Like all weight loss, you will lose lean muscle mass and bone density as part of it if you do not take corrective measures. And even with such measures, you will likely lose some either way for the medium term. It's very difficult to lose only fat during a long term calorie deficit.

The answer is getting into a regular schedule with resistance weight training. Obviously not all that many people will pull it off, but if you can pull it off you can stave off the worst of the side effects in many cases.