Why some people remain thin

Researchers have found that aerobic capacity is a major predictor of daily physical activity level among humans and laboratory animals.

Researchers including Chaitanya K. Gavini, in their new study, 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.

The researchers 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

This may be due to more lean rats having higher levels of proteins that support energyexpenditure and lower levels of proteins that encourage energy conservation and/or an increased sympathetic nervous system role in powering the muscles of lean rats.

The article has been published in the American Journal of Physiology-Endocrinology and Metabolism.