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.

well you can stay at maintainance at burn 500 cals a day n you will lose a pound every week.
2nd ques when you are burning fat look for when thing when you go to toilet to pee you will notice something white in your pee is like chicken fat thats how some of your fat comes out n some as a sweat. because when i did a diet few years back when the white thing came in pee i got scared that its something serious but found out its nothing just fat so a doctor knows best.

I found some white stuff in my poop, I wonder if I'm still losing fat?

I feel like since I have now read this thread, I have finally "arrived". Thanks MFP folks, my life is forever changed! :flowerforyou:

To whoever said you PEE it out:

"Ketonuria is a medical condition in which ketone bodies are present in the urine.
It is seen in conditions in which the body produces excess ketones as an alternative source of energy. It is seen during starvation or more commonly in type I diabetes mellitus. Production of ketone bodies is a normal response to a shortage of glucose, meant to provide an alternate source of fuel from fatty acids."

Note where it said DURING STARVATION or in Diabetics...

This is not a "common" type of "where does my fat go?". It is not something to strive for or be pleased to see. It is actually bad.

well you can stay at maintainance at burn 500 cals a day n you will lose a pound every week.
2nd ques when you are burning fat look for when thing when you go to toilet to pee you will notice something white in your pee is like chicken fat thats how some of your fat comes out n some as a sweat. because when i did a diet few years back when the white thing came in pee i got scared that its something serious but found out its nothing just fat so a doctor knows best.

FU Science!

WHHHHHHHHYYYY???