If you lose weight fast, do you lose more muscle too?

Short answer

Yes. Losing the same weight faster took off more non-fat mass and less fat. Pooled, the slow groups lost 1 kg more fat, and only those who lost slowly, about 0.7% of body weight a week, gained muscle.

If you lose weight fast, do you lose more muscle too?
'Lose it fast → lose more muscle' — does this formula hold?
You want the diet over quickly, but the warning that fast loss burns muscle makes you hesitate. We looked at studies that compared what the body actually loses when the same weight comes off fast versus slowly.
Losing the same weight faster took more muscle
Non-fat mass loss: 8.8% vs 1.3%
57 people dieted on 500 kcal a day for 5 weeks or 1,250 kcal a day for 12 weeks. They lost similar weight: 9.0 kg and 8.2 kg. But the loss of lean mass (muscle and everything except fat) was 8.8% in the fast group and 1.3% in the slow group.
Another trial found the same
About 3 times more lean mass lost, and less fat
36 people lost at least 5% of their weight over either 5 or 15 weeks. Lean mass fell 1.51 kg in the fast group and 0.52 kg in the slow group. Fat went the other way: the slow group lost 4.52 kg, more than the fast group's 2.92 kg.
The same held with strength training
Only the slow group gained muscle
24 people who lifted 4 times a week aimed to lose either 0.7% or 1.4% of body weight per week. Both lost similar weight, 5.6% and 5.5%. Lean mass rose 2.1% only in the slow group; in the fast group it stayed the same.
Pooled, the gap showed up in fat
The slow approach took off 1 kg more fat
A 2020 analysis that pooled studies losing the same weight fast or slowly found the slow groups lost 1 kg more fat, while the lean mass gap was small. In a trial where 35 people lost about 9% over 4 or 8 weeks, body composition was also similar. What split muscle was the daily calorie gap on the next card.
What decides muscle is the daily calorie gap
About 500 kcal short a day stopped muscle gain
An analysis that pooled trials of lifting while cutting calories found that people in a calorie deficit gained less lean mass. A deficit of about 500 kcal a day stopped muscle gain. Strength rose the same even with the deficit.
Bottom line: losing fast costs more muscle
7% of body weight a week is the benchmark
Even for the same weight, trials that lost it fast took off more non-fat mass and less fat. To protect muscle, losing about 0.7% of body weight a week (about 0.5 kg for a 70 kg (154 lb) person), keeping the daily deficit under 500 kcal and continuing to lift works.