Scientists say a gut hormone could explain why high protein diets can aid weight loss.
Fish is high in protein
PYY is released when a person eats protein-rich foods and sends signals to the brain indicating fullness.
The Medical Research Council team at University College London believe their findings could help tackle obesity.
But dietitians said much more work was needed before people could be advised to make long-term changes to their eating habits.
Many popular diets, including the Atkins, employ high protein foods.
Although there has been evidence to suggest high protein diets are effective in achieving and maintaining weight loss, the reasons for this have been unclear.
Although PYY was discovered more than 20 years ago, its role is still debated.
Dr Rachel Batterham and colleagues have shown that low levels of PYY can result in obesity.
High protein diets lead to highest PYY levels in the body and the greatest curb in hunger in both obese and normal weight people.
Overall, however, the PYY levels were lower in the obese individuals than in the normal weight individuals. The associated reduction in appetite was also smaller when the person was obese.
The mouse on the right lacked PYY
The researchers decided to engineer mice that were genetically unable to produce PYY in order to study the long-term effects of a high protein diet.
The engineered mice ate more than normal mice and became obese, even when they were offered protein-rich foods.
When the researchers gave the mice PYY to treat their deficiency, their food consumption went down to normal levels, as did their weight.
When they withdrew the PYY treatment, the mice ate more and became obese again.
The researchers said small tweaks to meals - a couple of spoonfuls less of potato and an extra slice of meat - could make a difference.
Dr Batterham said: "We have proved that if you do not have PYY you become obese, and if you give it back then you lose the weight again."
She said the next step was to do longer studies in humans.
"It's not a diet that you would go on for a few months. You would go on it for life.
"We need to check that it would be compatible with lifestyle and look at the safety aspect."
Dr Batterham said the changes required to the average person's diet would be small - 2-3% increases in protein intake and small reductions in carbohydrate intake.
"It's not drastic. It's subtle changes. Less potato and slightly more meat with your main meal, for example."
She suspects that there is an evolutionary explanation for why protein is most satiating, with protein making up a substantial part of hunter-gatherer diets.
Rachel Cooke from the British Dietetics Association said: "More research is certainly needed on this type of dietary regimen before firm recommendations can be made about its long-term safety and efficacy."