The body interprets a diet as a period of famine and will store extra body fat when more normal eating is resumed, say researchers at the University of Exeter.
But people who don't diet are telling their body that food supplies are reliable and so don't need to store so much fat.
That's why birds seem fatter in the winter months when seeds and insects are in short supply, say the researchers who used a mathematical model to assess the weight fluctuations of an animal during periods of food abundance and scarcity.
"Surprisingly, our model predicts that the average weight gain for dieters will actually be greater than those who never diet," said lead researcher Dr Andrew Higginson.
Instead of a drastic diet, people should take things more steadily, and eat just slightly less than they do normally, he says.