Of all the health conversations I have with African and Black patients in the UK, the vitamin D conversation is the one where I most consistently find myself saying: "Almost certainly yes, you are deficient. Let's check, but I already know what we'll find."
I say this not as a guess but as a near-certainty based on biology and geography. The combination of melanin-rich skin, the latitude of the UK, and the indoor nature of modern working life creates conditions in which adequate vitamin D synthesis from sunlight is essentially impossible for most of the year for dark-skinned people. This is not a lifestyle failing or a matter of not spending enough time outdoors. It is a physiological reality.
And it matters — because vitamin D does considerably more than support bone health, which is where the public conversation about it tends to stop.
The biology: why melanin changes everything
Melanin — the pigment that gives skin its colour — is extraordinarily effective at blocking UV radiation. This evolved as a protective mechanism against skin cancer and DNA damage in high-sunlight environments near the equator.
In equatorial Africa, where UV radiation is intense year-round, this is a significant advantage. In the UK — at 51–58 degrees north latitude — it becomes a liability. The UV radiation is weak for most of the year, and the melanin that evolved to protect against excess UV now blocks the limited UV that reaches UK skies.
The result: a person with dark skin needs approximately 5–10 times longer sun exposure to produce the same amount of vitamin D as a person with light skin in the same conditions.
In practical terms: the amount of sun exposure a fair-skinned person might need for adequate vitamin D synthesis in a UK summer (approximately 15–30 minutes of forearm and face exposure several times per week between April and September) becomes hours of exposure for someone with dark skin. Exposure that is rarely achievable outside of a beach holiday.
Between October and March, UVB radiation in the UK is so weak that no meaningful vitamin D synthesis occurs regardless of skin tone. Everyone in the UK relies on stores built up over summer and dietary sources during these months. For dark-skinned people whose summer synthesis was already inadequate, winter levels become critically low.
What vitamin D deficiency actually causes
Bone health: The most established consequence. Vitamin D is essential for calcium absorption. Severe deficiency causes rickets in children and osteomalacia (soft bones) in adults — a condition that can be mistaken for chronic musculoskeletal pain.
Muscle weakness and fatigue: Often profound and frequently dismissed or attributed to stress, depression, or overwork. Many of my patients describe vitamin D replacement as "transformative" for their energy levels.
Immune function: Vitamin D plays a significant role in immune regulation. Deficiency is associated with increased susceptibility to respiratory infections, and there is emerging evidence linking it to autoimmune conditions.
Mood: Low vitamin D levels are associated with depression, though the causal relationship is complex. Supplementation improves mood in deficient individuals.
Cardiovascular health: Associations between vitamin D deficiency and hypertension, heart disease, and stroke have been found in multiple studies, though whether supplementation reduces cardiovascular events in non-deficient individuals remains debated.
Increased cancer risk: Large observational studies show associations between low vitamin D and several cancers, including colorectal, breast, and prostate cancer. The evidence for supplementation reducing cancer risk is less clear.
Fibroids: As discussed in our fibroids article, vitamin D deficiency may promote fibroid growth — a particularly relevant finding for Black women, who have both higher fibroid rates and higher vitamin D deficiency rates.
Case study: Grace's "mystery fatigue"
Grace, 38, a secondary school teacher from Ghana based in Leeds, came to see me complaining of persistent fatigue for eight months. She was sleeping 8–9 hours and still waking exhausted. She had joint aches she had attributed to "getting older." She had two episodes of low mood that had lasted weeks each.
Her GP had checked thyroid function (normal), full blood count (mild anaemia attributed to low ferritin — iron was supplemented), and inflammatory markers (normal). Vitamin D had not been checked.
I added it to her blood panel. Her 25-hydroxyvitamin D level: 18 nmol/L. Severely deficient (deficiency is defined as below 25 nmol/L; insufficiency as 25–50 nmol/L; optimal is generally considered 75–150 nmol/L).
I prescribed a loading dose of vitamin D — 40,000 IU weekly for 7 weeks, then 2,000 IU daily maintenance. I also continued the iron supplementation.
Six weeks later, Grace described her energy as "completely different — I feel like myself again." Her joint aches had largely resolved. Her mood had lifted.
"I'm slightly annoyed it took this long for someone to check," she said.
I understand her frustration. Vitamin D is a cheap, simple test. It should be routine for every Black patient in the UK.
The numbers — understanding your result
Vitamin D is measured as 25-hydroxyvitamin D (25-OHD) in nmol/L (UK) or ng/mL (US — multiply ng/mL by 2.5 to convert to nmol/L):
| Level (nmol/L) | Status |
|---|---|
| Below 25 | Deficient — treatment required |
| 25–50 | Insufficient — supplementation recommended |
| 50–75 | Adequate for most people |
| 75–150 | Optimal range |
| Above 250 | Potentially toxic — though rare with standard supplements |
What to take — the supplement question
Vitamin D3 (cholecalciferol) is the form to take — it is significantly more effective than D2 at raising blood levels. Available cheaply at any pharmacy or supermarket.
Standard NHS recommendation: 400 IU (10 micrograms) daily for adults — this is the minimum to prevent deficiency. For most dark-skinned people in the UK, this is insufficient to correct existing deficiency or maintain optimal levels.
More realistic dosing for Black adults in the UK: 1,000–2,000 IU (25–50 micrograms) daily for maintenance, with a loading dose of 40,000–80,000 IU weekly for 6–8 weeks if severely deficient (this should be supervised by a GP).
Take with food: Vitamin D is fat-soluble — absorption is significantly improved when taken with a meal containing some fat.
Vitamin K2: Evidence suggests that vitamin K2 (MK-7 form) works synergistically with vitamin D3 to direct calcium to bones rather than arteries. Consider a combined D3/K2 supplement, particularly if you are at cardiovascular risk.
Toxicity: Vitamin D toxicity is rare but possible at very high doses (above 100 micrograms / 4,000 IU daily for prolonged periods). At recommended doses, it is extremely safe. If taking high doses for deficiency correction, recheck levels after 3 months.
Dietary sources — helpful but not sufficient
Dietary vitamin D sources cannot compensate for lack of sun synthesis in dark-skinned UK residents, but they contribute:
- Oily fish (salmon, mackerel, sardines, herring): The best dietary sources — 400–800 IU per portion
- Egg yolks: Modest amounts (~40 IU per egg)
- Fortified foods: Some cereals, plant milks, and margarines are fortified — check the label
- Mushrooms exposed to UV light: Useful source, particularly for vegetarians
A diet high in oily fish provides perhaps 1,000–2,000 IU per week — still insufficient for dark-skinned people in the UK without supplementation.
The bottom line
If you are Black and living in the UK, you are almost certainly vitamin D deficient or insufficient at some point during the year — and likely year-round if you are not supplementing. The consequences are real and broad. The solution is simple and cheap.
Ask your GP to check your level. Take a supplement — 1,000–2,000 IU of D3 daily is safe, evidence-based, and likely to make a measurable difference to how you feel.
Sources: NHS — Vitamin D (nhs.uk/conditions/vitamins-and-minerals/vitamin-d); Darling AL et al, American Journal of Clinical Nutrition 2021 (vitamin D deficiency in UK ethnic minority groups); Webb AR et al, Nutrients 2018 (UVB synthesis by latitude and skin tone); Holick MF, NEJM 2007; Scientific Advisory Committee on Nutrition — Vitamin D and Health Report 2016.