The most common type is acne vulgaris, characterised by inflamed, pus-filled spots. The facts that more teenage boys than girls suffer from acne and that people with no male hormones (eunuchs) do not suffer at all give some insight to the mechanisms behind it. The amount of the male hor mone testosterone in the body increases at puberty (in girls too, although not as much as in boys) and triggers the production of sebum and keratin. Keratin is the main constituent of the outer layer of skin, and an excess can block pores, as can too much sebum. It has now been found that it is not just the increase in testosterone - which happens to all teenagers - but excess conversion to an even more powerful version of the hormone called DHT (dihydrotestosterone), which may bring on acne.

With the increase in keratin, a blockage forms which in turn creates a build-up of sebum behind it and shows up as a blackhead. As the pore becomes blocked, it provides an ideal breeding ground for the bacterium Proprionibacterium acnes, which normally ' lives harmlessly on the surface of our skin. P acnes' s ideal party environment is one with no air and plenty of sebum to feed off so it is easy to see how it has a field day and creates an infection in the skin, causing the inflammation and soreness of a spot. If this inflammation gets out of hand, it can spread through to deeper tissues and, if it does not break through to the surface, it causes a cyst under the skin.

The main consideration diet-wise is to ensure an all-round healthy diet which focuses on plenty of fresh vegetables and fruit, and wholegrains with some protein in the form of fish, lean meat, soya products and beans or lentils. Sugary, processed and fatty foods should be avoided. He should drink six to eight glasses of pure water daily and keep alcohol to a minimum and use a pH balanced skin cleanser. As far as supplements are concerned, make sure he is supple - menting at least 3,000mcg (10,000ius) of vitamin A (retinol), 25mg of zinc and 200meg of chromium daily. Probiotic supplements can also help, especially if your son has been treated with antibiotics.