However the absence leads to an amino acid called 'leucine' that burns up excess calories, enabling the mice to stay thin. Obesity results from an imbalance in the simple equation of energy input versus energy output, because excess fuel is turned into fat.
Previous research has found that high-protein diets, or leucine supplements, lead to weight loss. To find out what happens when leucine is lastingly increased, Christopher Lynch of the Pennsylvania State University College of Medicine in Hershey and his team inactivated a gene that normally wipes out this amino acid from the blood.
Lynch said that the genetically modified mice 'seemed more hungry than the other mice' and ate more, but still stayed lean.
He added that the animals "went crazy" when the researchers changed their food pattern to lower the amount of leucine they could eat. "They were sitting next to their food, panting and eating," Nature quoted Lynch, as saying.
The animals on this reduced-leucine diet were soaked in sweat as they ate more and more, but still didn't put on weight. Lynch suggest that the animals might be using the amount of leucine in their systems to determine how much they 'ought' to eat to reach their optimum weight.
The study is published in Sept 5 issue of Cell Metabolism.