I understand that when the human body loses weight, the vast majority of that weight is lost as $CO_2$ (and a small bit is lost as water). I expect the predominant way $CO_2$ exits the body is through exhaling.

So, let's take a human with no influx of carbon (they are not eating); I expect such a person is slowly losing weight, a near infinitesimal amount with each breath (correct me if I'm wrong).

If the person breathes faster (more breaths per minute), that causes an increase in the rate at which $CO_2$ is expelled, correct (more mass per minute)? Is this rate expected to decrease, i.e., as one breathes faster, less $CO_2$ is contained in each subsequent breath? Or, does perhaps the body mobilize and oxidize carbon fuels (carbohydrates first and eventually lipids) to generate $CO_2$ in an attempt to restore homeostatic equilibrium?

Could breathing quickly and heavily possibly cause an increase in the $CO_2$ content of exhaled breaths? That is, would voluntary hyperventilation cause an increase in heart rate (and thus increase in carbon catabolism)? How much $CO_2$ is in a typical exhalation during exercise?

I realize that without exercise, the amount of $CO_2$ being lost during normal breathing is nearly negligible. But perhaps laying down and breathing quickly could cause a measurable increase in the net rate at which mass leaves the body. What do you think?

Thank you for your time.

  • 1
    $\begingroup$ Well, if you lay down and do situps & pushups, you will breathe faster and probably lose weight. (Unless you eat more to compensate for the calories burned.) Likewise if you stand up and walk faster, or run. But just breathing faster without some sort of exercise will most likely just cause you to hyperventilate and pass out. $\endgroup$ – jamesqf May 14 '17 at 18:38

The rate of CO2 production is normally set by the rate of metabolism and breathing rate is determined by the level of CO2 in the blood. What you are suggesting is known as hyperventilation. Because blood CO2 plays an important role in maintaining the pH of the blood, hyperventilation can lead to a dangerous increase in blood pH, referred to as respiratory alkalosis.

Apparently one response to respiratory alkalosis is an increase in metabolic rate as the body tries to compensate for the increased rate of loss of CO2. However, respiratory alkalosis is dangerous so I don't think hyperventilation is a viable strategy for losing weight.


There are two organs which govern pH: the lung and the kidney. If one is having some problem the other steps in to correct it. The body does not like pH to drift far from 7.4.

CO2 gas is blown off or retained by the lungs. More CO2 = more acidic. CO2 is in equilibrium with bicarbonate HCO3-. The kidney handles that. More HCO3- = more alkalinity.

Chronic respiratory acidosis is pretty common - people have bad lungs and they cannot get rid of CO2. If you have acute respiratory acidosis (like a heroin overdose, and you are not breathing enough) it is hard for the kidney to move quick enough to help. But for the chronic form you see the kidney holding on to a lot of HCO3- and not letting it go in the urine, to keep the pH up.

Chronic respiratory alkalosis is unusual but the kidney would do the same thing in reverse: dump bicarbonate to let the pH come down.

So in the short term you can screw up your pH and get rid of a trivial amount of calories but in the long term net CO2 loss will be the same.

A better way to exhale calories as CO2 is to produce more CO2. Your respiratory rate will automatically increase to get rid of it and keep pH normal. A good way to produce more CO2 is to oxidize fat and glucose molecules to produce ATP for your muscles.


Disclaimer: Not a doctor, a biologist, or a scientist.

This TEDx talk by Ruben Meerman explains the process of the mass loss via CO2 via exhaling.

The mathematics of weight loss | Ruben Meerman | TEDxQUT (edited version)

His explanation to increasing CO2 by-product of oxidizing fat was to increase O2 intake by raising your heart rate. This raises your respiration rate.

So increasing your O2 intake is one way. What about increasing your body's oxygen level as measured by an oximeter? And during exercise, does the oxygen level go down? How about during sleep? Would using an oxygen mask during exercise, or during sleep, increase your oxygen levels?


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