Vitamin D in Manchester: why most of us are deficient and what to do

The latitude problem

Cutaneous synthesis of vitamin D requires UVB radiation in the 290–315 nm wavelength. UVB only penetrates the atmosphere effectively when the sun is high in the sky — specifically, when the solar zenith angle is below about 60 degrees. In Manchester (latitude 53.5°N), that happens from approximately late March to late September. For the remaining six months — essentially the British autumn and winter — even on a sunny day at midday, the geometry is wrong. UVB doesn't reach the surface at intensities that drive vitamin D synthesis.

This is not a clinic-specific finding. It's the same calculation that drives Public Health England's universal recommendation that everyone in the UK aged five and over should consider taking 10 micrograms (400 IU) of vitamin D daily from October to March. The recommendation isn't conservative — it's a minimum.

Even during the summer months when synthesis is possible, several modifiers reduce it: cloud cover (frequent in Manchester), time spent indoors, sunscreen, clothing covering skin, dark skin pigmentation, and ageing skin that synthesises less efficiently. The 'just go out in the sun more' approach is not realistic for most adults.

The Public Health England / NHS recommendation

Public Health England (now part of UK Health Security Agency) and the Scientific Advisory Committee on Nutrition (SACN) published a comprehensive review of vitamin D requirements in 2016. The conclusion: 10 micrograms (400 IU) daily for everyone over the age of one year, throughout the autumn and winter at minimum, and year-round for at-risk groups.

At-risk groups deserve year-round supplementation:

• People with dark skin (African, African-Caribbean, South Asian, Middle Eastern)
• People with limited sun exposure: indoor workers, those who cover most of their skin for cultural or religious reasons, housebound or institutionalised people
• People over 65
• Pregnant and breastfeeding women
• Children under 5
• People with obesity (vitamin D is fat-soluble and sequesters in adipose tissue, leaving less available in circulation)

Who's most at risk

Looking at the practice population we see across Manchester, the highest deficiency rates are in:

• South Asian adults in Rusholme, Longsight, Cheetham Hill: Pigmented skin plus, in some cases, modest dress. Deficiency rates above 80% in some surveys.
• Older adults in care or housebound settings: Limited outdoor time plus reduced skin synthesis.
• Office workers across Trafford and central Manchester: Even in summer, eight hours indoors per day removes the main synthesis window.
• People with obesity: Vitamin D is sequestered in adipose tissue, leaving less in circulation. We screen routinely as part of our pre-Mounjaro baseline panel.
• People with inflammatory bowel disease, coeliac disease, or post-bariatric surgery: Reduced absorption.

Higher rates in South Asian adults link directly to the higher metabolic risk discussed in our NICE CG189 BMI thresholds guide — vitamin D deficiency, insulin resistance and type 2 diabetes risk are interconnected.

When testing matters

For most healthy adults, the public health recommendation is to supplement rather than to test. Testing every adult in the UK would be expensive, slow, and largely confirm the same advice (take 10 mcg daily). Testing makes sense when:

• Symptoms suggest significant deficiency: muscle aches, bone pain, persistent fatigue, low mood not otherwise explained
• Risk factors are stacked: obesity plus dark skin plus indoor work, for instance
• You've been supplementing but symptoms persist — confirms whether you're hitting therapeutic levels
• Planning starting a treatment where vitamin D matters (Mounjaro baseline, fertility planning, osteoporosis)

The test is a serum 25-hydroxyvitamin D, often abbreviated 25(OH)D or '25-OH-D'. It reflects body stores reasonably well.

Reference ranges and what they mean

UK guidance (using nmol/L; some labs report ng/mL where 1 ng/mL = 2.5 nmol/L):

• Below 25 nmol/L: deficiency. Treat with loading dose followed by maintenance.
• 25–49 nmol/L: insufficiency. Treat with maintenance dose, sometimes a shorter loading.
• 50–75 nmol/L: adequate but borderline. Continue or start regular supplementation.
• 75–150 nmol/L: optimal. No additional treatment needed beyond maintenance.
• Above 150 nmol/L: high. Reduce or stop supplementation; recheck.

For technical context on this and other markers see our blood test results plain-English guide.

Treatment doses

The standard approach has two phases for symptomatic deficiency:

Loading dose (for serum 25-OHD below 25, or below 50 with symptoms):

• 50,000 IU once weekly for 6 weeks, or
• 20,000 IU twice weekly for 7 weeks, or
• 4,000 IU daily for 10 weeks

All produce a similar total dose (around 300,000 IU) over a similar period. Choice is patient-preference and prescriber-preference. The once-weekly 50,000 IU approach is convenient and well tolerated.

Maintenance dose (after loading, or for non-deficient at-risk patients):

• 800–2,000 IU daily depending on body weight, ongoing risk factors, and follow-up levels
• Higher (up to 4,000 IU) for people with obesity or malabsorption

Why 'more is not better'

Vitamin D toxicity is rare but real. Doses above 4,000 IU daily long-term, or single mega-doses, can cause hypercalcaemia — raised blood calcium leading to kidney stones, kidney damage, nausea, confusion, and in severe cases cardiac arrhythmias. The therapeutic window is wide but not infinite.

Online vendors sometimes sell 10,000 or 20,000 IU daily doses marketed as 'optimised' — there's no credible evidence base for these doses in adults without confirmed severe deficiency and clinical monitoring. Stick to evidence-based dosing.

How we test and treat at Trafford Clinic

Vitamin D testing is part of our standard Manchester blood test menu. We use UKAS-accredited labs; results return in 24–72 hours. For confirmed deficiency, Haroon as an Independent Prescriber can issue a private prescription for the loading dose if appropriate, or recommend OTC supplementation if levels are insufficient rather than deficient.

We test patients from across Greater Manchester: Rusholme (the highest-volume site for South Asian patients), Old Trafford, Salford, Manchester city centre, Altrincham, Sale, Chorlton and others.

How to book

Book a vitamin D test (standalone or as part of a broader panel) at traffordclinic.co.uk/blood-tests or call 0161 258 6149. Empire Pharmacy is GPhC-registered (premises 1123966); Haroon Iqbal is an Independent Prescriber (reg. 2051093).

Related reading: Reading blood test results, NICE CG189 BMI thresholds.