Almost every diet book, nutrition guide and weight-loss manual includes the information that one pound of body weight equals 3,500 calories… and then comes the advice that for every pound that you want to lose, you must either eat 3,500 fewer calories or use up that amount with extra physical activity.

It’s a simple, straightforward formula — but there’s one problem: It’s simply wrong.

Why? Stretch out the numbers over time, and you can see that the math may make sense but the science doesn’t work. If you reduce your average daily caloric intake by 100 calories (one less snack pack per day, perhaps?), you should shed one pound of body weight every 35 days. This would add up to almost 10.5 pounds per year. Keep it going for 10 years and you’ll lose… almost 105 pounds? It can’t be true.

The New Math

While it has been the conventional diet wisdom for decades, this calculation is “obviously ridiculous,” I was told by Kevin Hall, PhD, a physiologist and investigator in the laboratory of biological modeling at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) at the National Institutes of Health. The problem with the 3,500-calorie formula, said Dr. Hall, is that it is predicated on the idea that metabolism remains constant — and it does not.

Metabolism is affected by gender, by age, by size and weight loss, and by stress and lifestyle. Your metabolism and my metabolism may be as unalike as are your eight-cylinder, four-wheel-drive SUV and my Mini Cooper — not to mention that even that SUV “metabolizes” gasoline differently while puttering around in town than it does at 60 mph on the highway.

So What Can You Do?

According to Dr. Hall, even when an eating plan is followed religiously, every dieter’s weight loss eventually slows and then reaches a plateau. The point at which this occurs is both highly individualized and affected by the changes in body composition. He said that people who are very obese, for instance, will lose weight faster than those who aren’t. “They tend to burn more fat during weight loss, which helps preserve their muscle mass,” he explained, “so they lose more overall weight than someone who is leaner.” Even so, Dr. Hall noted that the 3,500-calorie formula grossly overestimates how much weight will be lost in a year even for that population.

Dr. Hall said that a simple, quick formula for sustainable weight loss is to reduce intake by 10 calories per day per pound you’d like to lose (for instance, to lose 30 pounds, you could cut your calorie intake by 300 calories per day). But this will be slow, he cautions — at this rate, significant weight loss takes yearsto achieve.

You can lose weight faster if you cut back more calories and/or add more physical activity, of course — and you don’t have to be a sophisticated scientist to figure out how to do that.

But for those who really like to work with more precise calculations, Dr. Hall and his team have developed a mathematical formula that takes into account the various complexities of body composition and metabolism to more accurately predict how many calories a particular individual will have to cut back — or use up — in order to reach a weight goal after a desired period of time. The model also calculates what permanent changes of diet or physical activity will be required to maintain that weight for many years. Dr. Hall told me that he hopes to have his calculator, which is already being used by researchers and clinicians around the world, available to the public online by mid-2011. Stay tuned.