Study: Prolonged exercise to exhaustion by marathon bicyclists.

Participants: 10

Funding: Industry, the U.S. Department of Defense and the National Institutes of Health.

Study question? How does the body regulate glucose during long-term exercise?

Detail: Dopamine dosing turns off the body's normal regulatory process. Researchers then monitor blood sugar.

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Know those times when you're exercising and just can't go on?

Mayo Clinic anesthesiologist and physiologist Dr. Timothy Curry said that feeling is related to dramatic drops in blood-sugar.

Mayo researchers are using dopamine to turn off those normal regulatory processes in marathon bikers. The study subjects ride a stationary bike for three hours at 70 percent of their maximum ability.

If pedaling slows, the exercise bike automatically becomes harder to pedal.

The idea is to see what triggers the moment of max-out, and to see if there are ways to extend the time before that happens.

"How can you prolong their exercise tolerance?" Curry asked.

The findings might help athletes, soldiers and those with health issues such as diabetes and heart disease.

Curry said interesting research questions often produce more questions than answers.

"Sometimes that's where the best studies come from, when you learn something unexpected," he said.

As of February, 10 people had successfully completed the study. Only one was able to finish the full three hours, even though all of the subjects are highly fit.

Such studies are expensive and labor-intensive for research institutions, Curry said. For each cyclist's exercise study, about seven research workers are present, including one or two nurses, a technician, a physician, a post-doctoral fellow and two technicians.

The Clinical Research and Trials Unit at both the Saint Marys and Methodist campuses of Mayo Clinic Hospital, make researchers elsewhere envious.

"When we go to other places and tell them what we have here, they're just astounded by what we have here and all of our resources," Curry said.

Studies in animals help answer basic questions. But you can't understand how a treatment will work in humans until it's tried in them, Curry said. That requires altruism from volunteer research-study participants.

"We hope we are going to help the human race," Curry said. " It really is the reason we're here at Mayo, is to answer those questions and make it all better."