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DALLAS - The ideal runner's body is lean and lithe.

But 47-year-old Luis Serna is a marathoner who knows he defies that description.

I see all my friend runners and they are very skinny like this, said Serna, holding up his pinky finger. And then when I see myself, I see someone like this [holding up a thumb]. And it is interesting to see a big guy running alongside my friends.

The Texas Health Dallas Institute for Exercise and Environmental Medicine is studying Serna and other healthy, but medically obese, athletes to determine how they do it.

According to some estimates, one-third of the U.S. population is clinically obese. And, at any given time, up to 35 percent are trying to lose weight.

No matter how hard many of them try, they just can seem to exercise the weight off. Why not?

We're using this small group of obese and fit individuals, explains exercise physiologist Santiago Lorenzo, To try and understand how are they able to do it, so that we can help the regular obese people and fight this epidemic of obesity.

Santiago Lorenzo knows a thing or two about fitness. He was an Olympic decathlete for Argentina in the 2004 Olympics.

Since becoming an exercise physiologist, he's been fascinated with finding a reason why obese people often can't work off the weight.

One is, they're out of shape, Lorenzo said. And the other is because they have some sort of respiratory limitation, because of the extra weight they are carrying. So we're trying to figure out what's really the cause.

He believes the key is in the respiratory system -- which seems to work differently, more efficiently, in the fit-but-fat population.

Luis Serna hopes his part in the research makes a difference for others who are overweight, including himself.

I want to be faster, he said. I wish the people doing the study will give me like a pill that I will slim down and be much faster.

Anyone interesting in the IEEM Shortness of Breath and Obesity study should call (214)345-7574.

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