of 15 nmol/L, and both need a different approach from someone sitting comfortably at 70 nmol/L. Your GP can advise on the right dose based on your result.

When to see your GP

If your vitamin D is below 25 nmol/L, this is a clinical deficiency that your GP should know about. If you have symptoms like bone pain, proximal muscle weakness (difficulty getting out of a chair or climbing stairs), or recurrent fractures, these may indicate more significant vitamin D-related bone disease that needs further investigation. Persistently low vitamin D despite supplementation may also warrant checking for conditions that impair absorption, such as coeliac disease.

How Brooksby Medical can help

The standalone Vitamin D Blood Test (£59) is available as either a fingerprick or venous sample and gives you a precise measurement of your 25(OH)D level. If you want to check vitamin D alongside other common deficiencies, the Nutrients and Energy Profile (£99) adds active B12, folate, and a full iron profile. The Wellness Profile (£129) includes vitamin D alongside magnesium and a broader health screen. Every result comes with a GP-written report that interprets your level in context and advises on appropriate supplementation.

References

1. Scientific Advisory Committee on Nutrition. Vitamin D and Health. 2016. GOV.UK

2. Martineau AR, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis. BMJ. 2017;356:i6583

3. Public Health England. National Diet and Nutrition Survey: Years 1 to 9. GOV.UK

Written by Dr James Coleman, NHS GP and founder of Brooksby Medical. Dr Coleman is a practising NHS General Practitioner who founded Brooksby Medical to give patients direct access to the blood tests and clinical interpretation they need, without waiting lists.

Medically reviewed: March 2026 | Next review due: March 2027

Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Blood test results should always be interpreted by a qualified healthcare professional in the context of your individual symptoms, medical history, and clinical picture. If you have concerns about your health, please consult your GP.