LOSING weight just got twice as hard because of a new way of calculating the effects of dieting.
Conventional guidelines fail to take into account changes in metabolism, says US expert Dr Kevin Hall.
As a result, dieting is always going to be a struggle, he argues. Cutting calories leads to a slowing of metabolism which means it takes longer to lose weight.
At a rate of 100 calories lost per day, it would take the average person a year to shed 5lbs of body weight.
Under the "old" guidelines, based on the idea that each pound is equivalent to 3,500 calories, the same degree of weight loss should be possible in roughly half that time.
Speaking at the annual meeting of the American Association for the Advancement of Science (AAAS) in Vancouver, Canada, Dr Hall said: "People have used this rule of thumb to predict how much weight people should lose for decades now, but it turns out to be incredibly wrong.
"The reason it's wrong is because it doesn't account for the metabolic changes that take place when people change their diet.
"We know that if you cut the calories in somebody's diet their metabolism starts to slow down, and it slows down more and more the more weight that is lost. So eventually you'll reach a plateau.
"Some of my work has been to develop realistic mathematical models about what happens to metabolism when people start changing their diets and can we come up with some better rules and better predictions."
Because of the way metabolism slows weight loss, dieting away 100 calories daily would lead to the loss of just 10lbs in three years, said Dr Hall, from the Laboratory of Biological Modelling in Maryland.
Five pounds of that weight loss would go in the first year.
Dr Hall has developed an online slimming aid that explains what level of dieting is needed to achieve a weight loss target. It takes numerous factors into account including metabolism.
The website http://bwsimulator.niddk.nih.gov is designed to assist researchers rather than members of the public, though anyone can access it. "People can plug in what their goal weight might be in a specific time period," said Dr Hall.