Weight loss and fat loss are almost always assumed to be the same thing. Bodybuilders assume muscle gaining will translate in to weight gaining. And we all assume that weight gain in general means fat gain.
To be clear your body is mostly water (see picture above), and most of your day to day fluctuations in body weight are due to changes in water and have little or nothing to do with changes in fat mass or muscle mass.
It’s not unrealistic to have a 4-5 pound swing in bodyweight throughout the day due to changes in water content.
If you’re trying to lose 1 pound of fat in a week, you’ll never be able to measure your progess on the scale considering your goal of 1lbs lost is approximately 5 times less than your daily fluctuation (5lbs).
If you’re trying to measure fat loss then you must take a long view and use monthly measurements and pictures/mirror check vs daily or weekly scale measurements.
Daily and weekly scale measurements are more consistent with your current body water and food content.
The dramatic weight loss many people experience at the beginning of any diet is usually due to a large drop in body water content. Interestingly bodybuilders will ‘drop water’ with diuretics at the end of their diet before a contest and routinely lose an additional 7-10 lbs of bodyweight the day before a show.
* Muscles are full of water so there is some decrease in muscle volume/hydration but the protein content does not change. *
In both cases the dramatic changes in bodyweight are solely due to changes in water and have little or nothing to do with changes in fat mass or muscle mass per se