MacMadame, that is a great point, considering that you are a bypasser. Regarding lean muscle mass, everyone should be worried about losing muscle during weight loss.
The nutritional requirement for adult women is approximately 46g of whole Protein per day to avoid a deficiency. I don't think any of us posting on this board eat whatever we want to, whenever - we all have guidelines we use to control our eating, whether it's intuitive, counting calories, logging our food intake, etc. Part of however we lose weight or maintain our weight is to keep track of ourselves somehow - this likely includes making sure that we are healthy, along with the benefits of actual weight loss.
Here's an example. On a *typical* day, I eat 30% or less of my calories as fat (about 470 calories). 20% - 30% of my calories come from lean protein (about 200 calories), and about 30% of my calories come from complex carbs (another 550 calories). I eat roughly 1200 calories per day, depending on my activity level. While it isn't considered "low carb", it is low-calorie, low-fat, and generally HIGH in protein (slightly more than 46g per day). I work out and am very physically active, so my personal protein requirement is increased. This is true for anyone who is very physically active.
I have known people who have suffered horrible consequences as a result of losing weight quickly without paying attention to consuming enough protein. Protein deficiency leads to nitrogen deficiency, which can have disastrous effects on the body. I had a good friend with hair loss, immune deficiency issues, and tooth loss because she failed to eat enough protein. She lost weight, yes, but was extremely unhealthy in the long-term. Just my humble opinion, which I've formed from past experience.
Be well, everyone!