Researchers have found that YO-YO dieters are programmed to pile the pounds back on.
Experts say restricting calories will only make you crave them more.
They said that the reason so few people stay slim after a strict diet is because the more you restrict the calories, the greater the brain responds to foods such as chocolate milkshakes, the Daily Express reported.
The study by Dr Eric Stice, at the Oregon Research Institute in the US, looked at why people relapse after a diet, particularly if they have fasted or skipped meals.
Researchers studied the brain responses of teenagers who had restricted their calorie intake to photographs of various types of food.
Dr Stice said the results were the first to suggest that cutting calories increases the reward value of appetising high calorie food "and that the more successful people are at caloric-restriction dieting, the greater difficulty they will face in maintaining the restriction."
Talking about the report, he said the implications were "crystal clear," saying, "If people want to lose excess weight, it would be more effective to consume healthy, low-fat/low-sugar foods during regular meals, rather than go for long periods of time without any caloric intake."
The study is published in the journal NeuroImage.