I have lost 30 lbs. recently and I was thinking , where does the weight go? My guess is it comes out and ends up in the toilet. I guess maybe you could sweat some of it out too as water.
Also when you lose weight do you lose cells or are the cells just smaller?
You always retain the fat cells. They just get smaller. The only way to get rid of them is to have them sucked out. I read that somewhere.
The fat turns to fuel which is expelled as energy. I think.
Signed,
A dieter/exerciser
You guess correctly.
Maybe a little bit. Generally speaking fluid loss can lose you huge amounts of weight for extremely short periods. Try weighing yourself before and after a long run without drinking in between.
It went into your other cells to be used as energy. Where does all the food we eat every day go? Same place.
I don’t run, I walk and ride a bike but I will try that. My weight loss was not quick , it was over a period of about 4 months.
I think a noticeable amount of fat can also be breathed out - the carbon dioxide-richer air you exhale when burning calories weighs slightly more than the fresh air you breathed in.
Not most of it. Some of the weight loss is water, no doubt, but most of it will come from burning fat. Metabolism converts the fat mainly to carbon dioxide, which you lose by breathing. Almost all of the weight you lose goes into the air, not the toilet.
You breathe out that lost weight as CO[sub]2[/sub]. Fats plus O[sub]2[/sub] = H[sub]2[/sub]O + CO[sub]2[/sub] + energy.
So water (excreted as sweat, urine and as you breathe) and exhaled carbon dioxide.
Faeces contain undigestable material from the food you eat and bacteria that grow in the gut on that undigested material. So none of the lost weight ends up there.
Si
Simul-dioxide-post! 
What about people who have gastric bypass? Is the process the same for them? They lose a lot of weight pretty quickly.
The process of actual weight loss is the same. Gastric bypass just affects the amount you take in, altering the balance between inflow and outflow so that weight is lost faster.
I’m not an expert (and there’s some good answers in the thread already), but consider Michael Phelps. He eats 8,000 - 10,000 calories a day and clearly burns that up through exercise rather than endless sessions on the toilet!
This all makes sense, I forgot that fat cells can be burned as energy.
Yes, the number of fat cells your body contains does not change after puberty (though there are some who still think you can add fat cells during pregnancy). They simply get bigger and smaller if you gain or lose weight.
Although as implied by other posters, “burning” fuel whether through respiration or combustion has no effect on the total masses involved.
But some of the products of these processes are molecules which are in the gaseous state at standard temperature and pressure, and so in the case of respiration can be exhaled.
Interesting. I thought that the body created more fat cells when they were needed, and that those then stayed forever.
But if the number of fat cells stays the same over the course of your life, does it mean that when you reach puberty your “maximum possible weight” is somehow fixed (you’ll only be able to gain weight up to a given point because after that there won’t be any “storage space” left) or is it the case that fat cells have no upper limit in size, and will just keep growing as needed? IOW, did those people who are absurdly overweight have an abnormally high number of fat cells when they reached puberty, or do they just have absurdly large fat cells?
Wikipedia says that fat cells do divide when they reach about four times their normal size. I guess then that you would have to be substantially overweight before you gained many fat cells.
I believe you can ADD fat cells as you go through puberty. So your diet and exercise habits at that point are pretty important. I’m not sure about cells having an upward limit, but judging by the vast differences in human size, I bet it’s pretty darn big if there IS a limit. There’s probably some sort of limit to how much a person can consume though. That is, with more fat, you need more calories to maintain that fat. At some point, it would be difficult to take in enough to satisfy your caloric requirements to stay fat.
And I’m annoyed that I can’t find a good site for this. My only cite comes from seminars I personally attended on the subject, so relies on my (admittedly faulty) memory.
I don’t know. It depends on what their “normal size” is.
Be a little careful about this. Dehydration can range from uncomfortable to extremely dangerous depending on circumstances.
If you want to do it, weigh yourself before you start, hydrate normally during exercise and keep track of how much you are drinking - really easy if you bring a waterbottle with graduated marks (oz or ml on the side), then weigh yourself after.
For example if I weigh 170lbs before I start, run for a while during which I drink two 20oz water bottles, then get on a scale and I’m 169lbs, I know that I burned/sweated off (1lb+40oz).