In the study, published in the February issue of Diabetes, the researchers determined that middle-aged mice expended less energy i.e., burned fewer calories - to carry out the same physical activity -- scurrying to and fro -- than younger adult mice. The most dramatic findings were seen in a comparison of middle-aged and younger-adult mice genetically engineered to lack a brain cell receptor known as 5-HT(2c), which receives signals from the brain chemical serotonin; an effect, though less dramatic, was also seen in normal mice.
Traditionally, scientists have surmised that the increase in weight gain seen in middle-aged mammals, including mice and people, was due in part to declines in resting metabolic rate and physical activity levels. However, in the study, both sets of middle-aged mice ate the same amount, had the same resting metabolic rate and had the same level of activity as their younger brethren.
The only difference, the researchers discovered, was the increased efficiency with which both groups of middle-aged mice expended energy during exercise. In other words, they got more miles per gallon. (The increase was tightly correlated with the extent of activity and age, a fact dramatically illuminated on two graphs.)
The evidence that middle-aged mice burn fewer calories per dash across the cage and the implication that aging people may burn fewer calories per jog down the footpath -- are less than sanguine.
"This could mean that I'd burn less energy today running an 8-minute mile at a given weight than I would have 20 years ago," says senior author Laurence Tecott, PhD, UCSF associate professor of psy
'"/>
Contact: Jennifer OBrien
jobrien@pubaff.ucsf.edu
415-476-2557
University of California - San Francisco
6-Feb-2003