lifelines : Running mice are leaner mice
Taking regular exercise is a good way to lose weight. The same is true for mice as men - a mouse given a running wheel will lose more weight than a mouse forced to be sedentary. However, increasing evidence suggests that the relationship between body mass and activity is not so simple. For laboratory mice, at least, there is anecdotal evidence dating back to the 1940s suggesting that bigger, heavier animals are less active than smaller, lighter ones, irrespective of how much activity they take. Is there a genetic correlation between lightness and activity?
The answer, it seems, is 'yes', according to a long-term study published in the Journal of Experimental Biology. In the study, Theodore Garland Jr and colleagues from the University of Wisconsin, Madison, Wisconsin, report on the progress of mice bred for their high activity levels. In each generation, the mice showing the greatest fondness for the running wheel were used as breeding stock. Fourteen generations later, the researchers had produced a strain of mice which were, on average, more than two and a half times as active as regular mice, at least when measured by the number of revolutions of the running wheel the mice clocked up during the day. Activity levels varied depending on the ages and sexes of the mice studied, but in general, the selected mice were less massive at maturity than regular mice.
This could simply mean that mice bred to be more active lose weight simply because of their activity -- there need be no genetic correlation between activity and body mass at maturity. However, mice from the activity-selected line lost significantly more weight after a period of activity than did regular mice. And, in a crucial twist, the researchers looked at mice, selected and regular, kept in cages in which the running wheels were locked. Even in these mice, constrained to be sedentary, mice from the selected strains weighed less at maturity than regular mice. In other words, the researchers selected for active mice and got lighter mice, too -- more than they bargained for, because there is a strong genetic correlation between activity and lightness. This, say the researchers, is "the first clear evidence that activity levels are negatively genetically correlated with body size in house mice". In other words, the more active they are, the less heavy they are, irrespective of exercise.
Do these results mean anything outside the little world of running mice? As the researchers discuss, the findings could explain a general relationship between activity and lightness observed in several rodent species, but whether it can be applied to much larger animals -- such as people -- is moot. Nevertheless, it is tempting to speculate that less active people are heavier than more active people not simply as a consequence of inactivity, but because of some deeper, genetic cause.
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