Childhood obesity rates are rising

The research, published in the journal Pediatrics, adds to mounting evidence suggesting childhood obesity is causing the trend of earlier puberty in girls.

Studies suggest girls who reach puberty earlier than the "normal" age of 10 and above also start drinking alcohol and begin having sexual intercourse sooner. UK experts said early puberty could cause girls significant distress.

They are also at increased risk of certain cancers, including breast cancer.

Rights of passage

The "normal" age for a girl to show the first signs of puberty is generally accepted as 10 and above.

But Dr Joyce Lee and colleagues at the University of Michigan Health System found many girls, who had been overweight as children and toddlers, reached puberty aged nine.

Fat and hormones

Of the 354 girls they followed from the age of three up to 12, almost half (168) showed signs of puberty - breast development - by their ninth birthday.

Many of these girls had a high 'body mass index' - a calculation of metric weight divided by height squared, which doctors use as a measure of obesity - throughout their childhood.

The University of Michigan study follows University of Bristol research in 2000 which suggested one girl in six reaches puberty before the age of eight - 18 months earlier than their mothers.

One theory is that puberty is triggered by the hormone leptin, which is produced by fat tissue.

We are seeing girls of seven, eight and nine who are starting their periods and going through puberty

Helen Crocker, research dietician at University College London

Dr Lee explained: "Previous studies had found that girls who have earlier puberty tend to have higher body mass index, but it was unclear whether puberty led to the weight gain or weight gain led to the earlier onset of puberty.

"Our study offers evidence that it is the latter."

Researchers at the Karolinska Institute are investigating whether animals that are overfed produce more of the male and female sexual hormones that trigger puberty.

Helen Crocker, research dietician at University College London, said: "In our clinics we are seeing girls of seven, eight and nine who are starting their periods and going through puberty. It can be pretty distressing for them.

"It's another good reason for tackling the wider issue of childhood obesity."