This may explain why people who try low-calorie diets often overeat when not dieting and so don't keep the weight off.By contrast, people who don't diet will learn that food supplies are reliable and they do not need to store so much fat. The study, published in the journal Evolution, Medicine and Public Health, is based on observations of animals such as birds.

Animals respond to the risk of food shortage by gaining weight, which is why garden birds are fatter in the winter when seeds and insects are hard to find.The authors studied a mathematical model of an animal that knows whether food is currently abundant or limited, but does not know when things will change, so must learn about the changeability before deciding how fat to be. The model shows that if food supply is often restricted (as it is when dieting) an optimal animal -- the one with the best chance of passing on its genes -- should gain excess weight between food shortages.

Dr Andrew Higginson, Senior Lecturer in psychology at the University of Exeter, says: "Surprisingly, our model predicts that the average weight gain for dieters will actually be greater than those who never diet."This happens because non-dieters learn that the food supply is reliable so there is less need for the insurance of fat stores."