April 19, 2005 -- After a tough workout, you deserve a rest. But don't let that rest last too long.
Soon after exercise, fat cells start storing up energy. If you don't exercise again -- soon -- those fat cells will swell in size and weight, report University of Missouri-Columbia researchers David S. Kump and Frank W. Booth, PhD.
Why? Kump and Booth point to human evolution. Because food was often scarce, our bodies grew very efficient at storing away any food energy not burned off during exercise.
"Left on its own in nature, our body wants to store as much energy (fat) as possible because it doesn't know when the next meal will come," Booth says, in a news release. "If we don't burn that energy through regular physical activity the body makes it a priority to store that energy as fat."
48-Hour Exercise Window
Kump and Booth studied adolescent rats. When given exercise wheels, these teen rats grew much stronger than rats that didn't have exercise wheels.
But when the rats were kept from exercising -- Kump and Booth locked the exercise wheels for various lengths of time -- their bodies started to change.
The body changes came astonishingly fast. Just five hours after exercise stopped, the rat's abdominal fat cells started to send off chemical signals that made them start to swell. After two days of no exercise, the fat cells got 19% larger -- and the rats' stomach fat got 48% heavier.
This, Kump and Booth suggest, is just like what happens when people become couch potatoes. What's surprising is how very fast it happens. On the other hand, regular exercise offers a solution.
"We eat regularly, and we need to exercise regularly," Kump says in a news release. "There is a magic bullet for keeping weight down. It's daily physical activity."
A Warning for Teens
The teen rats that didn't exercise were much punier than those that did. Kump and Booth say this is a warning for humans. If kids stop getting daily exercise when they reach adolescence, they suggest, they may never develop proper muscle mass.
Muscle is, of course, the body's main way of using up excess energy stored as fat. Kids who don't add a lot of muscle during their teen years may be doomed to add a lot of fat during adulthood.
The findings appear in the online edition of the Journal of Physiology.