Rats might not seem like picky eaters, but if their diets lack key amino acids, they'll search for the right food. Now researchers believe they have discovered the molecular process that guides this finicky, but critical, behavior. The findings may help researchers figure out why people eat what they do.
Proteins are made from various combinations of 22 amino acids. Humans and rats produce most of these in their cells but have to get eight of them--known as essential amino acids--from their diets. Rats eating a diet devoid of an essential amino acid will stop eating after 20 minutes and look for other food.
The clues to this behavior originate in yeast. Experiments had shown that yeast detect nutritional deficiencies with the help of a gene called GCN2, which sends up a red flag when molecules that transport amino acids, called tRNAs, are empty-handed. Fast forward to 2003, when biochemist Tracy Anthony of Indiana University School of Medicine, Evansville, fed an amino acid-lite diet to rats whose GCN2 had been knocked out. The rats didn't notice the dietary deficiency, suggesting that, like yeast, they might rely on tRNAs to plan their menus. Anthony teamed up with neurophysiologist Dorothy Gietzen of the University of California, Davis, and colleagues to test the idea.
First, the team took normal rats and blocked a tRNA from grabbing its amino acid threonine. (They did this by injecting a chemical into a brain region important in eating.) Rats gave up regular food after 20 minutes, as if the diet lacked threonine. Next the team overloaded the diet with threonine, which interfered with the chemical's ability to block the tRNA; the injected rats went back to eating, indicating that an empty tRNA alerts the brain to problems with the diet.
Calling the work "wonderful," physiologist Larry Bellinger of Texas A&M Health Sciences Center in Dallas says that Grietzen "teased out how diet is sensed by rats in their cells." He says that understanding what makes people interested in eating certain foods and avoiding others will help nutritionists make healthy diet recommendations and fight obesity.