A Biological Difference – Exercise Affects Boys and Girls Differently

A recent study finds that the percentage of body fat and the amount of physical activity in girls are not related.

Physical activity provides numerous health benefits. However, physical activity affects boys and girls differently. A recent study looked at the connection between children’s physical activity and body fat.

“We looked at the connection between objectively measured physical activity and body fat ratio in girls and boys,” says Silje Steinsbekk, a professor at the NTNU (the Norwegian University of Science and Technology) Department of Psychology.

Instead of weight and height, the researchers assessed the individuals’ body composition. They addressed questions such as: does increased physical activity result in a reduced proportion of body fat over time? Or is it possible that people who accumulate more body fat over time become less physically active?

Body fat and physical activity in girls are not related

The researchers reviewed the children every two years from ages six to fourteen. They found that the degree of exercise had mixed effects on the sexes.

“In girls, we found no connection between their physical activity and the amount of body fat. Increased physical activity did not lead to a reduction in body fat in girls, and body fat had no effect on changes in their physical activity,” says Tonje Zahl-Thanem, former researcher and first author of the paper.

However, it is different for children. Your level of body fat affects your activity level.

More body fat in children results in less physical activity

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

With one exception, increased physical activity had no effect on changes in body fat.

“We found that children who are more physically active when they are 12 years old have a lower proportion of body fat when they are 14. This was not the case at an earlier stage of development,” says Steinsbekk.

Several possible reasons for the differences between the sexes.

The study did not investigate the reasons for these differences, but the researchers note that larger bodies are heavier and require more effort when exercising, which may explain why children whose body fat increases become less active over time . But why isn’t this 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,” says Steinsbekk.

We also know that children with large bodies are less satisfied with their bodies, and body dissatisfaction is associated with less physical activity in boys but not in girls.

“Boys’ physical activity is probably more competitively oriented than girls’, and higher amounts of body fat make it more difficult to succeed. Both conditions may help explain why increased body fat leads to less physical activity in boys, but not in girls,” says Lars Wichstrøm, professor in the NTNU Department of Psychology and also a co-author of the study.

It could also be that girls are more likely to be physically active when their body fat ratio increases because more attention is paid to girls’ bodies and appearance.

Body fat affects sedentary activity in children

The researchers also examined the link between inactivity or a sedentary lifestyle and body fat. In the same way that they objectively measured physical activity, they also measured how long the participants were sedentary during the day.

“The results show that children who had an increase in body fat ratio had a corresponding increase in sedentary activity two years later. This extended to all age groups studied, from 6 to 14 years.

In other words, children whose proportion of body fat increases become more sedentary.

For the girls, however, there was no link here either. Body fat percentage did not affect their level of inactivity over time, and they did not become less active as they gained more body fat.

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

Safe Early Trondheim Study

The researchers used figures from the Trondheim Early Secure Study ([{” attribute=””>TESS). They followed almost 1000 children at two-year intervals from when they were 4 years old. The participants are now 18 years old, and the eighth survey is underway.

In this study, the research group used data at five different times, when the participants were 6, 8, 10, 12, and 14 years old. The Trondheim Early Secure Study has provided data for a number of studies on children’s development and health.

Enter your journal: Reference: “Relations between physical activity, sedentary time, and body fat from childhood to adolescence: Do they differ by sex?” by Tonje Zahl-Thanem, Lars Wichstrøm and Silje Steinsbekk, 3 June 2022, International Journal of Obesity.
DOI: 10.1038/s41366-022-01156-6