Does losing 1 kg of fat really take 7,700 kcal?

Short answer

No. Each kilogram of weight lost held about 6,000 kcal, and the body burned fewer calories as weight came off. So real weight loss was smaller than the 7,700 kcal rule predicts. 103 people lost 9.1 kg instead of the 12.5 kg the rule predicted.

Does losing 1 kg of fat really take 7,700 kcal?
'Eat 7,700 kcal less → lose 1 kg' — does this formula hold?
3,500 kcal per pound, or about 7,700 kcal per kilogram, is a common rule in textbooks and on health websites. It says eating 550 kcal less a day for 2 weeks takes off 1 kg. We checked whether this rule matches real weight loss.
When researchers measured it
People lost 3.4 kg less than the rule predicted
Researchers pooled 7 experiments with 103 people whose food was strictly supervised. For 65 days on average, they ate about 1,440 kcal less per day. The rule predicted 12.5 kg of weight loss, but they lost 9.1 kg, and most lost less than predicted. A model that accounts for body changes predicted 8.3 kg, close to the real 7.7 kg.
Not all of the lost weight is fat
Each 1 kg lost held about 6,000 kcal
When 23 people cut calories for 24 weeks, each kilogram of lost weight held about 4,860 kcal at week 4 and about 6,040 kcal from week 6 on. In another study of 75 people over 13 weeks, it was 6,800 kcal for women and 6,120 kcal for men. Early on, more non-fat tissue was lost, so each 1 kg held less energy.
7,700 kcal fits only some people
Only those with more than 30 kg of body fat
A calculation that accounts for body composition found that 7,700 kcal matches 1 kg of weight loss only in people with more than 30 kg of body fat. The less body fat you have, the more of the lost weight is muscle and other non-fat tissue, so each kilogram holds fewer than 7,700 kcal.
So why did people lose less?
The body burned fewer calories
49 people ate about 270 kcal less per day for 4 months and their weight fell by 4.8%. The calories they burned at rest were 65-230 kcal per day lower than their new weight predicted. All 13 calculation methods showed this. As the body burned less, the deficit shrank.
Weight loss is a curve, not a straight line
23 kg a year from the same deficit? In reality it levels off
Researchers modeled a 100 kg person eating 480 kcal less per day. Weight leveled off at about 75 kg; it took about 1 year to lose half of that and about 3 years to reach 95%. By the 7,700 kcal rule, that person would lose about 23 kg in 1 year and never stop.
A calculation closer to reality
Every 24 kcal a day = about 1 kg in the end, half of it in year one
The same study offers a simple rule: every 24 kcal per day you steadily cut leads to about 1 kg of weight loss in the end. Cutting 240 kcal a day leads to about 10 kg eventually, with half, about 5 kg, in the first year. The 7,700 kcal rule would say about 11 kg in 1 year.
Bottom line: the 7,700 kcal rule inflates weight loss
Neither the kilogram lost nor the calories burned stay fixed
Each kilogram of weight lost held about 6,000 kcal, and the body burned fewer calories as weight came off. That is why 103 people lost 3.4 kg less than the rule predicted. Counting about 1 kg in the end for every 24 kcal a day, with half in the first year, comes close to reality.