Question: Could an elevated BUN indicate protein malabsorption and low stomach acid?
I get the Heidelberg test. That's the only accurate way to assess stomach acid. If you want to do something else, it would be better to use the kitchen techniques, like take half a teaspoon of baking soda and see how long you take to burp, or take HCl with your meal and keep adding capsules and see how many capsules you can take without reflux. That's probably both more accurate than using BUN.
I find it almost certainly the case that a slightly high BUN would never be a useful marker of low stomach acid and would never be a good marker of poor protein digestion. If you want to know if you have poor protein digestion, measure the protein in your stool. Get a GI stool test that looks at what you're not absorbing. That's how you test that.
The reason that this sounds nuts to me is maybe you are allowing the protein to ferment in your gut and generate urea from the microbes that you're absorbing, like maybe. But where does most of the urea come from that's in your blood? It comes from the urea cycle, which is how you get rid of ammonia. How do you get ammonia in your body that goes into the urea cycle? You digest protein into amino acids, you absorb the amino acids, and then you break them down so that you can either burn them for energy or turn them into glucose or turn them into certain neurotransmitters or whatever, and then you lose ammonia that you put into the urea cycle.
Why wouldn't the urea be a marker of having good digestion? I'm not even sure that we could say it could be an equally useful test of good digestion and bad digestion of protein. I don't know if it's as good a marker of bad digestion of protein as it is of good digestion of protein. But even if it were just as good a marker of bad digestion of protein as it is of good digestion of protein, something that's an equally good marker of two opposites is not a good marker of anything.
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