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When somebody loses weight, where does the fat go?

bmj.com

11–20 of 241 posts

Re: When somebody loses weight, where does the fat go?

#11
post #8

Why is CO2 the correct answer, the answer should be energy/heat + CO2 + H2O? The "burning" of fat in oxygen to CO2 and H2O is exothermic.

TFA does say "energy/heat + CO2 + H2O", but most of the output in terms of mass -- which is what one is interested in when it comes to losing weight -- is carbon dioxide.

Re: When somebody loses weight, where does the fat go?

#12
post #4

For anyone coming straight to the comments looking for an answer to the question: it turns into CO2 (and a bit of water) and you breathe it out. When you realize that you have to breathe out all the weight you lose, I think it gives you a bigger appreciation for how difficult weight loss can be.

Exercise makes more sense! Breathing hard == losing weight!

From the paper "Replacing one hour of rest with exercise that raises the metabolic rate to seven times that of resting by, for example, jogging, removes an additional 39 g of carbon from the body, raising the total by about 20% to 240 g. For comparison, a single 100 g muffin represents about 20% of an average person’s total daily energy requirement. Physical activity as a weight loss strategy is, therefore, easily foiled by relatively small quantities of excess food."

Re: When somebody loses weight, where does the fat go?

#13
post #4

For anyone coming straight to the comments looking for an answer to the question: it turns into CO2 (and a bit of water) and you breathe it out. When you realize that you have to breathe out all the weight you lose, I think it gives you a bigger appreciation for how difficult weight loss can be.

Exercise makes more sense! Breathing hard == losing weight!

It's easier to eat less calories than exercise.

Re: When somebody loses weight, where does the fat go?

#14
post #12

Earlier quoted context omitted.

Exercise makes more sense! Breathing hard == losing weight!

From the paper "Replacing one hour of rest with exercise that raises the metabolic rate to seven times that of resting by, for example, jogging, removes an additional 39 g of carbon from the body, raising the total by about 20% to 240 g. For comparison, a single 100 g muffin represents about 20% of an average person’s total daily energy requirement. Physical activity as a weight loss strategy is, therefore, easily fo…

The question is whether the body spends any energy outside of that hour after exercising (e.g. by growing muscle, etc.) than it would otherwise. That would make exercise more effective when considering longer periods of time.

Re: When somebody loses weight, where does the fat go?

#15
post #12

Earlier quoted context omitted.

Exercise makes more sense! Breathing hard == losing weight!

From the paper "Replacing one hour of rest with exercise that raises the metabolic rate to seven times that of resting by, for example, jogging, removes an additional 39 g of carbon from the body, raising the total by about 20% to 240 g. For comparison, a single 100 g muffin represents about 20% of an average person’s total daily energy requirement. Physical activity as a weight loss strategy is, therefore, easily fo…

200g are what would be "breathed out" in one hour without exercise?

So you can only increase your "burn rate" by 20%?

Re: When somebody loses weight, where does the fat go?

#17
post #4

For anyone coming straight to the comments looking for an answer to the question: it turns into CO2 (and a bit of water) and you breathe it out. When you realize that you have to breathe out all the weight you lose, I think it gives you a bigger appreciation for how difficult weight loss can be.

Exercise makes more sense! Breathing hard == losing weight!

Weight is lost in the kitchen.

Re: When somebody loses weight, where does the fat go?

#18
post #4

For anyone coming straight to the comments looking for an answer to the question: it turns into CO2 (and a bit of water) and you breathe it out. When you realize that you have to breathe out all the weight you lose, I think it gives you a bigger appreciation for how difficult weight loss can be.

How much weight does a typical person breath out in a day though?

(if you figure ~2000 calories worth, it doesn't seem like the breathing weight out is so much the issue...)

Re: When somebody loses weight, where does the fat go?

#19
post #9

Observations from Figure 1, "Responses of a sample of doctors, dieticians, and personal trainers to the question “When somebody loses weight, where does it go?”" 0 doctors were willing to admit they didn't know. 0 dieticians thought sweat was involved. Those results seem unsurprising. Only dieticians correctly answered CO2.

Overstating the case about “0 dietitians ... only dietitians.” It looks as though about 2 people thought sweat was involved, and only 1 dietician answered co2. Pretty much everyone answered “heat.”

The question itself is shit, too, which is what happens when you dont have survey epidemiologists eyeball your questions. In “losing weight,” you’re doing a number of things - chucking the actual carbons (mass), and removing the potential energy stored as carbon-carbon bonds (heat). Given the widespread colloquialism of “losing weight,” people failing to interpret it as regarding the removal of carbons specifically isn’t unreasonable.

Sweat and urine are also correct answers, for that matter. Some of the carbon is used to create bicarbon and carbonic acids, via co2, which are lost in urine and sweat. Whether you want to count that as co2 or not is an argument about semantics - it goes through co2, but isn’t lost -as- co2. Of course, from the perspective of “it’s all breathed out as the result of perfect oxidation,” it’s wrong... but why let biology get in the way of undergraduate chemistry, right?

This isn’t a reflection on docs, PTs, or dietitians. This is GIGO.

Re: When somebody loses weight, where does the fat go?

#20
post #15
post #12

Earlier quoted context omitted.

From the paper "Replacing one hour of rest with exercise that raises the metabolic rate to seven times that of resting by, for example, jogging, removes an additional 39 g of carbon from the body, raising the total by about 20% to 240 g. For comparison, a single 100 g muffin represents about 20% of an average person’s total daily energy requirement. Physical activity as a weight loss strategy is, therefore, easily fo…

200g are what would be "breathed out" in one hour without exercise? So you can only increase your "burn rate" by 20%?

Increasing your basal rate of caloric consumption (and, thus, CO2 exhaled) is hard.
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