Preemptive apology: I know discussing MSG is tired and overdone, and I'm not trying to rehash the old arguments. Instead, I want to see if anyone can help me solve a dilemma that I'm very motivated to solve by figuring out how my understanding is incomplete/wrong. I have a few hypotheses that might explain how I might be mistaken, and I am hoping some of you will help me weed out the sense from the nonsense. Of course, it would be sweet if my current understanding is correct, because then I have an argument to include MSG in the paleo diet. Which would be sweet.
The dilemma: Virtually all research studies have shown that MSG in normal concentrations has no effect on the overwhelming majority of people, BUT a small but significant percentage of people (who I respect and believe) are very adamant about its negative effects. This contradiction really bothers me.
The Motivation: I really, really enjoy that savory MSG deliciousness (I keep it on the table in a third shaker...unless we have company and my wife hides it), and it would be so cool if was no more dangerous than table salt. It would be even cooler if it was paleo.
My understanding: The best argument I can think of against the dangers of MSG is basic protein digestion and the presence of glutamate in most animal protein. Protein digestion begins in the stomach and is completed in the first section of the small intestine (duodenum), after which the resulting amino acids are absorbed into the blood stream as they travel further down the small intestine. Of course there are all sorts of specific enzymes and other details we could look at, but the relevant point (I could quote numerous textbooks on this) is that nearly all proteins are completely broken up into their constituent amino acids and that only individual amino acids are actually absorbed.**
The glutamate molecule found in monosodium glutamate is exactly identical to a very common amino acid found in the vast majority of animal proteins.**^ Breaking down these proteins releases the identical molecule that is supposed to be so nasty in MSG. Because of this, I can't understand why those affected by MSG don't get the exact same symptoms from eating proteins containing glutamate. Even very small portions of nearly any animal protein contain many times more glutamate than is found in even the heaviest MSG seasoning. See why MSG is paleo (hopeful tone? If it's in all animal meat, what's wrong with adding a negligible amount in predigested form to our diet? How is MSG production any different than cooking meat (which partially predigests the protein) and adding salt? ...Or maybe I'm wrong for one of the following reasons.
Certain flavors have been shown to produce systemic effects independent of their actual consumption, e.g. sweet flavor and an insulin response. The umami flavor might produce analogous effects and some part of the resulting physiological cascade could be responsible for negative symptoms in a small percentage of people.
If proteins are not always completely hydrolyzed into their constituent amino acids, then it is possible that a high percentage of glutamate from digested protein might be absorbed in the form of small peptides.
Proteins aren’t completely broken down until the duodenum, so maybe free glutamate is absorbed earlier and differently. (I was taught that proteins were only absorbed in the small intestine, but it might be possible that the stomach lining contains amino acid transport proteins--at least substantial levels of mRNA coding for the LAT2 subunit have been measured: http://jp.physoc.org/content/558/2/597.full.pdf.)
The supposed effects of MSG might be caused by differences between the commercial protein hydrolysis used to manufacture MSG and the digestive hydrolysis of humans and not by glutamate at all.
Any thoughts? Should I invest in selling shakers by the trio?
*I have seen attempts by supplement companies to defend protein supplementation by refuting this. For example, claims that the angiogenic inhibiting proteins found in shark cartilage (http://www.encognitive.com/node/12668 –study performed by supplement company) are absorbed in polypeptide form, but I am unaware of any study to that effect in a reputable journal.
** In fact, the inability to completely break down certain proteins (e.g. gluten, casein) results in disease (e.g. celiac disease, cystic fibrosis) because of the intestine’s inability to absorb the resulting incompletely digested protein fragments.
**^Some people mistakenly distinguish between glutamate and glutamic acid, but they really are the same molecule plus or minus a hydrogen ion depending on the pH of their environment. When glutamate enters acidic environments of low pH, it forms a bond with a hydrogen ion becoming glutamic acid. Glutamic acid loses that same hydrogen in alkaline environments with a high pH.-