I conjectured to a friend that some small amount of anything we smell is likely metabolized by the body. He disagreed. My thinking is that, if you are smelling something, some portion of molecules are being sensed by the olfactory tissue and some portion is taken into the lungs. Of the portion taken into the lungs, some portion is exhaled and some portion remains. I didn't propose a mechanism for the part that "remains" at the time, but it seems reasonable that some of it gets caught in mucus, some of which is then swallowed.

Obviously, a very, very small portion of whatever we're smelling would go through this process if my conjecture is even correct. For context, we were discussing people who have elected to restrict their diets for ethical or similar reasons. I was comparing what I proposed to when food products are labeled with "may contain trace amounts" due to "shared equipment".

So I guess my questions are these: was I right in my guess, and if so, was the mechanism correct or not?


1 Answer 1


Olfaction is the result of the neuronal impulse of ligand / receptor binding. To smell something requires 1) Specificity between the receptor and the ligand, 2) Concentration of the ligand molecule, and 3)The receptors that are on the surface of neurons in the olfactory bulb.

As for metabolize, then you need to be much more specific in defining what you are considering metabolism. Is the molecule broken down further; does it pass into the cells directly; does it need to be the unaltered molecule to qualify as metabolizing the molecule that you sensed as a smell.

The majority of our mucus in our airways is moved to the digestive tract as the result of ciliated epithelial cells "sweeping" the mucus and everything trapped in it to our esophagus, where it goes into the stomach and is digested. It is hard to say whether or not the molecules that you detect as smell are reactive, and if they are, whether or not they would chemically react with something else before it got to your stomach.

Also the volatile chemicals which you sense as an odor, aren't necessarily what would constitute the substance that they emanated from. So I would probably argue against your conjecture that someone who decides to be vegetarian or who has a peanut allergy is metabolizing something just because they inhaled it. It is likely the proteins in a peanut that cause an allergy, and that is not what you are inhaling, and as for the vegetarian, I think the justification is that it would be the protein that constitutes the consumption of a meat and not the molecules that come from the cooked or uncooked tissue.

If you want to broaden what you are calling metabolize, then you would only need to look at inhaled medicines such as corticosteroids or compounds to bronchodilate, to make a case for the fact that you are metabolizing what you inhale.

  • $\begingroup$ Thanks, @AMR. Can you clarify what you mean about volatile chemicals not constituting what they emanated from? Also, are you saying that the stuff we smell is non-nutrative and non-caloric? And since the precise context of the discussion was where an ethical vegan draws lines, can we define metabolism as breaking down the molecule and using its components? I apologize for any casual or incorrectly used language. I'm rather far afield of my discipline here. I appreciate the interest in such facile and impractical question. $\endgroup$
    – Becca
    Oct 9, 2015 at 18:01
  • $\begingroup$ What I meant by volatile chemicals are just chemicals that easily enter the gas phase are the molecules we are smelling. This article might clarify a bit, though if you are interested further I would suggest a Google Scholar search of Volatile Compounds in Food. en.wikipedia.org/wiki/Volatile_organic_compound To the extent they may not be used by the body as an energy source then you may or may not want to classify them as a undergoing metabolic processing. To the extent that they initiate a cellular response, i.e. the olfactory nerve impulse, then they do initiate a metabolic process. $\endgroup$
    – AMR
    Oct 9, 2015 at 19:10
  • $\begingroup$ I also think that an ethical vegan needs to draw the line at their choice of what they ingest and not inhale. If they are claiming that their right to be a vegan is being violated because they have to inhale compounds from animal products, then they are going overboard, in my opinion. And if they are arguing that they are vegan even if they inhale compounds from animals, then I would say that their argument is valid. The difference between what we inhale and what we ingest are very different things, which is why municipalities have a much easier time passing no smoking laws than soda taxes. $\endgroup$
    – AMR
    Oct 9, 2015 at 19:17
  • $\begingroup$ In the end, you are really defining the terms of the debate, my suggestion was that you clarify what your terms and conditions are. Like I said, inhaled medicines can even go as far as to turn on or turn off the transcription of genes in your cells, which is a metabolic process of the cell, so you need to specify the rules in order to get a definitive answer. $\endgroup$
    – AMR
    Oct 9, 2015 at 19:20
  • $\begingroup$ It sounds like, basically, the chemicals that enter the gas phase during cooking are VOCs and cannot be used as an energy source, though they might have other effects. So I'm going with I was wrong! This really was a hypothetical debate, since I wasn't suggesting people not breath in restaurants. Thanks again! $\endgroup$
    – Becca
    Oct 9, 2015 at 23:22

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