A more sophisticated measure of weight loss, which takes account of metabolic changes and of differences between fat and thin people has been developed by scientists.
For decades, doctors have based their advice to those who want to lose weight on the assumption that cutting 500 calories a day will see the weight fall off at the rate of 1lb a week.
"This is wrong," Kevin Hill, of the National Institutes of Health in the United States, said. "It does not happen."
The error has arisen because the calculation did not take account of changes in metabolism as weight falls. The body adjusts to reductions in energy intake (calories eaten) by slowing its energy output (calories expended).
The result is that forgoing that daily chocolate bar containing 250 calories will lead to about 25lb of weight loss if it is sustained for three years, much less than the 78lb predicted by the old dieting assumption.
The new study has shown that heavier people tend to lose weight faster than lighter people on the same diet, though they will take longer to reach the target weight than those who weigh less to begin with.
Most people on a diet achieve their maximum weight loss after six to eight months and it has been assumed this is a natural "plateauing" effect, resulting from slowed metabolism.
But evidence shows that people find it hard to stick to a diet for longer than six months and that is why they stop losing weight. Body-weight plateauing occurs much later, after two to three years.
There is nothing to choose between different diets that alter the fat, protein and carbohydrate balance, such as the Atkins diet that reduces carbohydrates, the researchers say.
The body adapts rapidly to changes in these constituents with the result that all diets result in the same loss of body fat, at least in the short term.
"Widespread past use of erroneous rules for estimation of human body-weight change have led to unrealistic expectations," the authors said.