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Fitness in a Bottle?

It's not yet clear from the studies what it is that makes a person more fit.

"What we still are sorting out is, are these fit people inherently different based on genetics -- people who are meant to be lean and who when they walk across the street burn more fat? Or by going to the gym three times a week, have they been able to alter their metabolism to burn fat more robustly?"

A tantalizing hint comes from further studies suggesting that the metabolites seen after exercise aren't just by-products of fuel burning. They may play an active role in promoting fitness.

"When we exposed muscle cells to some of the metabolites that increase after exercise, we found they turned on an important gene that regulates the ability to use glucose [sugar] in fats," Lewis says. "So exercise through these small molecules that are released can stimulate expression of genes that are important to our metabolism."

Indeed, one gene activated by these exercise-associated molecules is nur77, a gene that helps control how the body burns or stores sugar and fat.

"If you look at a sports drink label, you'll see the drink contains handfuls of small molecules. But think of supplementing that drink with all these molecules we now know the body uses up during exercise," Lewis says. "So you can imagine we might be able to use these findings to craft the next generation of sports drinks."

And by studying the metabolic profiles of people with heart disease and other conditions, the researchers also hope to learn whether certain metabolites -- or drugs that mimic their action -- can be therapeutic.

But if all we want to do is lose weight, all the small molecules in the world might not be enough, cautions Andrew S. Greenberg, MD, director of the obesity and metabolism laboratory and the center on aging at Tufts University.

"Just because you have these metabolites does not mean that if you put them into your muscles you would be fit," Greenberg tells WebMD. "You don't just get on a treadmill and burn fat. It takes a long-term process of resetting the thermostat of how your body responds to exercise."

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