When the days shorten and the sun sinks low in the sky, that familiar winter slump sets in. There’s a fundamental reason for this seasonal fatigue: low levels of vitamin D – the “sunshine vitamin”.

People in the UK simply can’t make enough of this vital nutrient from sunlight during winter, which is why many are advised to take a supplement.

So, what is vitamin D, why does it matter, and how much do you really need?

Vitamin D is a fat-soluble vitamin. This means it’s absorbed with other fats in our diet, and stored in the body’s liver and fatty tissue.

Vitamin D helps the body absorb calcium from the gut and regulates calcium and phosphate levels. These minerals are crucial for healthy bones, teeth and muscles.

Vitamin D plays a key role in bone development and in the continuous renewal of bone tissue. Without enough vitamin D, adults can develop osteomalacia (soft bones) and children can develop rickets (a bone deformity, often characterised by bowed legs).

Vitamin D supplementation, in conjunction with calcium, helps maintain bone mineral density and has been shown in trials to reduce fracture risk in people with osteoporosis.

Many tissues in the body have receptors that respond to vitamin D, as well. This means vitamin D can also affect neuromuscular function by helping us maintain muscle strength.

Low vitamin D levels have also been associated with a higher risk of several health problems,........

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