Question 1

Why is vitamin D deficiency so common?

Accepted Answer

Because the main source is sunlight and modern life keeps us indoors. Add to that sunscreen use (correct for skin health but it reduces synthesis), the small amounts in food, latitude (in winter sunlight is insufficient across much of Europe), and factors like age or darker skin. The result: very frequent deficiency even in sunny regions.

Question 2

How much sun do I need to produce vitamin D?

Accepted Answer

It depends on latitude, season, time of day, skin type and how much skin you expose, but as a rough guide, 10-20 minutes of sun on arms and face several times a week in spring and summer is usually enough for many people. In winter at high latitudes UVB radiation is insufficient and synthesis drops almost to zero, so diet and supplements become much more important.

Question 3

Which foods contain vitamin D?

Accepted Answer

Very few, and in modest amounts: oily fish (salmon, sardines, mackerel), egg yolk, liver, UV-exposed mushrooms and fortified foods (some dairy and plant-based drinks). It is very hard to meet requirements through diet alone, which is why sun and, failing that, a supplement are so important.

Question 4

Should I supplement with vitamin D?

Accepted Answer

It is one of the few supplements with broad scientific backing, especially in winter, at high latitudes, if you get little sun, in older people or those with darker skin. The ideal approach is to check your blood levels first and decide the dose with your doctor. The preferred form is D3 (cholecalciferol). Do not overdo it: more is not better and excess can be harmful.

Question 5

What are the symptoms of vitamin D deficiency?

Accepted Answer

Often none that are obvious at first, which makes it silent. In marked or prolonged deficiency there can be fatigue, muscle weakness or pain, greater susceptibility to infections and, long-term, bone problems. Because symptoms are non-specific, a blood test is the only reliable way to know.

Source	Contribution	Notes
Sun (skin)	The main one	10-20 min several times/week in summer
Oily fish	Moderate	Salmon, sardines, mackerel
Egg yolk	Small	Adds up within a varied diet
Fortified foods	Variable	Some dairy and plant-based drinks
D3 supplement	Reliable	Useful in winter or with little sun

Vitamin D: Why Almost Everyone Is Deficient and How to Fix It

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