Being more physically active does not help girls lose body fat, study says

TRONDHEIM, Norway — Girls do not shed pounds from being more physically active, according to new research. A team in Norway says their figures are not controlled by how much time they spend running, walking, or in the gym. While exercise has major health benefits, the effects differ between the sexes. Boys lose body fat but only after they reach adolescence.

“In girls, we found no connection between their physical activity and amount of body fat. Increased physical activity didn’t lead to less body fat in the girls, and body fat had no effect on changes in their physical activity,” says first study author Tonje Zahl-Thanem in a university release.

In boys, the amount of body fat influences exercise levels, with more resulting in less, respectively.

“Increased body fat in boys led to less physical activity two years later, when they were 8, 10 and 12 years old,” says Zahl-Thanem.

The findings are based on almost 1,000 Norwegian children who researchers examined every two years between the ages of six and 14.

“We looked at the connection between objectively measured physical activity and the proportion of body fat in girls and boys,” says Silje Steinsbekk, a professor at NTNU’s Department of Psychology. “We found that boys who are more physically active when they’re 12 years old have a lower proportion of body fat when they’re 14. This wasn’t the case at an earlier developmental stage.”

The Norwegian team points out large bodies are heavier and require more exertion when exercising. It may explain why boys whose body fat increases become less active over time. Surprisingly, however, this isn’t the case for girls.

“Here we can only speculate, but boys are generally more physically active than girls, so when boys reduce their activity level, the physical impact is greater,” Steinsbekk says.

Overweight children are more likely to be unhappy, with body dissatisfaction also having an association with less physical activity among boys.

“Boys’ physical activity is probably even more competitively oriented than girls’, and more body fat makes it more difficult to succeed. Both of these conditions can help explain why increased body fat leads to less physical activity in boys, but not girls,” adds co-author Lars Wichstrøm, a professor in NTNU’s Department of Psychology.

Girls may also be more likely to maintain physical activity when their proportion of fat increases. The researchers measured participants’ body composition rather than their weight and height. They then posed the question: does increased physical activity lead to a lower percentage of body fat over time? Or is it perhaps the other way around, that people who gain more body fat over time become less physically active?

They also examined the link between inactivity and body fat by measuring how long the participants were sedentary during the day.

“The results show that boys who had an increase in the proportion of body fat had a corresponding increase in sedentary activity two years later. This carried through all the age groups studied, from the age of 6 through age 14,” Steinsbekk explains.

In other words, boys whose proportion of body fat increases become more sedentary. For the girls, however, there was no link here either. The percentage of body fat did not affect their level of inactivity over time, and they did not become less active by gaining more body fat.

“In sum, we found a link between physical activity, sedentary lifestyle and fat percentage in boys, but not in girls,” Steinsbekk concludes.

The analysis in the International Journal of Obesity used data from the Trondheim Early Secure Study (TESS) which has been tracking participants since they were four years-old.

These children are now 18 years-old and the eighth survey is underway. The project has provided information for a number of studies on children’s development and health.

South West News Service writer Mark Waghorn contributed to this report.

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