One of the subjects in O'Connell's article, Julianne, had never suffered from spots as a teenager, but by the time she was 28 she had terrible cystic acne along her jawline and across her neck. An American, she had travelled to Europe to learn to do a cookery course.

Julianne had decided to open a restaurant and deli back in the States, so before she returned home she toured around Europe, sampling every cheese she came across. As she told her story to the dermatologist Bill Danby, something clicked: "Oh my God, it's the cheese," she said. For six months, she stopped eating all dairy products. During that time she became 85 per cent free of acne, and her skin cleared up.

According to Danby, a skin expert, "The ability to develop acne is partly genetic and partly the result of hormone exposure [e.g. hormones from dairy milk]. I tell my female patients that genetics are the key to the fact that Paris Hilton has lots of money and no zits and my patients have lots of zits and no money."

Although acne can affect people at any age, it usually peaks at between 16 and 18, when up to 98 per cent of the population of developed countries are affected. A link between dairy milk and acne has been observed because acne is significantly less common in parts of the world that consume less dairy milk. As well as being socially embarrassing, acne is costly - up to five $billion is spent worldwide each year treating the affliction.

This is how milk causes acne: hormones in dairy milk stimulate the glands of hair follicles, making them secrete more sebum than they normally would. As a consequence, hair follicles stick together and form a plug in the pore of the skin - the first visible sign of acne. As the plug 'pinches' the sebum canal it prevents the free-flow of sebum to the skin. The result is acne.

Milk is full of hormones: not only ones intended to help the calf grow, but also those produced by the placenta to aid the cow's pregnancy. Another worrying hormone, as far as acne is concerned, is IGF-1. Both humans and cows make IGF-1 in their bodies. This hormone peaks at age 15 in girls and 18 in boys, coinciding with peak acne levels.

IGF-1 is known to work with testosterone and DHT to cause acne. IGF-1 is found in all types of dairy milk: raw, pasteurized, organic and nonorganic. So when you consume any type of dairy milk it will significantly increase your levels of IGF-1 and be likely to cause acne.

There is plenty of evidence showing that milk is a major cause of acne. In a major study, Dr Walter Willett and his colleagues at the Harvard School of Public Health in Boston looked at the diets of 47,000 women who were part of a research project called the 'Nurses Health Study II'.

As part of the study, 47,000 women were required to complete questionnaires concerning their diet as teenagers, and to indicate whether they ever had severe acne. The study found no links between foods such as potato chips or chocolate and acne, but they did find a strong link between women who had acne and those who had regularly consumed milk.

This is confirmed by other research:

* IGF-1 [in cow's milk] contributes to the increase in sebum production during puberty. (Endocrinology, 1999 Sep, 140:9).

* About 80% of [dairy] cows are throwing off hormones continuously [milk is] implicated as a factor in the development of acne teenage acne patients improved as soon as milk drinking stopped. (Frank Oski, M.D., Don't Drink Your Milk, Teach Services, Inc).

* About 80% of cows that are giving milk are pregnant and are throwing off hormones continuouslyDr. Jerome has found that acne improved as soon as the teenagers stopped consuming milk. (Dr. George J. Georgiou, Ph.D., Clinical Nutritionist, Milk - A Recipe for Disease, Nov. 2002, worldwidehealthcenter.net).

There is, of course, a simple solution to help prevent acne: switch to non-dairy milk. You can buy commercial non-dairy milk such as soy and rice milk in some supermarkets. Better still, make your own non-dairy milk. When made correctly home made milk is super-nutritious and truly delicious, and of course, it does not cause acne.