Nutrients

Skin Tone, Latitude, and Why Vitamin D Advice Fails in Scotland

Stephen Duncan FDN-P BSc Hons MSc  ·  7 min read

Vitamin D advice in this country tends to arrive as a single number for everybody. Ten micrograms a day, October to March, and off you go.

That advice is not wrong. It's just calibrated for a population average in a country where the variation between individuals is enormous — and two of the biggest sources of variation, skin tone and latitude, interact in a way that makes the average close to meaningless for the people at either end of it.

Edinburgh sits at 55.9 degrees north. That's further north than Moscow. It's worth understanding what that actually means for your skin.

How the skin makes vitamin D

UVB radiation, specifically 290 to 315 nanometres, strikes 7-dehydrocholesterol in the epidermis and converts it to pre-vitamin D3. Body heat then isomerises that to vitamin D3, which goes to the liver to become 25-hydroxyvitamin D — the form measured on a blood test — and then to the kidney to become the active hormone.

Every step has requirements. The first one has a geographical requirement, and it's the one that catches people out.

UVB doesn't reach the ground when the sun is low. It's scattered and absorbed by the atmosphere, and the lower the sun sits, the more atmosphere it has to travel through. Below roughly 45 degrees of solar elevation, effectively no useful UVB gets through.

At Edinburgh's latitude, that means no meaningful vitamin D synthesis from around October to March, regardless of how bright the day looks or how long you spend outside. A crisp, clear January afternoon in Scotland produces essentially no vitamin D. The light is real; the relevant wavelength isn't there.

Even in summer, synthesis is restricted to the middle of the day. Early morning and evening sun, however pleasant, is doing very little.

Melanin

Melanin is a highly effective natural sunscreen. That's its function — it protects against UV damage, which is a substantial evolutionary advantage near the equator.

The consequence is that it also blocks the UVB needed for vitamin D synthesis. Estimates vary, and I'd treat any precise figure with caution, but deeply pigmented skin needs considerably longer UVB exposure than pale skin to produce the same amount of vitamin D — commonly cited as several times longer.

Now combine that with latitude.

Someone with dark skin living in Scotland has a very short window each year in which vitamin D synthesis is possible at all, and needs substantially longer exposure within that window than a pale-skinned neighbour. Those two constraints multiply rather than add.

This is the single most predictable vitamin D deficiency picture in the UK, and it follows directly from geography and physics. Nothing about it is mysterious. Yet the public health advice given to that person is the same ten micrograms given to everyone else.

The reverse case matters too. Pale skin evolved in exactly these latitudes precisely because it permits vitamin D synthesis from limited UVB — an adaptation to the same problem, running in the other direction.

What else blocks it

Glass blocks UVB entirely. Sitting by a sunny window produces no vitamin D. A conservatory in June is, for these purposes, indoors. This surprises people every time.

Sunscreen blocks it, by design. SPF 30 applied properly reduces synthesis very substantially. In practice most people apply far less than the amount used in testing, so real-world blocking is less complete — but if you're diligent with sunscreen you are not making much vitamin D.

I'm not telling you to skip sunscreen. Skin cancer is a serious risk and it doesn't go away because you'd like more vitamin D. The sensible resolution is short unprotected exposure around midday in summer, well short of any pinkness, with sunscreen for longer periods — not choosing between the two.

Age reduces it. 7-dehydrocholesterol in the skin declines with age, so older skin produces less from identical exposure.

Body composition affects it. Vitamin D is fat-soluble and distributes into adipose tissue, which lowers circulating levels for a given intake in people carrying more body fat.

Covering up for cultural or religious reasons has the same effect as sunscreen, and combined with darker skin at Scottish latitude produces the highest-risk group of all.

Why supplementing sometimes doesn't work

This is where it gets clinically interesting, and where I see people who have been taking vitamin D for two years with a level that has barely moved.

Vitamin D is fat-soluble, so it needs fat to be absorbed. Taken on an empty stomach with a coffee, much of it isn't absorbed. Take it with the fattiest meal of the day.

Absorption depends on bile. Fat-soluble vitamins require micellar emulsification, and that requires adequate bile acids. In genuine cholestasis this is dramatic — when intraluminal bile acids fall below the concentration needed to form micelles, absorption fails, and those patients need very high doses or specialised formulations. Lesser degrees of impaired bile flow produce a lesser version of the same problem.

Magnesium is required to metabolise it. The enzymes that convert vitamin D to 25-hydroxyvitamin D and then to the active form are magnesium-dependent. A randomised trial found magnesium supplementation modified vitamin D status — raising it when baseline was low. Supplementing vitamin D in someone magnesium-depleted underperforms, and magnesium depletion is common.

Gut integrity matters. Inflamed or compromised small intestinal epithelium absorbs less of everything fat-soluble.

And receptor sensitivity varies. Vitamin D receptor variants mean the same circulating level produces different biological effect in different people.

So "I take vitamin D and my level is still low" is a clinical finding, not a compliance failure. It usually points at fat digestion, bile, or magnesium — and testing tells you which.

Vitamin K2

If you're taking therapeutic doses of vitamin D, take K2 alongside it.

Vitamin D increases calcium absorption from the gut. Vitamin K2 activates the proteins that direct that calcium into bone and keep it out of arterial walls. Raising absorption without adequate K2 is a reasonable way to end up with calcium in the wrong place. MK-7 is the preferred form.

What I'd actually suggest

Test rather than guess. 25-hydroxyvitamin D is inexpensive and widely available. Sufficiency in lab terms starts around 50 nmol/L (20 ng/mL). My working range is 75–125 nmol/L (30–50 ng/mL), a clinical target rather than official guidance. Anything beyond that is a conversation with a practitioner, not a self-prescribing decision, because vitamin D is fat-soluble and genuinely can be over-supplemented.

If you have darker skin and live in Scotland, assume you need to supplement year-round unless testing tells you otherwise. Geography has made that decision for you.

Take it with fat, ideally the largest fat-containing meal of the day.

Check magnesium status if levels aren't responding.

Get midday summer sun on skin without burning, and don't imagine that a bright winter day is doing anything at all.

Retest after three months. If the number hasn't moved on a sensible dose, the question isn't the dose. It's absorption — and that's a different investigation.

Stephen Duncan is a Functional Diagnostic Nutrition Practitioner (FDN-P, BSc Hons, MSc) based in Edinburgh, working with people since 1988.

Educational content, not medical advice. Vitamin D is fat-soluble and accumulates; high-dose supplementation should be monitored. If you are on medication or have kidney or parathyroid conditions, speak to your doctor before taking high doses.