Many people who’ve lost weight often end up gaining it all back, but the trick to keeping the weight off has nothing to do with willpower but the number of calories consumed, according to health experts.

Dieters need to permanently cut out at least 300 calories from their daily intake to stay at their new weight, according to researchers who say that people who’ve slimmed down need to eat less than someone of the same weight but who has not dieted.

Three-hundred calories, which equates to a bag of chocolate candies, is just the sort of treats dieters look forward to eating after they’ve shed their pounds, but experts warn that during the course of the week the extra 300 calories consumed daily can amount to 15 cans of soda or 17 slices of buttered toast.

Professor Michael Rosenbaum of Columbia University in New York, who has monitored dieters for years, conducted experiments where male and female participants had been on a strict diet to lose 10 percent of their weight.

Tests were carried out before and after participants lost the required weight and researchers would monitor how participants tried to stay at their new weight. 

"The number of calories you are going to have to eat to maintain that weight loss falls by 22 per cent. That's 300 calories or more a day less than someone who looks exactly like they do," Rosenbaum said at a health conference, according to the Daily Mail.

“That’s 300 calories or more a day less than someone who looks exactly like they do,” he added.

Rosenbaum said that his findings suggest that the weight gain effect does not wear off, and that a person who has lost weight must eat hundreds of fewer calories a day for years if they want their lost weight to stay off.

This phenomenon can be largely explained by the effect dieting has on muscles. Muscles use up fewer calories to do their work in people who’ve dieted compared to those who are of similar weight but has not dieted, researchers explained in the European Congress on Obesity conference in Lyon.

Changes in hormones, metabolism and appetite also make it hard to keep lost weight off, and even the brain makes it tough of dieters to keep their new weight.

Rosenbaum found that areas of the brain that perceive food as rewarding are more active and parts that generate feelings of restraint and control are less active after a person diets, so dieters have to eat more to feel satisfied, even though they think they’ve eaten less. 

“You are creating the perfect storm for weight regain – energy expenditure is down and desire to eat is changed in ways that favor regain of lost weight,” Rosenbaum said at the conference.

“Weight loss is a relatively brief therapeutic intervention, but trying to keep the weight off requires a lifetime of diligent attention.”