Whether a person is a couch potato or hits the gym daily may be determined, a least in part, by the way his or her body converts food into energy, according to a new study.

The researchers propose a link between an animal's personality and its resting metabolic rate, the amount of energy an animal uses up while not doing anything.

"There are people that are always active and running around, and never gain weight, and one might assume that they might actually have higher [resting] metabolic rates than some people that are much more lethargic, and just sit around all the time," study researcher Judy Stamps of the University of California, Davis, told MyHealthNewsDaily.

Stamps' research was based on more than 500 previous studies of mice, birds, fish and other animals. Metabolic rates differ throughout the animal kingdom, but even within a species, they can vary. For example, some humans have higher metabolic rates than others.

Because an animal's metabolism can't change very quickly, its metabolic rate may be set to generate the energy needed for the times when the animal is most active. This would mean those with high metabolic rates have a high energy levels, even at rest. The researchers said these individuals may need to burn off some of this excess energy through exercise or other behaviors, such as aggression or courtship.

However, it's not clear yet if the findings apply to humans. None of the previous studies were designed to directly examine the link between personality and metabolism . Future work should specifically address this question to determine if there is indeed a connection, the researchers said.

Metabolism, aggression and exercise

Stamps and her colleague Peter Biro, of the University of New South Wales in Sydney, were curious about why behaviors in animals, such as aggressiveness, are so consistent over time.

"It's very easy for someone to be aggressive one minute, and not aggressive the next," Stamps said. "It's not immediately obvious why some individuals would be consistently more aggressive than others."

After researching the topic, the researchers also found consistent differences between individuals in their metabolic rates. They found enough evidence to suggest a link between the two, particularly between a high metabolism and high levels of activity and aggression.

Human personality

The results might explain why, for some people, their personalities seemed linked to their athletic inclination.

"We often associate the athletic 'jock' type or person with being aggressive and social, whereas the more sedentary 'nerd' often is seen as more socially awkward and submissive," Biro said in a statement.

Stamps suspects that if you measured people's metabolic rate and also measured their activity level, there might be a relationship.

But the link between aggression and metabolic rate may be less likely to apply to us, she said.

"In humans, it's not obvious that an aggressive individual is constantly aggressive, that they're doing it six to eight hours a day, whereas in some animals that might be the case," she said.

The study was published Sept. 13 in the journal Trends in Ecology and Evolution.