An expert on metabolism believes there may be a fundamental flaw in guidelines about weight loss through a change in diet. Dr Kevin Hall says in turn this may be deterring people from sticking with attempts to drop weight.

Speaking to the American Association for the Advancement of Science, Hall highlighted guidelines used by health agencies on both sides of the Atlantic. They work on the basis that if a dieter cuts 500 calories from their daily diet, they’ll lose one pound over the course of each week.

According to Hall, that theory doesn’t take account of changes in metabolism. He points to research showing that metabolism slows as a person is dropping weight and only becomes constant again once the person reaches and maintains a new stable weight.

The precise effects of this vary depending on the ratio of body fat to muscle when the person begins the diet. Hall gave the example of a person who cut daily intake by 100 calories. Under the current guidelines, the person could expect to drop 10 pounds in a year. Hall says in practice they would drop 5 pounds in a year, reaching a 10 pound drop after three years, at which point their weight would level off unless they made further dietary changes.

The immediate effect therefore is that an average dieter will initially drop weight at only half the rate of that predicted by guidelines. Hall describes this effect as “horribly demotivating.”

Hall, who works at the National Institute of Diabetes and Digestive Kidney Diseases, has now put together a body weight simulator that takes details of the person’s height, weight, age, gender and exercise level, then maps out the effects over time of a change in diet, exercise, or both. It also details what changes would have to be made permanently for a particular target weight to be maintained.