The goal of weight loss is almost always fat loss despite the phrases being used interchangeably by the general public. When you lose weight it isn't always clear what you are losing, but there are some things that you can keep in mind in order to better understand what's going on.
The "3500 calorie per pound of fat" model of weight loss isn't perfect and has some issues, but it's close enough to be useful. When your body is in an energy deficit, it releases stored energy to balance out that deficit. So if your body is 500 calories below maintenance on a particular day, your body will release energy from its stores equal to 500 calories to balance that out. Your body will not seek to use more or less than it needs, so a deficit of 500 calories always results in 500 calories worth of energy being broken down and released. So what is stored energy? Fat, glycogen, and muscle. Fat is the body's preferred method of energy storage, glycogen (carb storage) is meant to be used for immediate energy (running, lifting weights) as opposed to chronic maintenance like fat, and muscle (protein storage) isn't meant to be used for energy storage at all, but it will be used as such if the circumstances are right. How much is each of these worth?
- Fat contains 3500 calories per pound (2.2 kg).
- Glycogen contains ~1800 calories per pound.
- Skeletal Muscle contains ~600-700 calories per pound.
- Water contains ZERO calories per pound.
It's important to note that water fluctuations can and will often mask energy balance on the scale. This can occur by simply drinking more or less liquids than normal, but it can also occur by manipulating glycogen stores. Every ONE gram of glycogen requires THREE grams of water to store it. So while one pound of glycogen may be worth 1800 calories, losing that one pound of glycogen also means losing three additional pounds of water for a total of four pounds being lost. The first week of a diet (up to the first 2-3 weeks) is typically marked by huge shifts in water and glycogen stores in the body. Not only that, but the weight of the food sitting in your digestive system typically goes down as well. All this results in the initial 5-10 pounds of weight loss being short term adaptations as opposed to long term adaptations in weight. Another good rule of thumb is that for every five pounds of weight that you lose, one pound of that will be water (or put another way, 20% of your weight loss will typically be water).
What about muscle mass? How do you prevent the breakdown of muscle tissue? You give it what it needs. Exercise tells your body that the muscles being used are important and protein helps to maintain and repair muscle mass. You can actually lose weight faster if don't do any exercise at all, but that's because more of the weight being lost is coming from muscle. If you lost weight entirely from muscle mass, a daily deficit of 500 calories (weekly deficit of 3500) would result in nearly 6 pounds of weight loss! But that would carry a heavy negative impact on body composition and your capacity to do daily activities. The reality is that weight loss always comes from multiple sources and muscle loss can be minimized or even reversed by simply exercising appropriately.
So what's a reasonable expectation for the composition of weight loss? After the first three weeks of dieting where glycogen stores as well as water and food weight have stabilized, I would break it down as the following ESTIMATION for anyone above 15% bodyfat who is doing resistance training and feeding their body appropriately.
Finally, if you are gaining or losing weight much more rapidly than your calorie intake suggests that you should be, you can reasonably assume that it is water/glycogen. Unless you just went on a binge-fest all week. The best way to monitor this is by weighing daily first thing in the morning when you wake up from sleep. Doing this will allow you to take weekly means or medians which will be a good representative of your non-fluctuating weight each week as opposed to weighing once a week and hoping that it isn't higher or lower than it should be reporting. There isn't any benefit to weighing more than daily, but if that seems like too much, twice a week could also work. Personally, I just take a quick picture of the scale and write down the information later in a chart.