Okay! Found some answers! I thought it was written well (as in, I can understand the lingo lmao) and thought I'd share:
Any combination of diet and/or exercise that results in weight loss will cause you to lose all of the following three substances.
1) Fat
2) Muscle tissue
3) Glycogen (a carbohydrate) bound up with water that is normally stored in your muscles and liver. When people talk about "losing water", they're talking about this.
Here's some more detail about glycogen:
The other thing to keep in mind when you're using the bathroom scale is that when you first start limiting your calories, your body is going to start burning through its glycogen stores. Glycogen is basically a fuel stored in your body. It stores sugars together with water and locks them up in the tissues and organs of your body like an energy battery, ready for you to use at a future time.
There's water locked in with those calories. That water weighs a lot. So when you start restricting your calories, the first thing your body burns is this extra storage of energy, this extra glycogen. And the glycogen causes you, as it's burned, to shed water. You might look at the scale and think, gee, I lost 5 lbs, but you really lost no body fat whatsoever. It was just water, because your body released glycogen. What usually happens to people when their glycogen store has reached zero is they get really hungry, they think they're in a starvation panic, and then they overeat. Their glycogen stores fill right back up, they gain the 5 lbs back, and usually they overate to such an extent that they store another half a pound of body fat or so. Now they're half a pound heavier than when they began and they lost no body fat whatsoever. It was just a game of glycogen and water storage they saw reflected on the bathroom scale.
Any kind of diet and exercise program will cause you to lose some glycogen, muscle tissue and fat. However, the proportion can vary depending on your weight loss strategy.
Cutting carbohydrates (Atkins, South Beach, etc.) will result in relatively high glycogen loss in the first few weeks, along with some fat and some muscle.
Drastic calorie reduction, especially without exercise, will cause you to lose a lot of glycogen and muscle tissue.
Excessive cardio can also reduce muscle significantly. Many long-distance runners at the peak of training can look quite gaunt.
Moderate calorie reduction, moderate cardio or interval work, and moderate weigh training are probably the most healthful combination for most people. You will still lose some glycogen and muscle, but a larger proportion of the loss will be fat.
There is probably no way not to lose some muscle when you lose weight. This is why competitive bodybuilders never try to lose weight (fat) and gain muscle at the same time. They go through bulking cycles, where they eat, sleep and train a lot, and slimming cycles, where they reduce their calories to lose fat, then gain back the lost muscle the next cycle, etc.