Provided herein are methods of measuring glycogen and methods of diagnosing a disease. One method of measuring includes separating sugar monomers and sugar phosphates using gas-chromatography, and analyzing the monomers and phosphates using mass spectrometry. Another method of measuring includes adding an isoamylase to a sample, the isoamylase cleaving glucose chains from glycogen; applying a matrix-assisted laser desorption ionization (MALDI) ionization matrix to the sample; and analyzing the samples using matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). The method of diagnosing a disease includes determining an amount and location of glycogen accumulation in a subject; and diagnosing a disease when over-accumulation of glycogen is determined.

The current invention is a method of preventing and treating certain diseases by preventing the synthesis and/or breakdown of glycogen by the administration of an agent, guaiacol. Diseases that can be prevented and treated by the administration of guaiacol include but are not limited to glycogen storage disease type IV (GSD-IV), adult polyglucosan body disease (APBD), and Lafora disease (LD). The invention also includes methods and tools for screening for agents that prevent and treat these diseases as well as basic research, specifically in the form of cells and cell lines that produce detectable polyglucosan.