Updated: Mar 1, 2019
Weight loss is different than fat loss, usually how fast the weight drops is highly correlated with how much of that weight is muscle/water or fat. For example, if you are 190 lbs in the morning, then 186 after your workout, most of that loss is probably due to water loss...If you exercised fasted, you can add some muscle loss in there too. But how can you be sure or know if your body actually burned its fat. BIG DISCLAIMER! science has yet to get a full grasping understanding of our metabolism, if it did, fat burners would work...
How Fat burn works: Here is what we know for sure. Fat is stored in specific cells in your body, upon the release of appropriate enzymes and hormones by your body, those cells release their content (free fatty acids) into your bloodstream to be delivered where it is needed. As a result that fat cells shrinks, giving you that lean look you are probably after, (fat cells are never destroyed but endlessly created if need be).
What happens next?: Once fat is released from its storage, it oxidizes and the following formula takes place...Oxygen + Fat = CO2 + H2O + Energy. out of that outcome your body takes the energy it needs, releases the carbon dioxide (CO2) via your exhales, and the water (H2O) via sweat, urine, water vapor in your breath.
What does this mean?: It means that there is a high correlation between fat burn and water / oxygen consumption. As you start getting leaner or losing fat, the energy held withing your fat cells has been used up and its byproduct exhaled or sweated out. Meaning the intensity of your workouts matter, as well as the conditions in which you exercise in (fasted, not fasted, weight lifting, cardio, etc...); Those conditions bring this scenario to a more technical level. But the gist of it all is to drink plenty of water, get a good sweat, and don't be lazy.