Why vitamin D matters, how much you need, who's at risk, and when supplementation makes sense. A practical guide for people living in the UK.
Core Health
Core Health Team
The sunshine vitamin problem
Vitamin D is unique among nutrients — your body can produce it when your skin is exposed to ultraviolet B (UVB) radiation from sunlight. The problem? If you live in the UK, you're at a significant geographical disadvantage.
Between October and March, the sun in the UK sits too low in the sky for UVB rays to reach the earth's surface effectively. This means that for roughly half the year, you simply cannot make vitamin D from sunlight, regardless of how much time you spend outdoors.
The result: an estimated 1 in 5 adults in the UK has low vitamin D levels, and Public Health England recommends that everyone consider taking a supplement during autumn and winter.
What vitamin D does
Vitamin D is involved in far more than bone health, though that remains its most well-known role:
- Bone and teeth: Vitamin D regulates calcium and phosphorus absorption. Without adequate vitamin D, you can absorb as little as 10-15% of dietary calcium (compared to 30-40% with sufficient levels)
- Muscle function: Low vitamin D is associated with muscle weakness and increased fall risk, particularly in older adults
- Immune system: Vitamin D modulates both innate and adaptive immune responses. Multiple studies link deficiency to increased susceptibility to respiratory infections
- Mood and mental health: Low vitamin D correlates with higher rates of depression, though causation is still being studied
- Cardiovascular health: Emerging evidence suggests a role in blood pressure regulation and arterial health
How much do you need?
The UK government recommends:
| Group | Daily recommendation |
|---|---|
| Adults and children over 4 | 10 micrograms (400 IU) |
| Babies 0-1 year | 8.5-10 micrograms |
| Pregnant and breastfeeding women | 10 micrograms |
These are minimum recommendations to prevent deficiency. Some researchers and clinicians argue that higher intakes (20-25 micrograms / 800-1,000 IU) may be optimal for immune and muscle function, though this is debated.
Safe upper limit: The NHS considers up to 100 micrograms (4,000 IU) per day safe for adults. Toxicity is rare but possible with very high doses over extended periods, particularly from supplements (not sunlight).
Who's most at risk of deficiency?
Some groups are at higher risk:
- Everyone in the UK between October and March: UVB levels are too low for skin synthesis
- People with darker skin: Higher melanin levels reduce UVB absorption, meaning more exposure is needed to produce the same amount of vitamin D
- People who cover most of their skin: For religious, cultural, or occupational reasons
- Older adults: Skin becomes less efficient at producing vitamin D with age
- People who are housebound or work indoors: Limited sun exposure during peak UVB hours (11am-3pm)
- People with obesity: Vitamin D is fat-soluble and can be sequestered in body fat, reducing circulating levels
- People with malabsorption conditions: Crohn's disease, coeliac disease, and other gut conditions affect absorption
Food sources of vitamin D
Very few foods naturally contain significant vitamin D:
- Oily fish: Salmon (10-25 mcg per 100g), mackerel, sardines, herring
- Egg yolks: About 1.5-2 mcg per yolk
- Red meat: Small amounts
- Liver: Rich source but not recommended during pregnancy
- Fortified foods: Some breakfast cereals, plant milks, and margarine spreads are fortified (check labels)
- Mushrooms: Exposed to UV light, some varieties contain meaningful amounts of vitamin D2
Realistically, it's very difficult to get enough vitamin D from food alone, especially in the UK. This is why supplementation is recommended.
Supplementation: what to look for
Vitamin D3 vs D2
- Vitamin D3 (cholecalciferol): The form your body produces from sunlight. More effective at raising and maintaining blood levels
- Vitamin D2 (ergocalciferol): Plant-derived. Less potent and shorter-lasting
Recommendation: Choose vitamin D3 where possible. If you follow a vegan diet, look for D3 derived from lichen (a plant-based source), or use D2 at slightly higher doses.
When to take it
Vitamin D is fat-soluble, so take it with a meal containing some fat for better absorption. Morning or evening doesn't matter — consistency is more important than timing.
The supplement landscape
- Tablets and capsules: Most common, widely available, cheap. A year's supply of 10mcg D3 costs as little as a few pounds
- Oral sprays: Absorbed through the cheek lining. Useful for people with gut absorption issues
- High-dose weekly/monthly: Some people prefer less frequent, higher doses. This can work but daily supplementation provides more stable blood levels
- Combined supplements: Vitamin D + K2 is a popular combination. Vitamin K2 helps direct calcium into bones rather than arteries, though the evidence for this synergy is still emerging
NHS Healthy Start vitamins
If you're pregnant, breastfeeding, or have a child under 4, you may be eligible for free vitamins containing vitamin D through the NHS Healthy Start scheme.
Testing your vitamin D levels
If you suspect deficiency, you can ask your GP for a blood test measuring 25-hydroxyvitamin D (25(OH)D). Levels are typically interpreted as:
| Level | Status |
|---|---|
| Below 25 nmol/L | Deficient |
| 25-50 nmol/L | Insufficient |
| 50-75 nmol/L | Adequate |
| 75-125 nmol/L | Optimal (debated) |
| Above 250 nmol/L | Potentially harmful |
Home testing kits are also available through services like Thriva and Medichecks, though results should be discussed with a healthcare professional.
Summer sun: how much is enough?
Between April and September, when UVB levels are sufficient in the UK:
- Duration: 10-15 minutes of midday sun (11am-3pm) on exposed arms and face, several times a week
- Don't burn: Never let skin go red. The aim is brief, regular exposure — not prolonged sunbathing
- Sunscreen: There's ongoing debate, but most dermatologists recommend applying sunscreen after a brief period of unprotected exposure. Even SPF 15 reduces vitamin D production by about 99%
- Window glass: UVB doesn't pass through glass, so sitting by a sunny window doesn't count
Practical recommendations
- October to March: Take a daily vitamin D3 supplement of at least 10 micrograms (400 IU)
- All year round if you're in a higher-risk group: Supplement daily regardless of season
- Eat oily fish 1-2 times per week and include eggs and fortified foods
- Get brief sun exposure during summer months when possible
- Consider testing if you have symptoms of deficiency (fatigue, muscle weakness, bone pain, frequent illness)
The bottom line
Vitamin D deficiency is largely a consequence of UK geography and modern indoor lifestyles. The fix is straightforward and cheap: a daily supplement of 10-25 micrograms during autumn and winter (or year-round if you're at higher risk), combined with sensible sun exposure in summer and a diet that includes oily fish and fortified foods. It's one of the simplest, most evidence-backed steps you can take for your health.
Health disclaimer: This content is for informational purposes only and is not intended as medical advice. Always consult with a qualified healthcare professional before making significant changes to your diet or exercise routine.
