Why Doesn't The Fat Rat Want To Go Out And Play?

Enlarge this image toggle caption Joanna Servaes/Wikimedia Commons Joanna Servaes/Wikimedia Commons

Nobody likes being called a fatty, but if you're a particular kind of rat, maybe it's not so bad.

The Zucker Fatty Rat has been bred specifically as a genetic model for obesity in humans. Loads of researchers buy them, stuff them full of rich gourmet rat food, and then run tests. One study might help explain why these fat rats (and humans) may not be inclined to go out and play.

Rudolf Schilder, a postdoctoral fellow in physiological genomics at the Penn State College of Medicine, used Zucker rats in a study, published online this month in the Journal of Experimental Biology. He found that after the rats put on weight, the expression of a certain muscle gene didn't change, as it typically does during weight gain in early development. The gene looked the same in muscles of the obese rats as it did in the muscles of normal rats even though the obese ones were carrying much bigger loads.

"When they're not obese their muscles seem fine, but as they get progressively obese the muscles can't adjust to the amount of fat they carry," Schilder tells Shots.

Schilder believes this finding may apply to humans as well, though nobody's ever studied it, he says. The muscles of obese humans may not be suited to supporting their bigger bodies.

And it raises a question about whether muscle is playing a role in keeping people obese. "Maybe obese people aren't exercising because their muscles are not equipped to handle the extra weight," says Schilder.

That leads to the possibility of a vicious cycle. If muscles aren't going to work as hard, they won't burn as much fat, which means the person may gain more weight. And similarly if they can't exercise because they can't move well, their body could get heavier.

Schilder's finding, though just in rats at this point, may also help explain why intense exercise regimens don't work so well for obese humans as weight loss programs. "Studies show that obese people do get injuries more often especially in the balancing, supporting-type muscles like around the knee," says Schilder. "It would be a good idea to re-evaluate exercise regimes for them."

As intriguing as Schilder's finding is, it's only focused on one gene sensitive to muscle composition. He still needs to understand how muscles in obese rats function differently than those of normal rats. But that's what he's studying now.

Zucker rats, fortunately, have one thing going for them that humans don't: four legs to carry the heavy load instead of two.