Is PYY really the key to obesity? Batterham's team first looked at what kind of food best satisfies hunger. They studied nine obese men and 10 normal-weight men. After brief fasts, the men ate different meals. Each of the meals - a high-protein meal, a high-fat meal, and a high-carbohydrate meal - had the same number of calories.

All the men said the high-protein meal best satisfied their hunger. Interestingly, the normal-weight men found the high-fat meal more satisfying than the high-carb meal, while the obese men did not.

Measurements showed the high-protein meal triggered the most PYY in all of the men. In the normal-weight men -but not the obese men - the high-fat meal triggered more PYY than the high-carb meal.

Batterham's team genetically engineered a mouse strain that did not have the PYY gene. These mice ate huge amounts of food, and quickly became obese. Normally, obese mice fed a high-protein diet will eat less and lose weight. But a high-protein diet didn't help the PYY-defective mice lose weight - unless they also got PYY treatments.

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