"The vast majority of investigators who use rats and mice don't recognize that their normal conditions are relatively unhealthy," Mattson is cited as saying by Nature journal. "The most logical way to extrapolate is to say any data we obtain in the animal model would be more relevant to overweight, sedentary humans than normal-weight, active individuals."

Indeed, lifestyle of many lab animals is far from being healthy and normal for their species. They have abundant access to food and little to no incentive to burn off their calories. Naturally, these furry versions of coach potatoes balloon, with some rats reaching weights up to 1 kilogram (2.2 pounds).

The situation should concern not only animal rights activists, but also researchers themselves. Sedentary, overweight animals have their metabolism skewed, and any potentially promising cancer drug, for example, tested on them may fail to work, or perform worse on healthy animals.

Some research, like those that report the positive influence of caloric restriction on human life spans, may be simply wrong because they started with a wrong baseline, Mattson said.

With rats and mice being animal models of choice for hundreds of research projects, the impact of the problem may be huge.

The authors suggest providing running wheels in cages and restricted feeding schedules as part of good research practice for labs using rats and mice as test animals.