The truth about vitamin D — who really needs supplements

Why this matters

Vitamin D is one of the most discussed supplements of the last decade. You’ll find articles claiming it prevents cancer, treats depression, boosts immunity, protects against COVID, and accelerates weight loss. The market is enormous — UK consumers spend hundreds of millions a year on vitamin D supplements.

What’s the real picture? Vitamin D is genuinely important. Deficiency is genuinely common in the UK. Supplementation is genuinely useful for many people. But the evidence base is more limited than the marketing.

What vitamin D actually does

Vitamin D is unusual among vitamins because we make it ourselves. Sunlight on bare skin (UVB specifically) triggers its production. Small amounts come from food — mainly oily fish, egg yolks, and fortified foods.

Its main proven role is in calcium and phosphate metabolism — the system that keeps bones strong. Severe deficiency causes rickets in children and osteomalacia in adults. Modern deficiency is rarely that severe but is associated with:

• Lower bone mineral density and increased osteoporosis risk
• Possible muscle weakness and fall risk in older adults
• Possible immune system effects (less robust evidence)
• Possible mood effects (mixed evidence)

Why UK deficiency is common

The UK sits at a latitude (roughly 50–58 degrees north) where between October and March, sunlight is too weak to produce significant vitamin D in the skin — even on a clear winter day. We rely on stores built up over summer to carry us through.

Common factors that increase deficiency risk:

• Indoor working life — office-based, screen-based work
• Sun protection — appropriate to avoid skin cancer, but reduces vitamin D production
• Darker skin — melanin reduces vitamin D synthesis; people with darker skin need more sunlight to produce the same amount
• Older age — less efficient skin production
• Veiling or covering for religious reasons
• Pregnancy and breastfeeding — increased need
• Obesity — vitamin D gets sequestered in fat, less circulating
• Some absorption problems — coeliac, inflammatory bowel disease, post-bariatric surgery

By the end of winter, blood tests show clinical or subclinical vitamin D deficiency in around 30–40% of UK adults.

Who genuinely benefits

Everyone in winter

Public Health England recommends 10 micrograms (400 IU) daily for everyone aged 5+ in the UK from October to March. This is sensible. It’s a small dose, very safe, and addresses the seasonal deficiency reality.

Year-round groups

• People with darker skin (Asian, African, Middle Eastern descent)
• People who cover most skin when outdoors
• People who spend most time indoors
• Older adults (over 65), particularly if frail
• Children under 5 (different doses)
• Pregnant and breastfeeding women

Higher doses sometimes needed

People with established deficiency, malabsorption conditions, obesity, or on certain medications may need higher therapeutic doses — up to several thousand IU daily — for a defined period to correct deficiency, then a lower maintenance dose.

Should you get tested?

For most people: no.

A standard low-dose supplement (10 micrograms / 400 IU) daily is safe for everyone, costs almost nothing, and addresses the most likely scenario. Testing in a healthy person who can just supplement adds cost and complexity without changing the recommendation.

Testing is worth doing if:

• You have symptoms suggestive of deficiency (bone pain, muscle weakness, recurrent falls)
• You’ve had recent fractures
• You have a condition affecting absorption
• You’re on a high dose for medical reasons and need monitoring
• You’re unsure whether deficiency is contributing to your tiredness or low mood, and a test would change your approach

Understanding your result

• Below 25 nmol/L — deficient. Needs treatment-dose supplementation.
• 25–50 nmol/L — insufficient. Supplement.
• 50–75 nmol/L — adequate for most people. Maintain.
• 75–125 nmol/L — sufficient.
• Above 125 nmol/L — high. Reduce supplementation.

What dose, and what type?

Maintenance dose

10 micrograms (400 IU) daily is the public health recommendation. Many people take 20–25 micrograms (800–1000 IU) which is also safe.

Treatment dose for confirmed deficiency

Higher doses for a defined period under clinical guidance, then transition to maintenance.

D2 vs D3

D3 (cholecalciferol) is preferred over D2 (ergocalciferol). It raises blood levels more effectively. Most supplements sold in the UK are D3.

Can you take too much?

Yes, but it’s hard at sensible doses. Toxicity is generally from supplements above 4000–10,000 IU daily for prolonged periods. Standard supplements at recommended doses are very safe.

The over-hyped claims

Cancer prevention

Some observational data suggests lower cancer rates in people with higher vitamin D. Large trials haven’t shown that supplementation prevents cancer in the general population. Possibly a small effect in specific groups; not a general prevention strategy.

Depression and mood

Lower vitamin D is associated with depression in observational studies. Supplementation in deficient people may help mood. Supplementation in non-deficient people generally doesn’t improve mood. Not a treatment for established depression.

COVID-19

Early speculation that vitamin D might prevent severe COVID. Subsequent evidence is mixed; large trials haven’t shown major protective effects from supplementation in non-deficient people.

Cardiovascular disease

Observational links exist but supplementation trials haven’t shown clear reduction in cardiovascular events.

Weight loss

Not a weight loss treatment. Lower vitamin D is more common in obesity but causation is unclear.

In summary

For most UK adults:

• Take 10 micrograms (400 IU) daily through autumn and winter
• If you’re in a higher-risk group, year-round
• Skip the routine testing — supplement and move on
• Don’t expect transformation; expect baseline maintenance

For people with specific symptoms, conditions, or risks, individual testing and a higher dose may be appropriate — see our vitamin D deficiency page for the detail.