A high-protein diet increases the secretion of a hunger-curbing hormone, leading to reduced calorie intake and weight loss in mice. These results may explain why high-protein diets, such as Atkins, are so effective in helping people lose weight.

The composition of the typical Western diet includes about half as much protein as the diet thought to have been consumed by our hunter-gatherer ancestors, and this deficiency may, at least in part, be responsible for the skyrocketing rates of obesity seen in developed nations. Several studies have demonstrated that high-protein diets promote satiety and lead to weight loss in people, but the basis of this effect has remained unclear.

Now, Rachel L. Batterham of University College London (London, UK) and colleagues report in the September issue of Cell Metabolism that the anorectic hormone peptide YY (PYY) may play a key role in suppressing hunger. PYY is one of several hormones responsible for regulating energy homeostasis; it is released in the gastrointestinal tract in response to a meal and sends signals to the brain indicating satiety. Batterham's team found increased levels of the hormone in people following a high-protein meal, compared to high-fat or high-carbohydrate meals containing the same number of calories.

To further investigate the relationship between PYY and a meal's protein content, the researchers turned to a mouse model. They found that mice fed a high-protein diet consumed fewer calories and showed increased levels of PYY. Chronic consumption of high-protein chow led to decreased weight gain and reduced deposition of adipose tissue, as well as increased levels of PYY.

Next, to confirm the role of PYY in regulating calorie intake, the researchers created a line of PYY knockout mice. The mice that could not produce PYY ate more than wild-type mice and became obese, even when fed a high-protein diet. Chronic administration of PYY, however, resulted in dramatic weight loss—approximately 20% of body weight after 15 days of daily injections.

Protein's increased ability to reduce feelings of hunger as compared to carbohydrates or fat may have an evolutionary basis. As the authors write in Cell Metabolism, “Many of the physiological systems that regulate food intake were probably established and may function better under lower-carbohydrate and higher-protein dietary conditions.”

These results suggest that boosting your diet's protein content may be a viable weight-loss strategy. It should be noted, however, that large-scale clinical trials have yet to confirm this idea.