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Do we lose weight by respiration ? (Losing carbons)

  1. Feb 10, 2010 #1
    We take in Oxygen (O2) & give Carbon dioxide (CO2) .
    Say a guy somehow does not digest any food or fat for 5 min .. So will lose weight equal to all the "C" lost from his body in that 5 min ??
  2. jcsd
  3. Feb 10, 2010 #2


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    I recall having heard this story once (I'm going to abuse your thread to check whether it was actually true :-)) about someone who actually performed the experiment.
    He constructed a very accurate scale, and sat on it for 24 hours. All the changes in his mass were accurately recorded: how much food and water he took in, how much he sweat, urinated, ... At the end there was a small amount of mass actually missing from the balance. Allegedly, this was due to the fact of him breathing out more mass than he breathed in (namely, one molecule of C for every oxygen molecule).

    I actually wonder whether this story can be true (I suppose that the mass you lose in C-molecules is really, really, REALLY very tiny, possibly smaller than the uncertainties in your other measurements, like sweat excretion).

    It does contribute however, so if you wanted to make it really accurate, you would at least have to include this effect (based on an average lung volume, respiration rate, etc) you would be able to estimate this effect.
  4. Feb 10, 2010 #3
    thx .. i have no interest on losing weight .. lol .. just wanted to know . thx
  5. Feb 10, 2010 #4


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    Did a quick-and-dirty calculation, based on the following data:

    * 7,5 litres of air per minute (source, fig 1)
    * air weighs 28.97 g/mol (source) of which 20.95 % vol is O2 (source)
    * for every 5 molecules of O2 one molecule of C is exhaled (i.e. 20% of the O2 is converted to CO2 - could not find a source for this so it's a wild guess)
    * C weighs in at 12,01 grams per mole
    * all other properties of the air (non O2-components, density, moistness / saturation) are unchanged

    According to my rusty chemistry, punching in this numbers, you lose a little under 1 gram every 5 minutes, as long as you breathe. Since all the calculations are linear, it's easy to adjust for changes (e.g. if you want to convert all the O2, multiply by 5; if you want to double the intake of air volume, multiply by 2, etc.)

    Note that 1 gram, is also 1 ml of water. So although the number is not as small as I suspected, you probably lose more weight water every minute (through breathing in dry air and breathing out damp air, as well as sweating) than in C every 5 minutes :P

    You are right though, if you wanted to lose weight, this would be a bad way to start :P
  6. Feb 10, 2010 #5


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    Well when we think of losing weight, we think of burning calories. When we convert fuel (carbohydrates and fat) into energy, we produce carbon dioxide and water. For example, for glucose:

    C6H12O6 + 6O2 --> 6H2O + 6CO2

    Now most of the mass of the water produced comes from oxygen (during oxidative phosphorylation, hydride ions are transfered to oxygen to produce water). So, by this logic, when we are burning calories, most of the mass of the mass of the fuel molecules is going away as carbon dioxide!
  7. Feb 10, 2010 #6
    There is definitely a difference in the weight of the change, but it is probably too small for measure or significance.
  8. Feb 11, 2010 #7
    I thought this conclusion was common knowledge. We are in effect, "burning" our food as fuel. Whatever we don't excrete out the back or build fat and proteins with we breathe out as "exhaust".
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