The mathematics of weight loss aren't that simple. (Chris Radburn/AP Photo)

There's a popular rule you've probably heard before about losing weight: for every 3,500 calories you shed from your diet, you'll lose a pound. But just because everyone, including nutritionists with graduate degrees, keep repeating this doesn't make it true.

In fact, it's a total myth.

"I see dietitians using it all the time, making recommendations based off of it," said Kevin Hall, who is a researcher at the National Institute of Diabetes and Digestive and Kidney Diseases. "Unfortunately it's completely wrong."

The adage dates back to the 1950s, when medical researcher Max Wishnofsky measured how much energy a pound of fat tissue represents, and found that it was 3,500 kilocalories, otherwise known as calories. Theoretically, he had calculated how many calories a person had to burn—or forego—in order to lose a pound of fat. But Wishnofsky made a couple spurious assumptions.

First, he assumed that when you lose weight you only lose fat tissue. "That isn't true," said Hall. "It's a relatively minor error, because a lot of it is fat tissue, but it still isn't true." 

The much bigger mistake Wishnofsky made was misunderstanding how our bodies react to weight loss. As soon as we start cutting calories from our diet, the number of calories our body expends begins to fall. "It literally starts happening on the first day," said Hall. "And it continues to mount as you lose weight."

The reason Wishnofsky, and so many others since, have botched this biological fact is that it's fairly counterintuitive. The tendency is to assume that as you lose weight, the same calorie cut back should prove even more effective once you are lighter, and, presumably, in need of less food. At the very least, it should continue to produce the same results as it was when you were heavier. So cut 500 calories per day, and drag it out for a week, and you'll be roughly one pound lighter; double the decrease, and you'll drop two pounds; triple it, and do away with three.

But the reality is much harder for people trying to lose weight. In fact, the further progress you make, the tougher it gets.

"Over time, the more weight you lose, the more your metabolic rate drops," explained John Peters, a leading researcher at the Anschutz Health and Wellness Center at the University of Colorado. "In order to keep losing weight at the rate you started losing weight, you’re going to have to eat even fewer calories. A month in, you might have to eat another hundred fewer; a month after that you might have to drop it another hundred."

Hall has, in many ways, spearheaded the movement to shed the nutrition world of the 3,500 calorie rule. In 2011, he created a model, called the Body Weight Planner, that directly challenged the adage. Drawing from a vast pool of data, the tool approximated metabolic changes in people trying to lose weight, and showed how greatly the 3,500 calorie rule overestimates weight loss.

"What we found when we made the comparison between the 3,500 calorie rule and our revised prediction is that it over predicts how much people will lose by a sizable margin," said Hall. "If you’re just looking at diet changes alone it’s about two fold greater a year."

The disappointing reality dieters face is that our bodies work tirelessly to defend our weight, even when that weight isn't ideal. The metabolic changes are actually only one of three biological adjustments that follow severe cuts in calories—there are neurological and hormonal changes that happen too, both of which make losing weight and keeping it off a significant challenge. In fact, it can be nearly impossible. For these reasons some researchers say diets don't actually work.

Hall prefers to say that losing weight is difficult. Most people who try to lose weight, he says, end up back to where they started in less than a year. But he blames popular but misleading rules for the long-term failure of so many diets.

"People don’t have the time or energy or know-how to sift through myths like the 3,500 calorie rule," said Hall. "So they believe them, and tailor their behavior to them."

It's hard enough to tell tell fact from fiction in the nutrition world, where popular fads often speak louder than science. But it's nearly impossible when the same lie is printed in practically every nutrition textbook.