While it's true that dieters who forsake bread and gorge on bacon tend to drop weight (at least in the short-term), it's never been clear why. Until now, that is: Extra protein, it turns out, sends "stop eating!" messages to the brain.

Robert Atkins, the late originator of the eponymous and best known low-carbohydrate, high-protein diet, believed that cutting carbohydrates stops sugar from getting stored as fat, while increasing protein works the kidneys harder and thus burns more calories. But all that was ever shown scientifically was that protein feeding suppresses the appetites of both animals and humans, said Gilles Mithieux of the University of Lyon in France, one of the authors of the study.

Previous research from the University of Washington, for example showed that simply increasing the amount of protein in your diet helps you lose weight even if you don't shun carbohydrates one bit. Protein makes up 15 percent of most Americans' daily caloric intake, while fat accounts for 35 percent and carbohydrates for 50 percent. In the study, subjects bumped up their protein intake to 30 percent and reduced their fat intake to 20 percent. Within three months, they were 11 pounds lighter on average, even though half of the calories they ate still came from carbohydrates. The group also reported feeling satisfied with less food. In other words, they lost weight because they consumed fewer calories.

To figure out just how protein achieves this special effect, Mithieux and his colleagues fed rats a protein-enriched diet and measured metabolic changes. They found that the regimen sparked production of glucose in the small intestine, and that this increase, sensed in the liver and relayed to the parts of the brain involved in the control of appetite, caused the rats to eat less. Since the human intestine also synthesizes glucose, glucose metabolism may be a new target in the treatment of food intake disorders, the researchers concluded.

Protein's benefits go way beyond waistline trimming. The brain and its long spidery neurons are essentially made of fat, but they communicate with each other via proteins. The hormones and enzymes that cause chemical changes and control all body processes are also made of proteins. Carbohydrates, while essential as the brain's main source of fuel, can make you feel tired—and hungry for an energy boost—because they increase the brain's level of the amino acid tryptophan, which in turns spurs production of the calming neurotransmitter serotonin. Protein, on the other hand, prompts the brain to manufacture norepinephrine and dopamine, chemical messengers that promote alertness and activity.

Not all high protein foods are created equally, though—nutritionists recommend low-fat dairy products, beans, fish and lean cuts of meat, such as skinless chicken and turkey breasts. Even carb-heavy treats, like muffins and cookies, can be made protein-rich when baked with non-fat powdered milk and egg whites.