Alright so I apologize if this questions seems less "physics" based, but I promise there is a reason I'm posting it here- if you can thoroughly explain this, I'd much appreciate it! FIRST PREMISE: I was recently reading on the forum about the question of burning a log of wood (mass) and that if all of the parts of that wood (ask, smoke, water, CO2 etc) were collected that it would be almost the same weight, someone wrote "Burning a log does not convert mass into energy" what did this mean? SECOND PREMISE: In another post someone mentioned that energy is stored as mass in food. So for example 100 grams of FAT contains a certain amount of energy (we measure this in calories, and there are 8 calories in 1 gram of fat, 4 calories in 1 gram of protein or carbohydrates. THIRD PREMISE: The act of Jogging, Swimming, Running etc. causes the body's metabolic rate to increase which causes a "burning" of calories. If someone burns more calories than they consume by eating, they will lose weight. QUESTION: Since burning a block of wood really only releases a little energy that cannot be retrieved (heat and light) but in reality the water, CO2, ASH, smoke etc. weighs about the same as the original block of wood. AND since the amount of sweat that comes off a body that is working contains a minimal amount of weight, and is almost entirely water and salt (NOT any of the components in the macro-nutrients ingested) then how can it be that after consuming (say...) 100 grams of FAT, or 800 calories, that running or exercising in any amount (say 45 minutes of jogging) could "burn" off those calories of energy AND consequently reduce the physical mass of the person working. Sorry for the long post, and structure- I just want to be sure my question is complete and understandable. In brief, though, I'm wondering how by running for 50 minutes, I'm burning 500 calories which is about 120 grams of FAT, yet I am CERTAIN that I have no "lost" .12 Kg of weight. Thanks!