Understanding Vitamin D

We call it the sunshine vitamin, yet for much of the world sunlight is not a reliable source, and deficiency is strikingly common.

Vitamin D occupies an unusual place in nutrition: it behaves less like a vitamin and more like a hormone, and our main natural source of it is not food but sunlight.

What it does

Its best-known role is in the skeleton, vitamin D is required for the body to absorb calcium, and deficiency causes rickets in children and osteomalacia, or soft bones, in adults.¹ Beyond bone, it has been linked to immune function, respiratory illness, pregnancy outcomes, and chronic diseases of adulthood.¹

The trouble with sunlight

The body makes vitamin D in the skin under ultraviolet-B light, but that supply is unreliable. It depends heavily on latitude, season, skin tone, and how much time we spend indoors. People at high latitudes are far more likely to be deficient, and deficiency in winter and spring can run about 1.7 times that of summer and autumn.²

The sunshine vitamin, for a species that increasingly lives indoors.

How common is deficiency

Very. In a pooled analysis of nearly eight million people, roughly 48% had vitamin D levels below 50 nmol/L and about 77% below 75 nmol/L.² It is among the most widespread nutritional insufficiencies in the world.

What to do

Because sunlight is unreliable, food fortification and targeted supplementation are the main levers for at-risk groups.¹ Vitamin D is a case where a simple, inexpensive intervention can quietly protect a great deal of health.