Sunny countries have vitamin D deficiency too. Here’s what that actually tells you.

We spent September explaining why Britain runs short of vitamin D, and the explanation was essentially geographical. We're at 50 to 60 degrees north. From about October to March the sun doesn't climb high enough for UVB, the specific slice of sunlight your skin needs, to get through the atmosphere in any useful quantity. Wrong latitude, wrong months, no vitamin D.

All true. And it leads almost everyone to the same comfortable conclusion: this is a British problem, caused by British weather, and people in sunnier places don't have it.

They do. Quite badly, in places.

Which is much more interesting than it first appears, because once you know why sunny countries have vitamin D deficiency, you understand your own situation considerably better, wherever you happen to be reading this.

Start with the one that makes the point most cleanly.

Australia's National Health Measures Survey, part of the Australian Health Survey, measured vitamin D in blood across a nationally representative sample. The result: around 23% of Australian adults fell below the survey's adequacy threshold of 50 nmol/L: 17% mildly, 6% moderately, under 1% severely.

Nearly a quarter of a country that is, by any reasonable standard, extremely sunny.

And the seasonal pattern is the same shape as ours, just moved six months: 14% in summer, rising to 36% in winter. The state-level numbers are starker still. In Victoria and the ACT the winter figure approached 49%, against 16% and 13% in summer. Queensland and the Northern Territory stayed low all year round.

Look at where those places are. Melbourne sits at about 38 degrees south. Hobart is nearer 43. Those are the southern-hemisphere equivalents of southern Spain and central Italy, far enough from the equator that winter sun sits low, exactly as it does here. Darwin, at 12 degrees south, has no such problem and the data shows it.

So Australia isn't an exception to the latitude rule. It's the same rule, running on a different calendar. If you're reading this in Melbourne in October, you're heading into your good months, not out of them. Your version of this article belongs in April.

Before going further, the thing that trips up almost every article on this subject, including, nearly, this one.

"Deficient" does not mean the same thing in every country.

The UK defines vitamin D deficiency, in relation to bone health, as blood levels below 25 nmol/L. Australia's survey used 50 nmol/L, double that. Other countries and other studies draw the line somewhere else again, and the laboratory methods used to measure it vary too.

Which is why you won't find a league table of nations here. Put two countries' headline percentages side by side and you're usually comparing two different definitions, measured two different ways, in two differently-shaped populations. That's how you get the confident international rankings that circulate online and then fall apart the moment anyone reads the footnotes. It's the same trap as the "half of UK adults are deficient" figure we took apart in September: the number that sounds most quotable is usually the one built on a different definition.

What does travel between countries is the mechanism. Wherever a study draws its line, the same pattern turns up: deficiency climbs in winter, climbs with latitude, and climbs sharply among people who rarely get uncovered skin into high-angle sun. That part is consistent everywhere, and it's the part that's actually useful to you, because unlike your country's national average, it's about your week.

The pattern holds, and gets stronger, closer to the equator.

A 2021 systematic review and meta-analysis in BMC Public Health by Siddiqee and colleagues pooled 65 studies across South Asia and found 68% of adults below 20 ng/mL, the same 50 nmol/L line, with India at 67% and Pakistan at 73%. Reviews across the Middle East and North Africa report figures that vary enormously by country and population but frequently sit well above half.

These are regions with abundant, year-round, high-angle sunshine. Latitude is not the constraint. Something else is.

This is the point our co-founder Chris makes on the So That's Why episode about vitamin D: that your vitamin D status has far more to do with how you live than where you live. It's worth a listen, and what follows is the long version with the research laid out underneath it.

Four things. None of them is how much sunlight lands on the country.

One: are you outside in it? In a very hot climate, the middle of the day is when you go indoors, into air conditioning, into shade, into a car. The behaviour is sensible and it's the exact opposite of what vitamin D synthesis requires. Britons chase the midday sun because it's rare. Where it isn't rare, people avoid it, and the biochemistry doesn't care about the reason.

Two: is your skin uncovered? Clothing blocks UVB completely. People cover up for heat, for sun safety, for modesty, for work, for dozens of entirely reasonable reasons. Every one of them has the same effect on your skin's production line.

Three: is the sun actually high enough? Even in sunny countries, UVB only arrives in quantity when the sun is high, roughly the middle of the day. Early-morning and late-afternoon sunshine is mostly UVA, which is lovely and does nothing for vitamin D. Being outdoors at the wrong hours can feel like plenty of sun and deliver almost none.

Four: how much melanin do you have? Melanin is an extremely effective natural sunscreen, which is precisely what you want at the latitudes where it's most common, and a genuine handicap when UVB is in short supply. More melanin means more UVB exposure needed for the same yield. In the UK the effect is stark: the same Christie review found deficiency below 25 nmol/L in 12% of White participants, 35% of Black participants and 54% of Asian participants. This is a property working exactly as designed, meeting an environment it wasn't designed for.

On top of those: skin converts less efficiently with age, vitamin D distributes into body fat so a higher body fat percentage leaves less circulating, and sunscreen blocks UVB because blocking UVB is its entire job.

Vitamin D status is not a fact about your country's weather. It's a fact about your skin, your schedule, your clothes and your latitude, in combination, which is why two people in the same city can have completely different results, and why an office worker in Dubai and an office worker in Dundee can end up in a more similar position than either would guess.

The question isn't does the sun shine where I live. It's does UVB reach enough of my skin, at a high enough angle, often enough.

For a lot of people, in a lot of places, the honest answer is no. That isn't a warning, and it isn't a crisis. It's just a more accurate way of thinking about it than the weather map.

A thirty-second version you can actually run

Four questions, and the more of them you answer "no" to, the less the local weather is telling you:

1. In the last week, were you outside, in daylight, in the middle of the day? Not commuting in the dark at either end. Actual midday.
2. Was any meaningful amount of skin uncovered while you were? Forearms and face is plenty. A coat and a scarf is not.
3. Is the sun where you are currently climbing high in the sky? Above roughly 37° north or south, it doesn't in winter, whatever the sky looks like.
4. Is there anything that raises your requirement? Deeper skin tone, being over 65, a higher body fat percentage, covering up routinely, or working indoors through every daylight hour.

Four "no"s and a "yes" to question four, in a northern October, is about as clear as this gets without a blood test. And if you want the actual number, a vitamin D blood test is cheap, widely available, and the only thing here that replaces guessing with knowing.

We make vitamin D3 from lichen, which is worth a sentence because it's the reason this company exists.

D3 is the form your own skin makes. For decades every D3 supplement on earth was derived from lanolin, the grease from sheep's wool, which meant that for anyone plant-based the choice was a less efficient form, or nothing. Our founders are scientists, they thought that was a poor state of affairs, and they went and found D3 in lichen instead. That became Vitashine, the world's first vegan-certified D3. Same molecule, no sheep.

Our autumn dose for the northern locales is D3 2500iu, one small tablet a day. The rest of the range exists because people differ, not because bigger is better:

• 1000iu: a steady, modest daily dose and a sensible starting point
• 1000iu Spray: for anyone who finds tablets difficult
• 4000iu: for people who've been advised they need more
• 4000iu + K2: for anyone whose focus is bone health

Pick for your situation rather than for the biggest number on the box. And if you're somewhere the sun genuinely is high and you genuinely are out in it, uncovered, regularly, you may well not need us at all, and we'd rather say so than pretend otherwise.

Vitamin D deficiency isn't really a northern problem. It's an indoor problem, a covered-up problem and a wrong-time-of-day problem, and northern winters just happen to be the version of it that removes the option entirely.

Which means the useful question isn't where you live. It's what your week actually looks like.