Children in the west are principally becoming obese because their diets play havoc with their hormones, according to a new study by scientists.

Robert Lustig, a paediatrician at the Children's Hospital of the University of California in San Francisco, said: "Our current western food environment has become highly insulinogenic, as demonstrated by its increased energy density, high-fat content, high glycaemic index, increased fructose composition, decreased fibre, and decreased dairy content." More than a fifth of boys and a quarter of girls in the UK are classified as overweight or obese. Overweight children are more likely to become overweight adults, putting people at greater risk of heart disease and stroke.

In the latest issue of Nature Clinical Practice Endocrinology and Metabolism, Dr Lustig says the obesity epidemic rests on effects of processed food, which has sugar added to a wide variety of products that used never to include it and has fibre removed. This upsets the balance of two hormones which regulate how much we eat - leptin and insulin.