One of the Laws of Thermodynamics is the the prevelant natural law of physics that accounts for how our bodies gain and lose weight. Simply put, it states that Energy and Matter in the Universe are finite; meaning what exists cannot be destroyed nor can anything new be created. However energy can be converted to matter and vice versa.
What does this mean for me and my body composition?
Calories are a measure of heat energy. If we burn more Calories in a day than we consume, that means the additional Calories must come from somewhere. Remember, they can’t just poof, pop up from nowhere. The extra energy can come from matter, however. Matter that is stored in our body that contains stored energy. FAT. Now, muscle contains energy too and when Calorie deficits come from muscle it is called muscle atrophy. If you are following a solid regimen, then burning more calories than you consume in a day must lead to your body releasing the extra energy from stored fat.