Simply because fatty acids, amino acids and nucleic acids are acids that produce hydrogen +. In chemistry definitions, acids are substance that tastes sour that donates hydronium ions in a water solution. An acid is also a compound of substance that produce proton and embrace electrons. A proton according to science is something that has hydrogen but doesn't have electron. That's why when these acids are combined they increase the number of hydrogen + or (H+) in a certain substance.