The present disclosure describes improved adeno-associated virus (AAV) vectors for gene therapy applications in the treatment of glycogen storage disease, particularly glycogen storage disease type Ia (GSD-Ia). Described are recombinant nucleic acid molecules, vectors and recombinant AAV that include a G6PC promoter/enhancer, a synthetic intron, a G6PC coding sequence (such as a wild-type or codon-optimized G6PC coding sequence), and stuffer nucleic acid sequence situated between the G6PC promoter/enhancer and the intron, as well as between the intron and the G6PC coding sequence. The recombinant AAVs disclosed herein exhibit highly efficient liver transduction and are capable of correcting metabolic abnormalities in an animal model of GSD-Ia.