When is a pound lost and where does it go?

The original poster asked some interesting questions, and I like the way she thinks.

A "heat engine" takes in heat and converts a portion of it into mechanical work. When nutrient molecules in the body are combined with oxygen ("burned"), that's an exothermic reaction; it produces heat. Muscle cells use that heat energy to do work. The whole human body can be usefully regarded as a heat engine.

And yes, calories *are* energy, rigorously and exactly. A calorie is the amount of heat energy required to increase the temperature of a gram of water by a Celsius degree. (A nutritional calorie -- the sort we all talk about here -- is 1000 calories, as defined. It is also equal to 4186 joules, which is the work done by one newton of force over a parallel displacement of 4186 meters. A "typical" person weighs about 667 newtons, and can be lifted a little over six meters, straight up, by a nutritional calorie of mechanical work.)

A pound is lost when a pound of stored nutrients in the body (hopefully, fat) has been converted to ... well, not just energy. If an entire pound of fat were all converted to energy (according to E = mc^2), a dieter would be like an atomic bomb. But, as another commenter pointed out, chemical bonds in the pound of fat are broken, some of it is oxidized, heat is produced by that reaction; most of the rest of the broken-down nutrient molecules become waste and are eliminated in the same way as any food waste. The energy required by your body is supplied by the food you eat; if you don't eat "enough," you're in a calorie-deficit situation, and some stored body fat is internally "eaten" as well. As the OP said, about 3500 cal of deficit is equivalent to a pound of fat; as for WHEN this happens: as soon as your body requires a single calorie more than you've eaten, 1/3500 of a pound gets burned. When your cumulative deficit reaches 3500 cal, a full pound will have been used. (For the elimination of the unburned waste, I suppose there's a time delay measured in hours.)

But, in any case, that pound of fat didn't turn into nothing. A very small fraction of it is turned into heat and used by your muscles; the vast majority is turned into feces and urine, and returns to nature in the usual ways.

People ask me from time to time how much weight I've lost. Of course, I tell them that "lost" is probably too strong a term. I pretty much know where all of it is. Most of it's at the grocery store; quite a bit is at Burger King and Taco Bell and so on. I could go pick it up at any time. So, it's not really lost; it's more like "misplaced." I hope to leave it that way.

ok... now that i have picked myself back up off of the floor... **wipes away tears**

the question about burning calories IS a good one. and without getting too scientific (i'm a writer, Jim, not a science officer) i can break it down this way:

when you start a fire in your fire pit, barbeque or fireplace, you stack in fuel. this fuel can be logs or charcoal. then you light the fire. the logs and charcoal burn - technically calories, funny but true - and break down into gases, smoke, ash and i'm sure a lot of other things that i was asleep in chem when they explained. the energy is used to make the fire burn, and anything that is NOT that energy, is broken down to waste.

your body isn't that different. but it's your metabolic rate rather than a lighter that gets the fire going. you actually ARE expending heat energy - this is why your core temp goes up and you sweat and all the other stuff that happens when you're hot. Your body uses the stored energy by burning it. anything that is not needed to keep the fire going, is converted into waste - sweat, exhaled breath, poop and yeah, even some urine.

i hope that worked for you.