By Richard Black BBC science correspondent

The number of obese children in developing countries has tripled over the past 20 years, a new report says.

According to the study in the American Journal of Clinical Nutrition, the increase in overweight children is linked to growing social prosperity, which had led to children eating more and exercising less.

Researchers reviewed two decades-worth of national health data from Brazil, China, Russia and the USA.

In Russia, childhood obesity has fallen slightly; but in the other three countries, it has risen.

In China, it has gone up by one fifth; and in the USA, it has doubled, with around a quarter of children now qualifying as obese.

But the most startling figures come from Brazil, where childhood obesity has tripled over the last 20 years.

The researchers comment that in the same period, Brazil's per capita gross domestic product has also tripled, and television use has greatly increased.

Health warning

In recent years a number of bodies including the World Health Organisation (WHO) have warned that obesity is becoming a major issue for some developing nations, especially those such as Brazil which have seen marked economic progress in the last few years.

Last month, WHO Director-General Gro Harlem Brundtland urged people to fight what she called "the epidemic of inactivity", to avoid the problems of heart disease and diabetes which are linked to obesity.

The number of adults with diabetes is expected to double over the next 25 years, with the vast majority of that rise happening in the developing world.