I have been on Atkins for 7 days now and lost about 7 pounds. I have had 3 bowel movements and, without weighing them, am pretty sure that the food I’ve consumed has exceeded their combined weights. I should be gaining weight by that way of thinking.
Where does the weight go? Is fat or muscle converted into liquid or gas and expelled in those states?
In most diets, you reduce intake and increase output so that fat/muscle is metabolized for energy. In the Atkins diet a lot of the initial weight loss is simply water you pee out - low carbs means you have low glycogen in the muscles and glycogen is responsible for holding water. It’s why bodybuilders carb deprive and then carb-up immediately preceding a competition.
Fats are made from carbon, hydrogen and oxygen; they can be (pretty much) broken down into carbon dioxide (which is exhaled) and water (which is exhaled as vapour or passed in urine). But the fat storage function of your body also requires liquid water to make it work, so burning off the fat means you also lose some liquid water.
That said, much of your initial weight loss is likely to just be water that you’re losing for other reasons than actual loss of fat.
I’m sure a bit is radiated off as heat?
While it is true that much of the weight is eliminated as water in some form or other, a significant amount of fat is burned as a matter of course as the body adjusts its own scalar potential. In the mid-1920s this scalar potential was measured by the US Dept of Energy using an x-ray scanning phosphorgraph. In addition to some measurable heat being shed by exposed skin surfaces the scalar potentals observed on a 38 year old male of average build during a 45 minute aerobic exertion was determined to be only about 3 nanovolts, but at a startling 845 amps! Working with this energy measurement it was determined that an average individual could generate enough scalar energy during a 3 hour sprint to completely overcome the central nervous system, should that energy be stored and then released in the direction of a person “at close range” in a single burst! A device for such an application is thought to have been developed, however its actual use has never been confirmed, and the Department of Energy denies any such research ever took place.
Alright, you guys, lets get real. While fat, carbohydrates, and even proteins are ‘burned’ (oxidized) for energy, the resultant waste products still need to leave your body in some physical form. The mass decrease due to radiated energy is immeasurably small … If you lose a pound, a pound of matter has exited your body in your exhalation, excretion, or secretion processes.
Inigo Montoya: a ray. In the 1920. I wonder if that brings any cliches to mind? :rolleyes:
Nope. There are no nuclear reactions taking place in a standard metabolism. The energy you use is chemical and thus does not change your mass at all.
To answer the OP: Your original assumption about the weight of your bowel movements is incorrect. You have been voiding more mass in waste than you have been taking in.
Why, I have no idea what you’re implying, good sir. Perhaps it’s time to apply a new layer of ketchup to the inside of your hat! 
Not as bowel movements. Bowel movements contain mostly undigestible material, intestinal bacteria, water, and some cellular breakdown products. Their weight relative to your intake has comparatively little to do with whether or not you are gaining or losing weight.
As others have said, the main products of catabolic metabolism (breakdown of body tissues, including fats and proteins) are carbon dioxide, water, and heat, plus some urea in the case of proteins. The carbon dioxide of course is breathed out, while the water and urea are removed mostly by urination (plus breathing and sweating).
OK then. Does anyone have a breakdown of %ages? Even a rough approximation? Of a pound of catabolized fat, how much of it is going to be excreted/secreted as water, how much as faeces, and how much released as carbon dioxide? Heat (and scalar energy) I can understand would represent a negligible mass as they are simply energy released by the action of molecules breaking. Yeah, some mass, but barely.
Inigo I can Dig It!
Actually the fat, when lost, goes to a special storage place. Once you loose it, HAL 9000 takes over and, well, returns it to you. You’ll never know it, but at the darkest time of night, step by step, bit by bit, the fat is returned to you! (Except chocolate. That don’t count)
I’ll be rambling on now… 
Any release of energy inevitably results in an alteration of mass. That’s just as true of chemical energy as it is of nuclear energy. The binding energies being converted are different between nuclear and chemical reactions but the net result is the same.
What it comes down to is that E=MC^2 no matter what type of energy you are talking about. If energy is lost to the system (in this case the body) there has to be a concommitant loss of mass. If you can ever work around that problem you’ve got yourself an instant Nobel prize.
The amount of mass actually lost by the body due to energy loss is completely trivial, and probably not even trivial. What you need to realise is that the same is true of nuclear reations. The actual amount of mass lost from a nuclear reactor is also totally trivial. In both cases it’s only the mass actually contained as binding energy that’s lost. None of the mass that existed as ‘real’ matter is lost in either case.
Essentially none of it goes into the feces (except perhaps in the form of water). Pure fat is converted entirely into carbon dioxide and water.
I’m not going to try to figure out the relative weights of the CO[sub]2[/sub] and H[sub]2[/sub]O produced, since there are a very complex series of reactions involved. But you can get an overview of the process of fat metabolism here.
Actually, come to think of it, it should be possible to get a ballpark figure without working out the equations in detail. Take the example given in my link of a fat molecule with 58 carbons, 112 hydrogens, and 6 oxygens. Since carbon has an approximate atomic weight of 12, hydrogen 1, and oxygen 16, carbon makes up about 77% by weight of the fat molecule.
If the fat is completely catabolized, all of the carbon will be removed from the body in the form of carbon dioxide. Therefore out of every pound of fat lost, a minimum of 77% by weight is exhaled away. Most of the oxygen in the carbon dioxide comes from the air. Offhand, I don’t recall whether the relatively small amount of oxygen in the fat - 10% of the total weight - ends up in the carbon dioxide or the water. So, very approximately, between 77 and 87% of every pound of fat lost is removed as carbon dioxide, and the rest as water. (Note that the carbon dioxide and water excreted together will actually weigh more than a pound, since some of their weight will come from atmospheric oxygen.)
As the self-proclaimed champion of the OP I take back what I was muttering under my breath about the Handy-style “Google It Your Own Damned Self” answer in post #14 and humbly thank and applaud you for providing some Dope.
So the answer is: It’s mostly breathed away…unless there’s a bunch of carbonation in your urine, in which case you most likely work for a macrobrewery in Wisconsin.
I take exception to the suggestion that post #14 was handy-ish. The exact composition of fat is variable, and the metabolic reactions are complex. (And it’s been a very long time since I had to memorize the Krebs cycle.) I answered the part of the question I could respond to easily. I provided the info on fat metabolism mainly to demonstrate the fact that the products were CO2 and water, and to show how complex the process was.
:: still kissing Colibri’s backside ::
Yes, I’m aware of that. But since we’re talking body chemistry and not high-energy physics, Conservation of Mass is a convenient approximation for this level of abstraction.
Ok, ok, that’s enough. It’s just that the mere mention of handy’s name still makes me go cross-eyed.