Bad news for people trying to lose weight. A new assessment of how the body responds to dieting shows that it is about twice as difficult as previously thought to shed fat.

THE OLD MODELS of how much weight you can lose by reducing calories are seriously off the mark. It is a lot more difficult to lose weight than these models suggest, according to the authors of a mathematical approach to weight loss and obesity.

The calculations also puncture the myth that cutting calories will lead to continued weight loss. In fact, the effect of reducing the intake of calories levels off after about three years, scientists said.

Dr Kevin Hall and Dr Carson Chow from the US National Institute of Diabetes and Digestive and Kidney Diseases have improved on the old linear model that suggested a 250 kilocalorie (usually just referred to as calorie) cut in food intake would deliver a 25lb loss over a year.

This ignored the reality of human metabolism, however, they told a session at the American Association for the Advancement of Science’s annual meeting in Vancouver.

"People have used this rule of thumb for how to lose weight for decades and it turns out to be completely wrong," said Hall. "The reason it's wrong is that it doesn't account for the metabolic changes that take place when people change their diet. If you cut the calories in someone's diet, the metabolism slows down and it slows down the more weight that is lost until it reaches a plateau."

A popular misconception is that overweight or obese people have a slower metabolism, which would mean that they do not burn off the calories as fast as leaner people. In fact, the fatter someone is, the higher their metabolism is likely to be be, Dr Hall told the American Association for the Advancement of Science.

"Unlike the popular notion out there that heavier people have slower metabolism, the heavier you are the more calories you are burning," he said. "And the more of that mass is lean tissue compared to fat tissue, the more calories still you are burning – and obese people tend to have both an elevated fat mass and an elevated lean-tissue mass."

Weight loss when dieting comes quickly when you start but soon diminishes over time until finally hitting a plateau, as habitual dieters can attest. For this reason assumptions about energy intake and expenditure based on each pound of fat representing 3,500 calories was not useful for calculating energy balance, they said.

Hall pointed to a US study that indicated 67 per cent of people there were overweight or obese. It suggested that if the government introduced a heavy tax on sugar-sweetened beverages this would quickly drop to 50 per cent on the basis of the older models. The researchers ran a “dynamic simulation model” on the data and found that in fact the tax would only reduce obesity to 62 per cent because of the plateau effect.

The two decided to build a new, more accurate model of energy balance and weight loss. It took into account metabolic factors including the weight to lean ratio, sex, physical activity and other factors. They also built a simpler online version so people could log on and set up a weight loss programme. (bwsimulator.niddk.nih.gov)

This was very different from the older calculators that suggested reducing calorie intake by 500 a day would deliver a 1lb loss per week. Their calculator allows you to set goals – how much weight you want to lose over what period of time – and then does the calculations for changes to calorie intake and exercise.

A person should reduce food intake by 10 calories a day for each pound of weight change required. At that rate about half the weight change would be reached in about a year and about 95 per cent of the loss reached in about three years, the researchers calculate.

They also used their model to look at the US adult obesity epidemic. They showed that reaching the average population-based weight gain only required seven extra calories a day.

The calculations were much more sophisticated, however, and also took into account the energy demands required by an individual to carry around the extra average weight. The average person needed to burn up an extra 215 calories a day to haul around the weight. Changing the energy balance will deliver weight loss, Dr Chow said. Eating one fewer biscuit a day could deliver a 15lb weight loss over a year.

“The surprise is such a little change can deliver such a large result,” he said.

Dieting: how long it will really take

Male Average 35-year-old

Initial weight: 180lb

Target weight (in 6 months): 150lb

Calorie-per-day intake now: 3,167

Meet goal by eating: 2,353

Maintain goal by eating: 2,741

Female Average 35-year-old

Initial weight: 154lb

Target weight (in 6 months): 130lb

Calorie-per-day intake now: 2,542

Meet goal by eating: 1,882

Maintain goal by eating: 2,231