One in five American four-year-olds is obese

Many people love to see round, chubby children, once seen as a sign of good health. But it becomes worrying when it turns into obesity.

A new study, published in the Archives of Pediatrics and Adolescent Medicine, found that one in five four-year-olds is obese, and the rate is worse among ethnic minorities. It studied 8,550 children born in 2001, measuring their body mass index (BMI) according to US government guidelines: a child was considered obese with a BMI at or above the 95th percentile for their age.

The researchers found that 31.2% of Native American children, 22% of Hispanic, 20.8% of Black, 15.9% of white and 12.8% of Asian children were obese, and boys were more likely to be obese than girls. "The size of these differences is much bigger than we expected," said Dr Sarah Anderson of Ohio State University. It is surprising to see such ethnic differences at such an early age, so more study of the environment and family circumstances is needed.

"Children living in inner cities differ from those in the suburbs," said Dr Michael Rich of Harvard Medical School. "Children in dangerous neighbourhoods don't get to go outside, and their diet relies on fast food."

The differences mirror those in adults: 45% of Black, 36.8% of Mexican-American and 30% of white adults are obese, according to the Centers for Disease Control and Prevention. So the differences may also be partly biological.

Obese children are at risk of type 2 diabetes, once seen only in adults. "Four-year-olds have many complications, including high blood pressure and high cholesterol that can lead to heart attacks, and a risk of diabetes," said Dr Thomas Robinson of Stanford University.

Childhood obesity casts its shadow over a whole life: many teenagers now have surgery for life-threatening problems linked with obesity.

Scientists can't pin down every cause of obesity, but they agree that a healthy diet and physical activity from early childhood help, which is much better than trying to change a child at 8, 12 or 16. A third of obese children become obese adults.