Histidine HCl is simply histidine, an acid native to the stomach that normally helps to promote digestion, with a bit of HCl added to help balance out the pH. Histidine is an essential amino acid and needs to be taken regularly to help with digestion and to maintain digestive health. Because taking more than the average amount of histidine may sometimes result in a deflation in the amount of zinc in the body, it is suggested that zinc supplements be taken in tandem with any histidine supplements. Histidine is one of the top most common of all of the amino acids and can be found in great concentrations within the skeletal muscle.