What is the yo-yo effect?

In one line

The yo-yo effect is when weight lost by dieting comes back, returning to where it was or going even higher.

What is the yo-yo effect?
Why does hard-lost weight come back?
Diet talk almost always includes "the yo-yo came back." Weight that took months to lose is suddenly back on the scale. What exactly is the yo-yo effect, and how common is it?
In short
Lost weight coming back
The yo-yo effect is when you lose weight by eating less, and then your weight goes back to where it was or even higher. Losing and regaining weight over and over is also called yo-yo dieting.
Where the name comes from
Like a yo-yo going down and up
Just as a yo-yo on a string drops and climbs back up, body weight goes down and then back up. In research, losing and regaining weight again and again is called "weight cycling." It means body weight going around in circles.
The opposite
Keeping lost weight off for over 1 year
The opposite of the yo-yo effect is "weight loss maintenance," keeping the lost weight off. A common research definition is losing at least 10% of your starting weight on purpose and keeping it off for at least 1 year.
How common is it?
After 5 years, most of the lost weight was back
Studies that followed people for years after weight-loss programs found that more than half of the lost weight came back within 2 years. After 5 years, most of it was back. Even so, 5 years later the average weight was still more than 3 kg below the starting weight.
The body's response (1)
Losing weight makes you hungrier
Hormones are substances made in the body that travel in the blood and carry signals. In a study where people lost weight by eating very little for 10 weeks, hormones that raise hunger went up and hormones that signal fullness went down. After 1 year, these hormones had still not returned to their pre-diet levels, and people felt hungrier.
The body's response (2)
A lighter body uses fewer calories
A lighter body takes less energy to move and to keep running. So when weight goes down, your maintenance calories, the calories that keep your weight the same, also go down. If you go back to eating what you ate before the diet, you are now eating more than you use: a calorie surplus.
Common myth
Does each yo-yo pile on more fat?
Many people believe that each round of yo-yo dieting adds more and more fat to the body. But when studies of healthy adults were pooled, most found that people who had yo-yo dieted did not have more body fat. Not a single study found that their lean mass (muscle, bone and water) went down.
Summary
The yo-yo effect is lost weight coming back
The yo-yo effect is when weight lost by dieting comes back, returning to where it was or going even higher. In most studies, people who went through the yo-yo did not keep piling on more body fat. Related terms: weight loss maintenance, maintenance calories, calorie deficit and surplus.