Rats that were “born to run” not only outpaced their less-talented cousins but also were naturally less prone to heart disease, a finding that may help explain why exercise prevents heart death, researchers said on Thursday.

The study may be bad news for people who hate to exercise, suggesting that not only laziness but also their genes may put them at higher risk of heart disease.

“The reality of having a genetic determinant of our existence is that there are some people who are born with less ability to take up oxygen and transfer energy than others,” said Steven Britton of the University of Michigan.

“These people may have to work harder and will never reach the level of a professional athlete, but almost everyone can improve their aerobic capacity and health status with regular exercise.”

Britton and colleagues bred rats for 11 generations to be good or poor runners.

Then they tested their ability to exercise, without training them first, so that differences could not be attributed to practice.

Their high-capacity runners can exercise on a little rodent treadmill for 42 minutes on average before becoming exhausted, while the low-capacity runners average only 14 minutes. It is a 347 percent difference in capacity, the team reports in Friday’s issue of the journal Science.

Colleagues in Norway examined the rats for heart health factors.

“We found that rats with low aerobic capacity scored higher on risk factors linked to cardiovascular disease - including high blood pressure and vascular dysfunction,” said Ulrik Wisloff, a professor of exercise physiology at the Norwegian University of Science and Technology in Trondheim.

Vascular dysfunction means abnormalities of the blood vessels.

“Rats with low aerobic capacity also had higher levels of blood fat disorders (such as High cholesterol), insulin resistance (a pre-diabetic condition) and more abdominal fat than high-capacity rats,” added Sonia Najjar, of the Medical College of Ohio in Toledo.

It looked like the mitochondria, the powerhouses in each individual cell, were involved, the researchers said.

“Compared to high-capacity rats, the low-capacity rats had lower levels of oxidative enzymes and proteins used by mitochondria to generate energy in skeletal muscle,” Najjar said in a statement.

Many experts point out that it is possible to be fit and fat, or to be thin and to die of heart disease.

Studies have shown that a poor ability to exercise aerobically - the kind that makes for heavy breathing - is a very strong predictor of heart disease, the researchers noted.