Where are -HCl supplements absorbed in the human digestive system?
The hydrochloride ions are not what facilitate their absorption into the body. The HCl notation is from organic chemical reactions of a weak base (usually amines are involved here) with HCl to form a salt. The salt is what gets purified as a powder (otherwise the chemical remains in solution and can't be made into a pill/supplement). The bottle may say: "compound-HCl" or "compound HCl" implying that the two are chemically bonded together, but that is incorrect. What actually happens is that the compound is an ionic salt, in the form [compound]H+ Cl-. As far as absorption, that is entirely dependent on the compound(s) being absorbed. Once you swallow them, they dissociate back into the weak acid form and H+,Cl- ions.