For half a century, dieters have adhered to a simple doctrine: to lose a pound of weight, you must burn 3,500 more calories than you take in. The 3,500-calorie rule has become a universal constant in the complex calculus of weight loss, but the truth may not be so simple: a new mathematical model predicts that the energy deficit required to lose a pound depends on how much fat a person has to start with.
According to the model, 3,500 calories per pound is a reasonable estimate for those who are moderately obese, but not necessarily for others. When dieting, those with a high percentage of body fat will mainly lose fat, whereas leaner people will lose a higher proportion of lean body mass, such as muscle or liver tissue.
Because lean body mass stores less energy per pound than fat, the model predicts that trimmer people require less of a caloric deficit to shed weight. Not that this will always be easy to achieve.
The results do not necessarily mean that it is easier for skinnier people to lose weight, says Kevin Hall, a biophysicist at the National Institutes of Health in Bethesda, Maryland. Lean body mass uses up more of the body's energy than fat does, so losing lean body mass should reduce the body’s overall energy needs. Therefore a person who maintains a steady diet may actually put weight on, as their smaller body needs less fuel to maintain itself.
Food for thought
Hall first began to wonder about the 3,500-calorie rule when he embarked on a project to model weight regulation. “It took a long time to track it down,” he says. “Dieticians are tested on this idea; textbooks quote it. But no one could tell me where it came from.”
It came from a study published in the 1950s that was heavily skewed towards overweight women. Since then, scientists have learned that not everyone loses weight in the same way. “One of the big assumptions was that when people lose weight, they lose fat tissue,” says Hall. “We now know that’s not true. They lose lean body mass as well.”
Hall decided to calculate the energy stored in lean body mass. He found that the energy density of lean body mass was five-fold smaller than the energy density of fat. His results are published in the March issue of the International Journal of Obesity1.
When lean body mass was incorporated into his weight-loss models, a clear difference emerged. For someone who weighed 100 kilograms and wanted to lose 5 kilograms, the rule of thumb worked well: they would need to burn nearly 39,000 more calories than they use. Assuming no change in energy use and an average daily intake of about 2,000 calories, that means avoiding the equivalent of 2 chocolate bars a day (about 500 calories) for 2.5 months. But to achieve the same weight loss in a 50-kilogram person required a caloric shortfall only two-thirds as large.
Growing concerns about obesity have fuelled a push to better understand such issues, says Steven Heymsfield of the New Jersey-based pharmaceutical company Merck. Heymsfield notes that several companies, including Merck, have launched large-scale weight-loss studies that include measurements of body composition. “There is a lot of interest in understanding the regulation of long-term changes in body weight,” says Heymsfield. “You’d like to know: are the dynamics for a 60 year old who goes on a diet the same as for a 20 year old? Because they probably aren’t.”
In 2002, Heymsfield and his collaborators showed that the energy deficit required to lose weight varied between men and women, and between young and old2. Their results were in line with what Hall found: women — who have a higher percentage of body fat than men — required a higher caloric deficit to lose weight. Similarly, a person aged about 70 needs a larger calorie deficit to achieve the same weight loss as someone who is 35 years old.
Does all of this mean that the textbooks should be amended? Hall notes that for obese people, the original equation works reasonably well. “For those who are recommended to lose weight, the rule isn’t that bad,” he says. Adding subtleties to this advice might unnecessarily complicate matters.