I lost about 40 lbs recently.
When I first got married I was in pretty good shape — muscular, fit. After we had kids, I started to get out of shape. I lost a lot of the muscle I had. And, started to get fat.
Recently I lost about 40 lbs in about 2 to 2½ months. Which is rapid weight loss.
I did it mostly through: calorie deficit.
I probably went a bit too fast, as I lost some muscle, too.
But, since then I have been weight lifting and putting the muscle back on.