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Vitamin D is critical for building strong bones and teeth, and has extremely wide-ranging influences on most of your bodily systems—and your brain.

Vitamin D is the ‘sunshine vitamin’, formed naturally by your skin when it’s exposed to the ultra-violet light that accompanies bright sunlight. In summer, you should make enough to last you through the darker days of winter. It’s fat-soluble, so we can store it—but only if we’re exposed to enough sunlight to build up a surplus, or get plenty in our diets. Few foods provide much vitamin D directly. Organ meats (liver, kidneys, etc.), oily fish or full-fat dairy produce are the main dietary sources.

A severe lack of vitamin D causes rickets (softening of the bones, leading to physical deformities). Less obvious deficiency is a major cause of osteoporosis, and is implicated in unexplained muscle and bone pain. That’s because you need vitamin D—along with magnesium—to actually get calcium into your bones.

Active vitamin D is also one of the most powerful pre-hormones in the human body. Deficiency can contribute to heart disease, stroke, hypertension, various autoimmune diseases, diabetes, depression, chronic pain, osteoarthritis, muscle weakness, muscle wasting, birth defects, periodontal (gum) disease, and 17 varieties of cancer!⁸ There’s also some evidence that vitamin D deficiency in pregnancy may contribute to hyperactivity and mental health problems in the next generation.⁹

This being so, it’s not good news that almost one in four children and adults in the UK are seriously deficient in vitamin D each winter, and those who rarely go outside are at risk all year round. What’s more: people with dark skins need up to 10 times the exposure to UV light that fair-skinned people do to make the same amount of vitamin D. This needs to be recognized, as indoor lifestyles (and even some habits of dress) can make it difficult to achieve enough exposure to sufficiently bright sunlight in the UK (or any other countries at a similar, or greater, distance from the equator).

If more than a quarter of our children and adults are frankly vitamin D deficient for large parts of the year, why haven’t we heard more about this? Could it be because no one can patent sunshine, and there are no big profits to be made from selling vitamin D? Looking on the bright side (sorry)—at least sunshine is free: just make sure that you and your child get enough. Take care never to burn (if skin turns even slightly pink, that’s more than enough), but the evidence suggests that moderate exposure can bring real health benefits, with no serious additional risks of skin cancer, which we have all heard plenty about.