We've often come across people who tend to eat a lot yet remain slim, dodging the effect of the extra calories consumed. The answer lies in our muscles. The way the muscles of the inherently thin work may give them the edge, research reveals. "Daily physical activity is an inherited trait with a strong association to how fat or thin a person is," said Chaitanya K. Gavini, a researcher with Kent State University's School of Biomedical Sciences. Aerobic capacity or the Genetic Tendency is a major predictor of daily physical activity level among humans and laboratory animals. In the study, researchers compared female rats with high aerobic capacity (genetic tendency toward leanness) or low aerobic capacity (genetic tendency toward obesity) to investigate how muscle physiology affects leanness. Though the rats in each group were similar in weight and lean body mass, the rats with a high aerobic capacity were consistently more active than the low capacity rats. While all the rats had similar energy expenditures when at rest, big differences in energy expenditure (calorie burn) occurred during mild exercise. "We found the muscles of rats with lean genes demonstrated 'poor fuel economy', meaning that they burned more calories when performing the same exercise as those with fat genes," Gavini added. This may be owing to more lean rats having higher levels of proteins that support energy expenditure and lower levels of proteins that encourage energy conservation. The research has implications for how we consider metabolism when attempting to prevent or treat obesity, Gavini noted in the study published in the American Journal of Physiology-Endocrinology and Metabolism.