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Health experts generally recommend that children get at least one hour of moderate exercise each day, but that may not be enough to counter the problem of childhood obesity, a British study suggests.

Researchers found that among more than 200 British schoolchildren followed from age 5 to 8, 42 per cent of boys and 11 per cent of girls met government-recommended exercise levels.

But even among these children, there was no positive effect on weight control over time, the researchers report in the journal Archives of Disease in Childhood.

That's not to say that exercise has no health benefits for children.

The study also found that compared with their more sedentary peers, children who met the recommended activity levels had healthier numbers when it came to blood pressure, cholesterol, triglycerides and insulin resistance, a precursor to Type 2 diabetes.

"We would certainly encourage children to be active, as our study showed that being active improved metabolic health even without improving BMI [body mass index]," said lead researcher Brad Metcalf of the Peninsula Medical School in Plymouth.

However, he said, the findings suggest that physical activity alone will not reverse the current trends in childhood obesity in countries such as Britain and the United States.

"We found no association between the amount of physical activity a child did ... and the amount of excess weight they gained," Dr. Metcalf explained.

He said improving children's diets - which have "changed markedly" over the past 20 years - is likely to have a greater impact on their weight and overall health.

Dr. Metcalf and his colleagues based their findings on 212 children from 54 schools who were followed for four years. Once a year, the children wore small monitors that recorded their physical activity levels.

The researchers found that the children varied widely in their spontaneous exercise levels. Some spent only about 10 minutes a day being moderately active, while others topped 90 minutes.

The results for girls are in line with past research showing that young girls are habitually less active than boys, Dr. Metcalf said.

To some degree, he noted, any child's activity level is likely to be affected by biology; some children are naturally more active than others. And the consistently lower activity levels studies have found among girls as a whole may have biological underpinnings, according to Dr. Metcalf.

It's unclear, he and his colleagues write, whether exercise guidelines should be adjusted for girls, or whether girls should be encouraged to do more.