I went on a crash diet in 2019. Dropped from 300# to 260# in 4 months. About as fast as you can go without getting unhealthy (1% current weight per week).

I was a powerlifter for a time, so I carry around a lot of muscle. My lean mass before the diet was 210#.

BMR is how many calories a person will burn in a day by being alive, but staying in bed all day. All my lean mass puts my BMR around 3k calories a day.

1# of body fat loss requires about a 3.5k calorie nutritional deficit, so 3# a week loss is 1.5k a day deficit.

I don't like big daily deficits on my diets, so I kept mine moderate (eating 2.5k, or 500 deficit a day), and used exercise to create a bigger deficit when I could.

I ran (ok, jog/walk intervals) a 5k every day for 4 months. This generally created the other 1k deficit each day.

Generally, about 1/3 the weight you lose on a diet will be muscle mass. With higher protein intake, it can be lower.

Somehow, I lost almost no muscle.

I don't have a point to this thread. Self centered musing I guess. Apparently with the right diet and exercise and body type and... I dunno what else... You can break common sense a little bit.