Fingernails help protect the underlying skin from injury, and they also come in handy when you want to scratch an itch or peel a satsuma. But what do they reveal about your health?

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There's no shortage of folk wisdom about how to glean the state of your overall health from your nails, such as the pervasive idea that the white flecks that sometimes turn up – known as leukonychia – are a sign of calcium deficiency. But is there any truth to these ideas?

First up some basic anatomy. Nails are an extension of the skin. They are made from keratin, a super tough protein that shields the toes and tops of your fingers from trauma. The half-moon shape seen at the base of the nail is the lunula, which serves as the "growth centre" for the nail, producing the cells that will eventually harden into the nail plate . It sits above the cuticle, a layer of dead cells joining the base of the nailbed to the skin. The cuticle offers extra protection by acting as the nail's security guard, stopping bacteria, fungi and other pathogens in their tracks.

While the eyes might be the windows to the soul, to a doctor the nails can be the windows to your health. Physicians can use them to diagnose all sorts of conditions, from dermatological problems to kidney disease or even autoimmune disorders.

"One of the first things I learned in medical school was to look for something called clubbing, where there is this loss of the angle between the nail and the nail bed itself," says Dan Baumgardt, a general practitioner in medicine and lecturer in neuroscience and physiology at the University of Bristol.

With clubbing, the nail beds soften, and the nails almost seem to "float" instead of being firmly attached to the finger. The fingertips also appear large or bulging. "It causes an unusual sort of swollen finger appearance where the finger looks a little bit like a drumstick," says Baumgardt.

Clubbing is a sign of extremely low blood oxygen levels. It is most commonly associated with lung cancer, but it can also indicate an infection of the lining of the heart chambers and heart valves, among other things. Other conditions where it is observed include celiac disease, cirrhosis of the liver and lung infections amongst others.

"If you do see a patient with clubbing the rule of thumb is to get them an x-ray as an urgency, because it could be an underlying lung cancer that could be explaining it," says Baumgardt. "Having said that even though it's one........

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