Your body can make Vitamin D from sunlight — so why is nearly half the global population still deficient in it?

In this episode of So That's Why, Jen, Chris, and Matt unpack what Vitamin D is actually doing inside the body, why the sunshine route fails so many people, and why deficiency shows up as fatigue, frequent illness, and muscle weakness rather than just weak bones. Along the way, they bust the sunscreen myth, explain why D3 is not the same as D2, and make the case for why Vitamin D supplementation is one of the most cost-effective health decisions available.

Timestamps 00:00 — Introduction 02:14 — Why Vitamin D is also classified as a hormone 05:18 — How the body produces Vitamin D from sunlight 06:07 — Why so many people are deficient despite sunshine 09:32 — Food sources, fortification, and supplementation 11:03 — How much Vitamin D do you actually need? 14:46 — The bigger picture: sleep, immunity, and muscle function

Vitamin D Is a Hormone as Much as a Vitamin

[02:14]

Most people know Vitamin D as a bone health supplement. What fewer people know is that it functions as a hormone — one that regulates over a thousand genes and has receptors in virtually every cell in the body.

"Vitamin D regulates over a thousand genes. It coordinates calcium absorption, it manages immune function, it helps maintain muscle strength and influences cellular processes throughout your body." — Jen

This is why deficiency produces such a varied range of symptoms. It is not one system failing — it is the body's master regulator running below capacity.

Why Sunlight Alone Is Not Enough

[06:07]

The UV radiation needed to produce Vitamin D requires sunlight at the right angle — typically midday sun. Anyone living above around 35 degrees latitude, roughly north of Los Angeles or Southern Spain, cannot produce Vitamin D from winter sunlight at all. For UK listeners specifically, Chris shares a striking statistic: if you live north of Milton Keynes, the average year does not deliver enough UV to consistently maintain Vitamin D production.

But latitude is only part of the picture. The Middle East records a 65% deficiency rate despite abundant sunshine, because staying indoors or covering up to escape heat means the skin never gets adequate exposure. Parts of Australia and some areas of India show similarly high rates. Skin pigmentation matters too — melanin reduces Vitamin D synthesis, meaning people with darker skin need significantly more sun exposure to produce the same amount. As Jen points out, darker skin in a northern climate is a genuine double challenge and a health equity issue that deserves more attention.

"If you live north of Milton Keynes, on the average year, you don't get enough UV for the entirety of the year to consistently produce Vitamin D." — Chris

On sunscreen: Chris is unequivocal — the idea that sunscreen blocks Vitamin D production is a myth. Sunscreens reduce UVB rays but not completely; enough UV still gets through for Vitamin D synthesis. Please do not avoid sunscreen for the sake of Vitamin D.

What Deficiency Actually Does to the Body

[02:23]

The effects of Vitamin D deficiency extend well beyond bone health.

1. Calcium absorption: Without adequate Vitamin D, the body absorbs only around half the dietary calcium it should. Bones do not mineralise properly — structurally present but soft and weak.
2. Immune function: Vitamin D controls antimicrobial peptides — the first line of defence against bacteria and viruses — while simultaneously preventing the immune system from overreacting and attacking the body's own tissues. As Jen puts it: it is about balance, not just strength. This is why deficiency links to both increased infection risk and increased autoimmune disease risk.
3. Muscle strength: Deficiency causes proximal muscle weakness — the large muscles in the thighs and upper arms — affecting everyday activities like stair climbing and standing from a chair. Chris notes this happens regardless of training level.
4. Sleep quality: Vitamin D receptors exist in the brain regions that regulate sleep-wake cycles. Deficiency links to poor sleep quality, difficulty falling asleep, and reduced deep sleep.

"There are multiple systems running below capacity — and understanding the why helps prioritise it." — Matt
D3 vs D2 — Why the Form Matters

[09:32]

Food sources of Vitamin D are limited and unreliable. For most people, supplementation is the most practical and consistent option. When choosing a supplement or fortified food, the form matters: Vitamin D3 is nearly 90% more effective than Vitamin D2, and is the form the body naturally produces from sunlight. Many fortified foods use D2 as a default, often labelled as the vegetarian option — but plant-source D3 is now available, and as Chris confirms, is directly equivalent to animal-derived D3. The body cannot tell the difference.

The European safe upper limit for ongoing daily intake is 4,000 IU per day.

"Consistency beats perfection — daily supplement, weekly high dose, or fortified foods plus supplementation. Whatever you'll actually stick to." — Jen
About So That's Why

So That's Why is a weekly podcast where Jen, Chris, and Matt unpack the science behind everyday health questions. No jargon, no judgment — just genuine curiosity and proper research.