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Here's my theory:

If you consume x calories each day with y amount of caloric expenditure each day you gain zero weight, regardless of x and y being out of balance.

If you then mechanically remove a large percentage of body fat (surgically), the fat should not come back.

Is this a true theory or not? It seems to me that if the body is stable at the current consumption of x and expenditure of y then the removal of excess weight/fat should not affect anything.

Anyone have thoughts on this? Is my thinking flawed? Why? Even if x and y were equal why might the fat come back if there is no net gain when the fat exists?

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The fat that you removed has some non-zero metabolic cost that you need to account for. Your x-y=0 balance will be off. –  kmm May 15 '12 at 15:16
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What makes you think your theory is true? Definitely your first premise is false, otherwise nobody would ever gain weight... –  nico May 15 '12 at 16:45
    
Kevin- so if you consume less calories after the removal with all things being the same the removed fat should not return right? –  helicopterjeff May 15 '12 at 16:55
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@helicopterjeff I would, however, recommend clarifying your wording. Currently, it sounds like your assumption is that x and y can be completely off balance and yet you always maintain weight equilibrium. I don't think this is what you're trying to say. If I understand, you're suggesting that for some, weight equilibrium is possible even if x and y aren't balanced. Is this correct? –  Daniel Standage May 15 '12 at 17:34
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@DanielStandage: that is exactly my point. As it stands it looks like with any combination of x and y you will never gain weight. I am not doubting that SOME people may not gain weight for y<<x but that would definitely not be the norm. –  nico May 15 '12 at 17:53
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I'm not quite sure what you mean by

regardless of x and y being out of balance.

Surely it would make more sense to assume you gain 0 weight if x and y are balanced? The prime reason why people get obese is probably because x and y are not balanced for them (there are other reasons though, as well as reasons why people can have it imbalanced and still not gain weight).

  1. Even if you consume exactly as many calories as you expend in a day, you may gain weight depending on how (ratio of protein/fats/carbohydrates) and especially when you consume the calories. The body may for example store consumed amounts as fat after eating and then use carbohydrate storage from liver and muscle when starving, so long term you would still gain weight.
  2. Adipose (fatty) tissue requires energy too. If you consume in the same manner after as before cutting a large amount of it away, you may just regain it until x=y again.

  3. Also consider: if people ate just half an apple a day more than the exact amount of energy they need, they should gain more than a kilo on average every year. But they don't, and you can hardly assume that people manage on average to consume the exact amount their body needs.

The reason for that is probably leptin. It is a hormone produced by adipocytes (fat cells) and essentially indirectly reduces food uptake and increases expenditure, among others by decreasing your appetite. This has been regarded as a sort of "adipostat"; your leptin system creates a genetically determined level of body fat that you will tend to obtain. Dieting or cutting away fat will cause a drift to return to the previous weight because of a lower level of leptin (due to reduced fat cells). Of course it's not as simple as that though, so it's also possible that you will actually long-term reduce weight through those methods. E.g. exercise seems likely to be an exception somehow.

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x and y being out of balance meaning that most of us, in the USA anyway consume WAY more calories that we expend. Yet in many cases no weight gain is seen. I am personally in that boat. Despite diet, I'm not gaining or losing. So I am wondering why then if the fatty tissue is removed why does it come back? The leptin system is probably what is at work. The body wants to be at a certain weight. If you starve it you're at a lower weight. –  helicopterjeff May 15 '12 at 18:20
    
There are many reasons why that can happen. Regarding leptin, something could be wrong with its production or its reception at the target sites. The same is the case for me though even though my leptin is probably ok. It's most likely due to a kidney malfunction which my dad and brother have as well (we're all slim the same way). There's probably other different causes as well. –  Armatus May 15 '12 at 18:24
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