Betaine HCL has a molecular weight of 153.61 g/mol, betaine anhydrous has a molecular weight of 117.148 g/mol. The difference is one hydrogen atom with a molar mass of roughly one and the bound chloride with the mass of 35.453 g/mol. The water in the stomach causes a protolysis of the hydrogen from the hydrochloride version of betaine, forming hydrochloric acid and betaine. Theoretically, 76.26% of the Betaine HCL should be available for absorption as betaine anhydrous (trimethylglycine).
Am I missing something? I don't see any reason why the remaining betaine shouldn't be equally absorbed (76.26% that is) as oral betaine anhydrous is. The carboxylate shouldn't react with the stomach acid any further. Neither should the methyl groups react with water or the hydrochloric acid.
Now the question is, will taking so much betaine throw off my methylation work, or will the body only use what it needs. I am currently doing the HCL challenge test, and I'm up to 10 x 700mg capsules at each meal, without feeling any heartburn. So for the moment, I'll be taking at least around 5000mg of betaine (based on 75%) twice a day.
Do you also take pepsin? Or vinegar? These are also acidifying without having betaine as a side product. In theory, you're taking a very high dose of betaine at the moment. This overdosing might hinder the folate cycle and the replenishment of cysteine.
The hurtburn challenge test isn't always reliable. You might not feel your hurtburn or you might just have insufficient fluid in it and we well working lower esophageal sphincter. Heartburn also depends on how much you eat and how much protein it contains.
I could also look into another HCL source. Creatine HCL might work, although I can't find how much of it is HCL vs Creatine.