Vitamin D: does it work, and how to take it in the UK

The short answer

Vitamin D is one of the few supplements the NHS actively recommends for the entire UK population — and for good reason. At the standard dose of 10 µg (400 IU) per day from October to March, the evidence is Grade A for bone health, Grade B for immune function, and Grade B for muscle strength in people who are deficient. Verdict: Worth it.

What vitamin D is

Despite its name, vitamin D behaves more like a hormone than a conventional vitamin. Your skin synthesises it from UVB sunlight, and it regulates hundreds of genes involved in bone mineralisation, immune defence, muscle function, and cell growth. The problem in the UK is latitude: from October to March, the sun sits too low to trigger meaningful skin synthesis north of approximately 51°N — which includes all of England, Scotland, Wales, and Northern Ireland. Public Health England data consistently shows that around 40% of UK adults are deficient or insufficient in winter, rising to over 70% in some high-risk groups.

The primary circulating form measured by blood tests is 25-hydroxyvitamin D [25(OH)D]. Deficiency is generally defined as below 25 nmol/L; insufficiency as 25–50 nmol/L; sufficiency as above 50 nmol/L. Many experts recommend 75–100 nmol/L for optimal musculoskeletal health.

Does it work? The evidence

Bone health — Grade A

The strongest and most consistent evidence is for the skeleton. Vitamin D enables calcium absorption in the gut; without adequate levels, the body draws calcium from bone, raising the risk of rickets in children and osteomalacia or osteoporosis in adults. Multiple meta-analyses confirm that vitamin D supplementation — particularly combined with calcium — reduces fracture risk in older adults and corrects osteomalacia reliably. This is the primary reason behind NHS universal winter guidance.

Immune and respiratory function — Grade B

A 2017 individual-participant data meta-analysis in The BMJ (Martineau et al.) covering 25 RCTs and over 11,000 participants found that daily or weekly vitamin D supplementation reduced the risk of acute respiratory tract infections, with the greatest benefit in those who were severely deficient at baseline. The overall protective effect was modest for replete individuals, which is why the evidence sits at Grade B rather than A — but for the large proportion of the UK population who are low, the benefit is clinically meaningful.

Muscle strength and physical function — Grade B

This is the benefit most people overlook. Skeletal muscle contains vitamin D receptors, and deficiency is associated with muscle weakness, impaired power, and increased fall risk — particularly in older adults. Supplementation in deficient individuals consistently improves grip strength, leg power, and balance in RCTs. If you are training seriously and running low on vitamin D, you are leaving real performance on the table. This is especially relevant if you train indoors, train during winter months, or have darker skin.

How to take it

Dose: 10 µg (400 IU)/day is the NHS baseline for all adults from October to March. If a GP blood test reveals confirmed deficiency (25(OH)D below 25 nmol/L), short-term loading doses of 25–50 µg (1,000–2,000 IU)/day are commonly prescribed to restore levels, followed by a maintenance dose. Do not exceed 100 µg (4,000 IU)/day without medical oversight — this is the EFSA-established upper tolerable intake level for healthy adults.

Form: Choose vitamin D3 (cholecalciferol) — it raises and sustains blood 25(OH)D levels more effectively than D2 (ergocalciferol). Softgel capsules in an oil base have better bioavailability than dry tablets.

Timing: Take with a meal that contains fat — avocado, eggs, nuts, olive oil, or oily fish all work. A study in The Journal of Bone and Mineral Research found that vitamin D absorption increased by around 50% when taken with a fat-containing meal versus a fat-free one. Otherwise, time of day does not matter materially.

Duration: October through to the end of March is the minimum. Those in higher-risk categories (darker skin, covered skin, housebound, over 65) should supplement year-round.

Who should take it — and who should not

Take it if: You live in the UK and it is between October and March. Additionally, supplement year-round if you have darker skin (Fitzpatrick types IV–VI), spend most of your time indoors, cover most of your skin for cultural or religious reasons, are aged 65+, or rarely eat oily fish (salmon, mackerel, sardines) or fortified foods.

Think carefully if: You have hypercalcaemia, sarcoidosis, lymphoma, or primary hyperparathyroidism — conditions that dysregulate calcium metabolism. Vitamin D supplementation can raise blood calcium dangerously in these cases. See your GP first. If you take thiazide diuretics (a class of blood-pressure medication), the combination can also elevate calcium — worth flagging to your pharmacist.

Blood test first: If you suspect genuine deficiency rather than routine winter insufficiency, ask your GP for a 25(OH)D test before taking doses above 1,000 IU/day. This is especially worth doing if you have persistent fatigue, bone pain, or muscle weakness.

Safety and interactions

At the standard NHS dose of 400 IU/day, side effects are essentially zero. Toxicity requires chronically high doses — typically above 10,000 IU (250 µg)/day for months. The most serious consequence is hypercalcaemia: excess calcium in the blood causing nausea, vomiting, weakness, confusion, excessive thirst, kidney stones, and in severe cases, cardiac arrhythmia and kidney failure.

The EFSA sets the upper tolerable intake for adults at 100 µg (4,000 IU)/day. Staying below this without a GP-monitored reason is the sensible ceiling. Be aware that some high-dose vitamin D products marketed online significantly exceed this. Read the label.

General information, not medical advice — check with a GP or pharmacist before starting, especially if pregnant, breastfeeding, under 18, or on medication.

How to buy it well

Vitamin D3 softgels are one of the least expensive supplements on the market — a year's supply costs as little as £3–£6 from reputable UK retailers. Look for:

• Vitamin D3 (not D2) in a vegetable oil or olive oil base
• 400–1,000 IU (10–25 µg) per capsule for daily use
• A third-party tested product if you are a competitive athlete subject to anti-doping rules (Informed-Sport certification)
• No unnecessary fillers or proprietary blends — this is a single-ingredient supplement; simplicity is a feature

Leading UK pharmacies (Boots, Holland & Barrett) and online brands (BetterYou D Lux drops are convenient for sublingual delivery; standard D3 softgels from reputable suppliers are equally effective at a fraction of the cost).

The verdict

Vitamin D is the rare supplement where the NHS, EFSA, and virtually every major nutrition body agree: take it. The UK's latitude makes winter deficiency near-universal, the evidence for bone health is watertight, the immune and muscle benefits are meaningful for the deficient, the dose is cheap and safe, and the downside of not supplementing — degraded bone density, compromised immunity, impaired muscle performance — is entirely avoidable.

That said, supplements account for roughly 5% of the total result. The other 95% — your training programme, your calorie and protein targets, your sleep, your consistency — is where coaching makes the real difference. If you want a plan that gets all of it right, not just the supplement stack, book a free consultation or take the free Blueprint quiz to find out exactly what your body needs.

For more on building a strong, resilient body after 50, see our guide to building muscle after 50. If you are also working on fat loss, the TDEE calculator is a useful starting point for understanding your energy needs.