Why do diets hit a plateau?

Short answer

Not because your metabolism stops. Early on you lose water, appetite rises by about 100 kcal a day for every 1 kg lost, and people log 25-47% less food than they actually eat. Together these create the plateau.

Why do diets hit a plateau?
'Your body adapts and shuts down your metabolism' — is that right?
Weight comes off fast at first, then suddenly stops. People often say the body has adapted to starvation and switched off its metabolism. We looked at each possible cause of the plateau.
Much of the early weight is water
Each gram of stored carbs holds 3-4 grams of water
The body stores carbs as glycogen, and glycogen is stored with 3-4 times its weight in water. When 11 people went on a very-low-calorie diet for 4 days, most of the early weight drop came from this store. Once that water is gone, the rate of loss slows sharply.
The drop in metabolism was smaller than you'd think
About 92 kcal a day, even after losing 14 kg
71 people lost an average of 14 kg in 8 weeks on 1,000 kcal a day. The calories they burned at rest were 92 kcal a day lower than expected. After 4 weeks of holding their weight, that gap shrank by half. It is too small to explain a plateau.
So why does it stop? People under-log their food
They logged 47% less than they ate
10 people who said they ate under 1,200 kcal a day without losing weight were measured closely for 14 days. Their energy use was normal, within 5% of predicted. But they logged 47% less food than they actually ate and 51% more exercise.
The same held for people who had lost weight
They logged 605 kcal a day less
Even 26 people who had lost 13.6 kg or more and kept it off for more than 1 year logged 605 kcal a day (about 25%) less in their food diaries than they actually burned. Judging by your log alone that 'I eat this little and still don't lose' is easy to get wrong.
Appetite grows as weight drops
About 100 kcal more a day for each kg lost
An analysis of 153 people over 52 weeks found that for every 1 kg lost, they ate about 100 kcal more per day without noticing. That is more than 3 times the drop in calories burned (about 30 kcal per 1 kg). Lose 5 kg and intake rises by about 500 kcal a day, wiping out the original deficit.
Adding exercise raised food intake too
They lost 2.7 kg less than expected
171 people exercised at set doses for 24 weeks. The higher-exercise group lost 2.7 kg less than predicted. The cause was eating about 124 kcal more per day; changes in metabolism or everyday activity were not the cause.
Bottom line: a plateau is about intake more than metabolism
Water loss ends, then appetite and logging errors add up
In a computer model too, the plateau around 6 months was explained by an occasionally slipping diet, not by metabolic adaptation. Loss slows after the early water is gone, appetite grows, and logs come in 25-47% low. At a plateau, re-measuring what you eat is the place to start.