"We call this effect 'banking' because the kids benefit later on, similar to having a savings account at a bank. The protective effect is independent of what happens in between," said lead author Kathleen Janz, professor of health and sport studies in the UI College of Liberal Arts and Sciences.
"The implication is that even 5-year-olds should be encouraged to be as active as possible because it pays off as they grow older," she added.
During the study, the University of Iowa researchers tested the body fat and activity level of 333 kids at ages 5, 8 and 11.
The kids wore accelerometers to record their activity level for up to five days, providing much more reliable data than relying on kids or parents to track minutes of exercise.
It showed that kids who are active at age 5 end up with less fat at age 8 and 11, even when controlling for their accumulated level of activity.
The average 5-year-old in the study got 30 minutes of moderate to vigorous exercise per day. For every 10 minutes on top of that, kids had one-third of a pound less fat tissue at ages 8 and 11.
However, Janz insists that further investigation is needed to learn what happens to the active kids' bodies that keeps them in better shape down the road.
One possible explanation may be that the active 5-year-olds didn't develop as many fat cells, improved their insulin response, or that something happened metabolically that provided some protection even as they became less active.
In addition, boys were more likely to experience the sustained benefit from being active as preschoolers, possibly because they are more active at age 5 than girls, highlighting a need to especially encourage young girls to exercise.
The study is published in the American Journal of Preventive Medicine.