If you are trying to lose weight, you’ve definitely heard this rule: Cut 3,500 calories from your diet and you’ll lose one pound.

But that old adage is based on faulty science, diet researchers told the Washington Post.

In the 1950s, medical researcher Max Wishnofsky found that a pound of fat is equivalent to 3,500 calories, according to the paper.

But that doesn’t translate into how many calories you need to burn to lose a pound.

When you lose weight, you don’t only lose fat tissue, said Kevin Hall, a researcher at the National Institute of Diabetes and Digestive and Kidney Diseases.

“It’s a relatively minor error, because a lot of it is fat tissue, but it still isn’t true,” Hall said.

Besides, the number of calories required to lose weight has more to do with how a person’s metabolism functions, researchers explain. It gets progressively harder to lose weight the more pounds you cut because your metabolic rate drops.

As University of Colorado researcher John Peters explains:

“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.”

So cutting 3,500 calories may work at first, but it won’t stay that way.