I've heard that most of the weight you "lose" is actually carbon dioxide released when you exhale. In percent its much more than what is released in pure heat energy and feces. Does anyone have statistics on what's the split between where the energy leaves your body?

How many percentage of food weight is actually released through carbon dioxide when its burned in your body? For example when you eat a hamburger and fries, is all the carbon released later when you exhale? How much is heat energy? Provided it does not go into fat reserves.


3 Answers 3


Let's start with:

How much is heat energy?

2000 kilocalories per day, considering $E=mc^2$, is $9.3 \times 10^{-8}$ grams

is all the carbon released later when you exhale?

No. Brainstorming, the others ways would include: feces, urine, perspiration, flatulence, shedding skin, hair and nails, mucous from nose and mouth, for women: menstruation, vaginal discharge, lactation, and giving birth, and for men ejaculation.

According to The Value of Human Feces:

Per day the amount of carbon excreted in feces is 21 grams and in urine is 7 grams.

According to The Composition of Human Perspiration Proc R Soc Med. 1934 May; 27(7): 839–848, a typical amount of perspiration a day is 700mL, and per 100mL the main carbon containing compounds are 20 mg urea, 20 mg glucose and 5mg amino acids. So clearly less than 1 gram of carbon loss per day from perspiration. If we were considering loss of salts instead of just carbon, that would be about 3 grams per day from perspiration.

For menstruation, blood loss is about 40mL per cycle, and blood is 92% water, so maybe 1 gram of carbon on a heavy day of a period.

Skin loss per day is 1.5 grams according to Cells for life, cells for an instant.

A highly controlled study having a person in a chamber for 24 hours and analyzing the air found 653g of CO2 or 178g carbon "A Respiration Calorimeter with Appliances for the Direct Determination of Oxygen" Carnigie Institution of Washington Publication 42.

So, for a person that is not lactating or giving birth, I would estimate about 30 grams per day of carbon loss is other than from exhaling carbon dioxide. Therefore:

About 86% of carbon loss is through exhaling carbon dioxide.

However, this will certainly vary depending upon the amount of indigestible carbon such as fiber, in ones diet.

Also, if mass loss through nitrogen and phosphorous (but not hydrogen and oxygen) were also considered the loss through breathing would come down to about 80%. Inclusion of salts would bring this percentage down further.

The sum of water from feces, urine and respiration is about 1.8 kg per day and the total mass emission of the body is 2.6kg, according to the Carnigie source above.

  • $\begingroup$ You forgot to mention, that most of the weight is water, strictly speaking $\endgroup$
    – permeakra
    Mar 6, 2015 at 13:02
  • $\begingroup$ I guess I didn't say that directly, but that's why I said "not hydrogen and oxygen". I will edit to make it more clear, thanks. $\endgroup$
    – DavePhD
    Mar 6, 2015 at 13:43
  • $\begingroup$ Arguably, whatever leaves the intestines came directly from the stomach so faeces only contain the bits of the food we can’t do anything with and the living things from our gut that digest bits of what we can’t use. (At least according to my current knowledge according to which there is no significant transfer of mass from the bloodstream to the intestines.) $\endgroup$
    – Jan
    Dec 18, 2019 at 13:44
  • $\begingroup$ @Jan, bile enters downstream of the stomach, and the intestines themselves also emit secretions. $\endgroup$
    – DavePhD
    Dec 18, 2019 at 21:48
  • $\begingroup$ @DavePhD True that; it’s something I forget. $\endgroup$
    – Jan
    Dec 19, 2019 at 3:54

Yes, nearly all of the weight you "lose" when your body mass goes down is lost as $\ce{CO2}$.

Infintessimally small -- essentially zero -- weight is lost from "heat". We have Einstein's theory that $E=c^2m$, but if you plug in the numbers, the mass change from burning off a pound of fat is less than a microgram.

Your body could also lose weight as water, but in the long term, if you kept doing this, you'd dry up and die from dehydration.

The only way to lose the weight is to "burn off" fat by converting it to $\ce{CO2}$ and exhaling the $\ce{CO2}$.

A great article which goes through the math -- and which also shows that many dieticians and doctors do not understand this point! -- is this one: http://www.bmj.com/content/349/bmj.g7257

  • $\begingroup$ It's not appropriate to use E=mc^2 for a chemical change. $\endgroup$
    – jerepierre
    Mar 6, 2015 at 5:48
  • 2
    $\begingroup$ @jerepierre Why not? Combustion causes a net loss to the environment of potential energy stored in the chemical bonds. $E=mc^2$ always applies, regardless of whether the mass difference is easily measurable or not. $\endgroup$ Mar 6, 2015 at 12:25
  • 4
    $\begingroup$ @jerepierre "Nevertheless, when a chemical reaction emits energy to its surroundings, its reactants lose an equivalent quantity of mass in the process." pubs.acs.org/doi/abs/10.1021/ed082p1636 $\endgroup$
    – DavePhD
    Mar 6, 2015 at 12:26
  • $\begingroup$ Sorry, I should have read to the end of the paragraph. Unfortunately my vote is locked unless there's an edit to the answer. $\endgroup$
    – jerepierre
    Mar 6, 2015 at 14:55
  • 1
    $\begingroup$ @CurtF. I have 1 lb corresponds to 4000kcal = 16,736,000 J = 16,736,000 kg m^2/s^2. (16,736,000 kg m^2/s^2)/(300,000,000m/s)^2 = 1.86 e-10 kg = 1.86 e-7g = 186 nanograms $\endgroup$
    – DavePhD
    Mar 12, 2015 at 14:20

Sorry but E=mc2 is not applicable. Chemical reactions do not result in a change in mass of the components but a change only in the molecular combinations. Yes, the heat generated in oxidation comes from the potential energy of the chemical bonds so one would have to put a lot of energy into again separating the oxygen and carbon in CO2. A change in mass only occurs in nuclear reactions which also results in a change in the atoms of the elements involved to a new element.

  • 4
    $\begingroup$ Hello, welcome to Chem.SE! I'm afraid this answer is partially incorrect. Just because there are experimental difficulties in measuring such slight changes in mass, that doesn't mean relativity stops applying. Check out the article (or even just abstract) linked in one of DavePhD's comments. $\endgroup$ Dec 18, 2019 at 13:31

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