Why does weight come back, and how do you keep it off?

Short answer

For every 1 kg lost, appetite grows by about 100 kcal a day and the body burns less, so weight comes back. People who kept exercising 4 times a week were 4.1 kg lighter 1 year later, and higher-protein diets kept off 1.5 kg more.

Why does weight come back, and how do you keep it off?
'Weight comes back because of weak willpower' — is that true?
When lost weight comes back, it is easy to blame willpower. We looked at what happens inside the body after weight loss, and what actually kept the weight off.
Most lost weight came back
After 5 years, 17 to 29% of the original loss remained
An analysis that pooled 29 US weight-loss program studies found that after 5 years, people on a regular reduced-calorie diet kept 17% of their original loss (2.0 kg), and those on a very-low-calorie diet kept 29% (7.1 kg). On average, people were still more than 3 kg lighter after 5 years.
Reason one: appetite grew
For every 1 kg lost, people ate about 100 kcal more a day
In a 52-week study of 153 people taking a drug that made them lose weight without noticing, appetite grew so that they ate about 100 kcal more per day for every 1 kg lost. That was more than 3 times bigger than the drop in calories the body burned. Losing 5 kg means about 500 kcal a day.
1 year later, hunger had not settled
Appetite hormones did not return to normal
50 people lost 13.5 kg on average over 10 weeks. 1 year later, the hormone that triggers hunger (ghrelin) was still high and hormones that signal fullness (such as leptin) were still low. People also felt hungrier than before the weight loss.
Reason two: the body burned less
Even 6 years later, about 500 kcal less a day
14 people who lost 58.3 kg on average over 30 weeks on a TV weight-loss show were checked 6 years later. Even though 41.0 kg had come back, they burned about 500 kcal less per day at rest than expected for their body size. Those who kept more weight off had a bigger drop.
So what kept the weight off?
The group exercising 4 times a week was 4.1 kg lighter 1 year later
After 195 people lost 13.1 kg on average over 8 weeks, those who exercised 4 times a week for 1 year were 4.1 kg lighter than those who did nothing. In the 1 year after the exercise program ended, they regained 3.6 kg, less than the 9.6 kg regained by those who stopped a weight-loss drug injection.
On the food side, protein helped
Higher-protein diets kept off 1.5 kg more
An analysis of 20 studies comparing ways to maintain a large weight loss found that higher-protein diets kept off 1.5 kg more over 5 months on average. An analysis of 45 studies (7,788 people) found that programs managing both eating and activity cut regain by 1.56 kg over 1 year.
Bottom line: regain is the body's response, not weak will
A bigger appetite needs something to hold the line
After weight loss, appetite rises and the body burns less, so most lost weight came back within 5 years. What held it off was exercising 4 times a week, a higher-protein diet, and programs managing both eating and activity. Keeping these going after the diet ends is what works.