In fact, it’s a total myth.
“I see dietitians using it all the time, making recommendations based [on] 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 kilojoules, 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 of 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 cutback 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 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.”
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 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 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.