Vitamin D in a sunny country: why deficiency still happens

Saigon has sun nearly all year, so "vitamin D deficiency" sounds implausible at first. But deficiency in tropical regions is well documented — and the reason lies in how people live, not how much sun there is.

Why sun isn't enough

Skin makes vitamin D on exposure to UVB in sunlight. Several conditions have to line up, and in a city they usually don't:

• Indoors nearly all day. Leaving early, returning late, air-conditioned rooms in between. Window glass blocks most UVB, so sitting in a sunlit room by the window produces very little.
• Thorough sun protection. Jackets, face masks, gloves, sunscreen — a very common habit in Vietnam, and it blocks exactly what skin needs.
• Avoiding the hours with UVB. UVB peaks around midday, which is precisely when nobody wants to be outside because of the heat.
• Air pollution reduces the UVB reaching ground level.

Together, someone living in a tropical city can easily get less sun exposure than someone in a cold country who spends time outdoors.

The extra difficulty for fully plant-based eaters

Food isn't anyone's main vitamin D source, meat eaters included. But the food sources that exist are almost all animal-derived: oily fish, egg yolk, liver, fortified milk.

For someone fully plant-based, that leaves almost no natural food source. Two remain:

• Sun-exposed mushrooms. Mushrooms contain a vitamin D precursor that converts under sunlight into vitamin D2. Indoor-grown mushrooms — most of what markets sell — contain very little.
• Fortified foods. Some plant milks have vitamin D added. Only the label tells you, because not all of them do.

D2 and D3 — what plant-based eaters should check

Supplements come in two common forms:

• D2 — plant-derived (from yeast or mushrooms), always suitable.
• D3 — more common, but mostly made from lanolin in sheep's wool, so animal-derived.

There is lichen-derived D3 made for plant-based diets, but it has to be sought out and the label read carefully — don't assume any given product qualifies.

What to do before buying supplements

The first step isn't buying pills, it's a blood test to find out where you actually stand. Why:

• Deficiency is measurable, not something to guess at by how you feel.
• Vitamin D is fat-soluble and accumulates in the body rather than clearing quickly like water-soluble vitamins. High doses over a long period without monitoring carry risk.
• The dose depends on how low you are, on weight, age, and other medications. That's a doctor's job, not an article's.

This article gives no dosages. If you suspect deficiency, get tested and ask a doctor — mentioning that you eat fully plant-based so the right form can be chosen.

Getting useful sun

It doesn't take long, and it shouldn't go as far as burning:

• Bare skin without sunscreen on the exposed part — face, arms, legs all work. With sunscreen the purpose is defeated.
• Short periods, several times a week. Longer exposure raises skin risk without a matching rise in vitamin D.
• Avoid the harshest hours if your skin burns easily, and weigh it carefully with any history of skin problems — a question for a dermatologist.

Older adults synthesise vitamin D from skin less efficiently, so sun exposure doesn't solve as much as it does for younger people.

Two common misconceptions

• "Drinking milk covers it." Only fortified milk, and only if the label says so. Home-made plant milk contains none.
• "Sitting in a sunny window works." Glass blocks most UVB, so the effect is close to nil.

This is general information, not a substitute for diagnosis or medical advice.

Quick answers

Can you be deficient in a sunny country?

Yes, and it's fairly common. The causes are lifestyle: indoors all day, thorough sun protection, avoiding the hours when UVB is strongest, plus air pollution reducing UVB at ground level.

Does sitting by a sunny window help?

Almost not at all, because glass blocks most of the UVB that skin needs.

Where do plant-based eaters get vitamin D from food?

Very limited sources. Natural food sources are mostly animal-derived. What remains is sun-exposed mushrooms — indoor-grown ones contain little — and fortified foods such as some plant milks, which only the label confirms.

What's the difference between D2 and D3 here?

D2 is plant-derived and always suitable. D3 is more common but mostly made from lanolin in sheep's wool; lichen-derived D3 exists for plant-based diets but the label must be read carefully.

What should I do before buying a supplement?

Get a blood test to find the actual level, then ask a doctor. Vitamin D is fat-soluble and accumulates, so high doses over time without monitoring carry risk.

How much sun is enough?

Not long, and never to the point of burning — short periods several times a week on skin without sunscreen. Longer exposure raises skin risk without a matching gain.