A "marathon" mouse which can run twice as far as a normal rodent has been bred by a US-South Korean team of scientists
Mice can go further with the modified gene
The genetically engineered animal has been given an enhanced protein that turns it into an "endurance athlete" and makes it resistant to weight gain.
Changing a gene that codes for a specific protein boosted the molecule's activity, leading to an increase in so-called "slow-twitch" muscle fibre.
The findings have been published in the open-access journal PLOS Biology.
They could be used to help people with muscle or weight problems, say the researchers. The scientists also acknowledge their studies could be abused by athletics cheats.
The body's skeletal muscle - the muscle that acts on the bones in the manner of a system of levers to enable locomotion - is composed of two types of fibres: slow-twitch and fast-twitch.
Slow-twitch muscle fibre is by far the most fatigue-resistant of the two types. Fast-twitch muscles are capable of large bursts of power, but get tired more quickly.
In this latest research, scientists changed a gene called PPAR-Delta in order to enhance its activity. PPAR-Delta is a so-called "master regulator" of a number of other genes.
When the researchers produced mice with enhanced PPAR-Delta activity, they saw an enhancement in "slow-twitch" muscle fibres and a decrease in "fast-twitch" muscle fibres in the mice.
"In previous work, we had shown that in various tissues, particularly adipose tissues, activating PPAR-Delta increased fat burning and, as a result, increased muscle mass," said co-author Ronald Evans of the Salk Institute in San Diego California, US.
"Going into this experiment, the possibility of an effect on muscle fibres was not on our radar screen."
Professor Evans and his colleagues tested the animals' endurance on the treadmill and found they could run twice the distance of normal mice.
Normal mice can run about 900m before dropping out due to exhaustion. The PPAR-enhanced animals were able to run 1,800m - more than a mile - before they ran out of steam, even though they had never been near a treadmill.
Enhanced mice can also run for about an hour longer than the average 90 minutes a normal mouse can run for before it maxes out.
The scientists also found that the mice were resistant to weight gain, even when placed on a high fat, high calorie diet that caused normal mice to become obese.
Professor Evans is aware his research could be abused
"Significantly, the increased number of fat-burning muscle fibres appears by itself to be protective against a high-fat diet," said Professor Evans.
The research may provide scientists with new routes to counter human health problems.
"The particular use would be for people who are overweight or who have problems in exercising," Professor Evans told the BBC.
"Almost by definition, athletes and elite runners would have an interest in this because it might make their exercise more efficient. So there is a potential that athletes might want to abuse this compound, like other compounds."
Researchers and sports officials are very aware that this sort of knowledge could be abused to create GM athletes.
"In terms of these genetic applications, we're at the beginning of the cycle," Dick Pound, the chairman of the World Anti-Doping Agency, told the BBC earlier this year.
"In the 60s, 70s, 80s and even the 90s, sport rather lost control of drug use. We've been playing catch-up ever since - with some success. What we'd like to do with this new branch of science is be there early to help in the formation of the regulation of it."