Dear 100 Hour Board Of The Strong Gag Reflex,
In the tradition of Board Question #65283 I ask you another source about the same magical substance that brings us gummi bears--yea, mucus.
I learned that mucus is composed of glycoproteins and water. If it indeed contains proteins, then does phlegm have any caloric/nutritional value? What sorts of chemical changes might have to occur to allow these proteins to be broken down by gastric acids?
What attribute or substance might these dried mucus solids contain that cause them to be consumed by toddlers, childrens, and--yes, sadly, many adults--the world over?
Needless to say, I don't think I'll get rid of my food storage any time soon.
Yes, the proteins in mucus are completely digestible, and they do contribute to nutrition, although rather minutely. Estimating the caloric value of mucus is a bit tricky, since its composition varies so much depending on things like genetics, environmental conditions, the body's immune state, and hydration status. Additionally, there are other components of mucus besides just glycoproteins and water; in a healthy individual, there are also proteoglycans (which are actually different from glycoproteins, believe it or not—isn't biology wonderful?), free proteins, dissolved ions, and sometimes lipids.
The 1978 British Medical Bulletin article, "Constituents of Mucus and Their Separation," reports that typical mucus contains, by weight, 0.5-1% glycoprotein and 0.5-1% free protein. So in 1 gram of mucus (that's pretty small—think a 1 cm x 1 cm x 1 cm cube), there would be about 0.01-0.02 grams of protein and glycoprotein. Proteins and carbohydrates both clock in at 4 calories/gram, which would bring the total up to 0.04-0.08 calories for that particular amount.
Now, if your body is making mucus and then recycling it again by swallowing and digesting it, that's not adding any new calories to the system, so it doesn't make sense to calculate something like how many calories of mucus are swallowed per day. I do think it's fair to say, however, that every time you blow your nose, you probably cost your body up to a tenth of a calorie in proteins that it could have recycled but now doesn't get to. Blowing your nose would be a terrible diet strategy, but it does have a tiny effect on overall nutrition.
And now, regarding your stomach acid question: proteins (and glycoproteins) are actually digested in several steps. The first step, being exposed to stomach acid, mostly just opens up the protein structure so that later, enzymes in the small intestine will be able to get to the entire protein and chop it up into small enough pieces to absorb. I don't think that mucin glycoproteins would react substantially different in stomach acid than other dietary proteins. As far as I know, the main difference in digestion between glycoproteins and straight proteins is that glycoproteins (since they have a carbohydrate component from the added sugars) require both sugar enzymes (glycosidases) and protein enzymes (proteases) in the small intestine before they can be absorbed.
And why do kids eat their boogers? I think it's just because they're salty and readily available. To the undiscerning toddler's palate, it's like having a snack factory in your own face. Charming, right?