4

votes

Stupid Question: So we're losing fat, but where does it go?

Answered on August 19, 2014
Created March 05, 2010 at 5:57 AM

People throw that phrase around all the time, "We're burning fat." And I understand much of the physiology involved, but where does the fat go after it's metabolized?

Do we sweat it out?

Does it pass through the urine or feces?

This question becomes even more absurd if you consider the fact that I have a Bachelor's degree in Exercise Science and Master's degree in Sport Mgmt., lol.

7255a87872b75e6f691d84dca769b87e

on September 11, 2011
at 04:40 PM

What happens when you're eating a high fat and low carb diet but don't experience ketone-breath or ketone bodies in your urine? Are you still ketotic, but burning all the ketone bodies produced?

7255a87872b75e6f691d84dca769b87e

on September 11, 2011
at 04:36 PM

If you're pooping it out, it's undigested dietary fat, not burned body fat.

6b73f0c4b971e2dde7147920e329fe7f

(2041)

on March 06, 2010
at 04:13 AM

Poop is mostly bacterial cells by volume, the remainder being roughage.

Af12951fdf48cf67d71605c1a7f9a492

(324)

on March 06, 2010
at 02:30 AM

The stuff in our poop is what never crossed the gut in the first place. At least according to my very primitive understanding of things.

7bea72ef073e8f76b5828727f1460900

(2718)

on March 06, 2010
at 01:46 AM

I'm not an expert in this area, so I'll defer to you, Acton, and modify my post accordingly. Thanks for the education!

6b73f0c4b971e2dde7147920e329fe7f

(2041)

on March 05, 2010
at 11:34 PM

Water is yielded in overall fatty acid oxidation. You left out the most important part, the electron transport chain, where the bulk of the energy is actually produced. Reduced protons from beta oxidation and the Krebs cycle donate their electons to oxygen via the electron transport chain to form water, producing ATP. "Burning" 100 grams of fatty acids produces roughly 100 grams of water.

7bea72ef073e8f76b5828727f1460900

(2718)

on March 05, 2010
at 07:56 PM

According to Wikipedia, the overall pyruvate (a ketone) oxidation reaction is: Pyruvate ion + 4 NAD+ + Q + GDP + Pi + 2 H2O → 4 NADH + 3 H+ + QH2 + GTP + 3 CO2 I'm sure the H+ will cycle back to become H2O later with some OH-, but I guess that's considered outside the oxidation reaction. I haven't fact-checked Wikipedia. :)

5cd18bfcafadc56292971e59f2f1faf6

(2475)

on March 05, 2010
at 07:35 PM

I thought that some of the released H+ ends up as H2O in the blood stream thus resulting in a net release of H2O? Krebs takes H2O as input but releases a heck of a lot of H+.

77877f762c40637911396daa19b53094

(78467)

on March 05, 2010
at 04:30 PM

Acton, you are a veritable encyclopedia of information!

A89f9751a97c3082802dc0bcbe4e9208

(13978)

on March 05, 2010
at 07:48 AM

Really? We don't poop it out? I have no idea. I'm just askin'...

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5 Answers

7
6b73f0c4b971e2dde7147920e329fe7f

(2041)

on March 05, 2010
at 06:06 AM

After fatty acids are metabolized, they are CO2 and water.

77877f762c40637911396daa19b53094

(78467)

on March 05, 2010
at 04:30 PM

Acton, you are a veritable encyclopedia of information!

Af12951fdf48cf67d71605c1a7f9a492

(324)

on March 06, 2010
at 02:30 AM

The stuff in our poop is what never crossed the gut in the first place. At least according to my very primitive understanding of things.

6b73f0c4b971e2dde7147920e329fe7f

(2041)

on March 06, 2010
at 04:13 AM

Poop is mostly bacterial cells by volume, the remainder being roughage.

A89f9751a97c3082802dc0bcbe4e9208

(13978)

on March 05, 2010
at 07:48 AM

Really? We don't poop it out? I have no idea. I'm just askin'...

7255a87872b75e6f691d84dca769b87e

on September 11, 2011
at 04:36 PM

If you're pooping it out, it's undigested dietary fat, not burned body fat.

3
7bea72ef073e8f76b5828727f1460900

(2718)

on March 05, 2010
at 05:29 PM

The Wikipedia article on fatty acid metabolism contains the full answer to your question. To summarize in laymen's terms:

  • fatty acids are broken down into acetyl-CoA molecules
  • acetyl-CoA is consumed in the TCA cycle
  • the TCA cycle uses acetyl-CoA + 2 H2O + stuff to produce 2 CO2 + energy + stuff

From Acton's comments below:

"Water is yielded in overall fatty acid oxidation. You left out the most important part, the electron transport chain, where the bulk of the energy is actually produced. Reduced protons from beta oxidation and the Krebs cycle donate their electons to oxygen via the electron transport chain to form water, producing ATP. "Burning" 100 grams of fatty acids produces roughly 100 grams of water."

If you're asking about the fat that we eat but don't store on a low-carb diet, some of the fatty acids get broken down into ketone bodies, and the ketone bodies can be expelled through breathing (keto breath) or expelled through urine. I believe both of these are rather minor amounts, however; most of the fat is converted to energy, making you feel "more energetic" and/or boosting your body temperature.

6b73f0c4b971e2dde7147920e329fe7f

(2041)

on March 05, 2010
at 11:34 PM

Water is yielded in overall fatty acid oxidation. You left out the most important part, the electron transport chain, where the bulk of the energy is actually produced. Reduced protons from beta oxidation and the Krebs cycle donate their electons to oxygen via the electron transport chain to form water, producing ATP. "Burning" 100 grams of fatty acids produces roughly 100 grams of water.

5cd18bfcafadc56292971e59f2f1faf6

(2475)

on March 05, 2010
at 07:35 PM

I thought that some of the released H+ ends up as H2O in the blood stream thus resulting in a net release of H2O? Krebs takes H2O as input but releases a heck of a lot of H+.

7bea72ef073e8f76b5828727f1460900

(2718)

on March 05, 2010
at 07:56 PM

According to Wikipedia, the overall pyruvate (a ketone) oxidation reaction is: Pyruvate ion + 4 NAD+ + Q + GDP + Pi + 2 H2O → 4 NADH + 3 H+ + QH2 + GTP + 3 CO2 I'm sure the H+ will cycle back to become H2O later with some OH-, but I guess that's considered outside the oxidation reaction. I haven't fact-checked Wikipedia. :)

7bea72ef073e8f76b5828727f1460900

(2718)

on March 06, 2010
at 01:46 AM

I'm not an expert in this area, so I'll defer to you, Acton, and modify my post accordingly. Thanks for the education!

7255a87872b75e6f691d84dca769b87e

on September 11, 2011
at 04:40 PM

What happens when you're eating a high fat and low carb diet but don't experience ketone-breath or ketone bodies in your urine? Are you still ketotic, but burning all the ketone bodies produced?

1
D15d6820ef1545edac65e975cc2d8949

on March 07, 2010
at 11:02 AM

There is no functional difference, in terms of what eventuall happens, between the fat you've got stored in your fat cells and the fat that you ingest through your mouth.

In fact, most fatty acids you eat are first transported to your fat cells and then used.

1
06d21b99c58283ce575e36c4ecd4a458

(9948)

on March 06, 2010
at 04:14 PM

Dr Eades, in his newsletter email, references his response to a question by Ryan http://www.proteinpower.com/drmike/weight-loss/a-legitimate-use-for-orlistat/ from 2007 regarding "Where does the fat go." It is buried in the comment section.

Richard Nikoley references the same at http://freetheanimal.com/2010/03/isnt-it-time-for-anthony-colpo-to-get-a-life.html in his blog about Anthony Colpo and Dr Eades.

Edit: Dr Eades ties the whole thing up at http://www.proteinpower.com/drmike/metabolic-advantage/thermodynamics-and-the-metabolic-advantage/#more-4034

0
33974ac55e5240bcc34a067a5644726c

(260)

on March 05, 2010
at 08:17 AM

The amount of fat cells stays almost the sama for all adult life. The fatty acids themselves disintegrate into CO2, water and energy (heat). The CO2 is breath out and I think you can guess how the water comes out. Energy is used by the body.

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