London, Dec 5 (IANS): Instead of helping you slim down, repeated 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, a study says.

The finding may explain why people who try low-calorie diets often overeat when not dieting and so do not keep the weight off.

By contrast, people who do not diet would learn that food supplies are reliable and they do not need to store so much fat, the study said.

"Surprisingly, our model predicts that the average weight gain for dieters will actually be greater than those who never diet," said Andrew Higginson, Senior Lecturer at the University of Exeter in England.

"This happens because non-dieters learn that the food supply is reliable so there is less need for the insurance of fat stores," Higginson said.

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 researchers 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 showed that if food supply is often restricted (as it is when dieting) an optimal animal should gain excess weight between food shortages.

The researchers' model predicts that the urge to eat increases hugely as a diet goes on, and this urge would not diminish as weight is gained because the brain gets convinced that famines are likely.

"Our simple model shows that weight gain does not mean that people's physiology is malfunctioning or that they are being overwhelmed by unnaturally sweet tastes, Professor John McNamara of the University of Bristol in England said.

"The brain could be functioning perfectly, but uncertainty about the food supply triggers the evolved response to gain weight," McNamara noted.

So how should people try to lose weight?

"The best thing for weight loss is to take it steady. Our work suggests that eating only slightly less than you should, all the time, and doing physical exercise is much more likely to help you reach a healthy weight than going on low-calorie diets," Higginson pointed out.