Vitamin D (25-OH)
| Clinical | Optimal | |
|---|---|---|
| Male | 25 - 175 nmol/L | 75–150 nmol/L |
| Female | 25 - 175 nmol/L | 75–150 nmol/L |
What is Vitamin D (25-OH)?
The 25-hydroxyvitamin D test (often written as 25-OH vitamin D) measures the main storage form of vitamin D circulating in your blood. This is the form your body creates when sunlight hits your skin or when you consume vitamin D through food or supplements.
Your liver converts vitamin D into 25-OH vitamin D, which then travels through the bloodstream to your kidneys, where it is activated into its final, hormone-like form. Measuring the 25-OH version gives the most accurate picture of your overall vitamin D status.
Despite being called a vitamin, vitamin D functions more like a hormone. Virtually every cell in your body has a receptor for it, which explains why it influences so many different aspects of health.
Why Vitamin D (25-OH) Matters for Your Health
Vitamin D deficiency is extremely common in the UK — some estimates suggest up to 40% of the population has insufficient levels, particularly during the autumn and winter months when UVB sunlight is too weak for the skin to produce it.
Adequate vitamin D is essential for calcium absorption and bone health, but its role extends far beyond bones. It supports immune function (both fighting infections and regulating autoimmunity), muscle strength, mood regulation, cardiovascular health, and even gene expression related to ageing.
From a longevity standpoint, maintaining optimal vitamin D levels is one of the most impactful and cost-effective health interventions available. Research consistently links adequate levels with reduced risk of fractures, respiratory infections, depression, and several chronic diseases.
Vitamin D (25-OH)& Your Wearable Data
Vitamin D (25-hydroxyvitamin D) is essential for bone health, immune function, muscle strength, and mood regulation. Wearable data provides relevant context: outdoor activity tracked by your device, particularly time spent in natural daylight, correlates with vitamin D synthesis. GPS and light exposure data from your wearable can help estimate your sun exposure patterns, especially during winter months when deficiency risk increases.
Vitamin D status directly affects physical performance metrics tracked by wearables. Deficiency is associated with reduced muscle strength, increased injury risk, lower VO2 max, and impaired recovery. If your wearable shows declining athletic performance, increased muscle soreness (longer recovery times), or stress fracture history alongside low vitamin D, supplementation may significantly improve these metrics.
Sleep quality tracked by your wearable has a bidirectional relationship with vitamin D. Low vitamin D is associated with poor sleep quality, shorter sleep duration, and increased daytime sleepiness. Monitoring improvements in your wearable sleep scores after vitamin D supplementation helps demonstrate treatment effectiveness and may guide dose adjustments.
What High Vitamin D (25-OH) May Suggest
Vitamin D toxicity from natural sunlight exposure is essentially impossible — your skin has a self-limiting mechanism. However, excessive supplementation can push levels above 220 nmol/L, which may lead to hypercalcaemia (too much calcium in the blood).
Symptoms of vitamin D excess include nausea, vomiting, frequent urination, weakness, and in severe cases, kidney damage. This typically only occurs with very high-dose supplementation (above 10,000 IU daily) taken over extended periods without monitoring.
If your levels are between 150 and 220 nmol/L, results suggest you are at the upper end of the range. This is generally not harmful but there is no additional benefit beyond optimal levels. Consider reducing your supplement dose and retesting in 3 months.
What Low Vitamin D (25-OH) May Suggest
Low vitamin D is one of the most common nutrient deficiencies in the UK. Results below 50 nmol/L are considered insufficient, and below 25 nmol/L is classified as deficient. Many longevity-focused practitioners consider anything below 75 nmol/L as suboptimal.
Symptoms of deficiency can be subtle and develop gradually: fatigue, low mood (particularly in winter), frequent colds and infections, muscle weakness, aching bones, and slow wound healing. Severe deficiency over time increases the risk of osteoporosis and fractures.
Low levels are particularly common in people with darker skin tones (more melanin reduces vitamin D synthesis), those who spend little time outdoors, older adults, people who cover most of their skin, and anyone living north of Birmingham during winter months.
How to Optimise Your Vitamin D (25-OH)
Food
Include vitamin D-rich foods such as oily fish (salmon, mackerel, sardines), egg yolks, mushrooms exposed to UV light, and fortified foods (some milks, cereals, and spreads). Cod liver oil is one of the richest natural sources. However, food alone rarely provides enough vitamin D, especially in the UK, making supplementation important for most people during autumn and winter.
Lifestyle
Aim for 10–20 minutes of midday sun exposure on bare arms and face during the spring and summer months (April to September in the UK) without sunscreen to allow natural vitamin D synthesis. During autumn and winter, the NHS recommends all adults consider a supplement. Spend time outdoors during peak daylight hours where possible. Maintain a healthy body weight, as vitamin D is fat-soluble and can become sequestered in excess body fat.
Supplements
Vitamin D3 (cholecalciferol) is the preferred form, as it raises blood levels more effectively than D2. The NHS recommends 400 IU (10 mcg) daily as a minimum, but many practitioners suggest 1,000–2,000 IU daily for those with suboptimal levels. Taking vitamin D with a meal containing fat improves absorption. Vitamin K2 (MK-7) is a useful companion supplement, as it helps direct calcium to bones rather than arteries.
⚠️ Supplement Warning
Warning: Vitamin D supplementation requires careful dosing. While deficiency is common, excessive vitamin D supplementation can cause hypercalcaemia (dangerously high blood calcium), kidney stones, and kidney damage. The upper tolerable limit is generally 4,000 IU/day for adults, though higher doses may be prescribed under medical supervision. Vitamin D is fat-soluble and accumulates in the body, so toxicity can develop gradually. Always test your levels before and during supplementation, and consult a healthcare professional for appropriate dosing, especially if you have kidney disease or take certain medications.
When to Speak to Your GP
See your GP if your vitamin D level is below 25 nmol/L (deficient), as you may need a higher loading dose. Also consult your GP if levels remain below 50 nmol/L despite supplementation, as there may be an absorption issue worth investigating (such as coeliac disease or inflammatory bowel disease). Seek advice if you experience bone pain, significant muscle weakness, or recurrent fractures.
References
- NHS. Vitamin D — Overview. Updated 2024. nhs.uk
- NICE. Vitamin D: supplement use in specific population groups. PH56. nice.org.uk
- SACN. Scientific Advisory Committee on Nutrition. Vitamin D and health. London: Public Health England; 2016. gov.uk
- NEJM. Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266-281. pubmed.ncbi.nlm.nih.gov
- Endocrine Society. Holick MF, et al. Evaluation, treatment, and prevention of vitamin D deficiency. J Clin Endocrinol Metab. 2011;96(7):1911-1930. pubmed.ncbi.nlm.nih.gov
Medical Disclaimer— This content is for general educational and informational purposes only. It does not constitute medical advice, diagnosis, or treatment. Omniwo Ltd is a wellness information service and is not a medical device, clinical laboratory, or regulated healthcare provider under MHRA guidelines. The “optimal ranges” presented on this page are based on published clinical guidelines (WHO, NICE, NHS) and peer-reviewed research; they represent functional wellness targets and may differ from standard laboratory reference ranges. Individual results should always be interpreted by a qualified healthcare professional (such as your GP) who understands your full medical history. Do not start, stop, or change any medication or supplement based solely on this information. If you are experiencing symptoms, seek medical attention promptly.