Repeated 'yo-yo' dieting may lead to weight gain because the brain interprets the diets as short famines and urges the person to store more fat for future shortages.

Research by the universities of Exeter and Bristol, published in the journal Evolution, Medicine and Public Health, suggests this explains 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 don't need to store so much fat.

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."

With more and more people becoming obese, scientists are looking for evolutionary reasons to explain why many find it hard to resist overeating. Humans evolved in a world where food was sometimes plentiful and sometimes scarce – and in the latter case those with more fat would be more likely to survive. Today, people can get into a vicious cycle of weight gain and ever more severe diets – so-called yo-yo dieting – which only convinces the brain it must store ever more fat. The researchers’ findings show that the urge to eat increases hugely as a diet goes on, and this urge won't diminish as weight is gained because the brain gets convinced that famines are likely.

“Weight gain does not mean that people's physiology is malfunctioning or that they are being overwhelmed by unnaturally sweet tastes,” says Professor John McNamara, of the University of Bristol. “The brain could be functioning perfectly, but uncertainty about the food supply triggers the evolved response to gain weight.”