PROBLEM: Lab animals don't care about heart health or weight loss, I'm pretty sure. So what could it possibly be that motivates a caged rat to run rapidly nowhere on its wheel each night? And why is that some humans, who we at least try to make care about heart health and weight loss, are so much more willing to jump on the treadmill than others?
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METHODOLOGY: Researchers at the University of Missouri took lab rats that spent the most time voluntarily running, and bred them with other highly active rats. They then did the same for the least active rodents. Eight generations later, they analyzed the behavior and physiology of the original rats' great-great-great-great-great-great grandchildren.
RESULTS: The rats had subtle differences in body composition and in the levels of mitochondria in their muscle cells. But those bred to be lazy chose to spend significantly less time on the wheel, ran shorter distances, and were generally slower. By the tenth generation, the active mice were running ten times as much as the lazy ones. And after six days of that, they had lost considerable body fat, while the others neither gained nor lost weight. The researchers narrowed down the discrepancy in motivation to 36 genes that appeared to be responsible.
IMPLICATIONS: The answer to promoting physical activity in humans probably isn't selective breeding, despite the best efforts of Fitness Singles ("where relationships workout!'). But if we can isolate the genetic basis of laziness in humans, perhaps we can better understand how to motivate those for whom the urge to exercise isn't innate. Or we can at least be more understanding about their indifference toward the idea of a bicycle desk.
"Phenotypic and molecular differences between rats selectively-bred to voluntarily run high versus low nightly distances" is published in The American Journal of Physiology.