Broadband dielectric relaxation studies were performed on D-glucose and 1,6-anhydro-D-glucose. In the liquid phase of both systems, one can observe the cluster relaxation and structural relaxation. In the glassy state of D-glucose two secondary relaxations were recorded. The slower one, hardly detectable in the loss spectra, was identified as the Johari-Goldstein type (JG) relaxation. The faster one, the gamma-relaxation, is visible as a well pronounced peak. For the past few years the origin of this process has been a subject of hot debate. Different authors have speculated about the source of this relaxation, but no consensus was reached. Moreover, application of more sophisticated method, such as NMR and MD simulations have not resolved this problem yet. Comparison of the dielectric loss spectra measured for D-glucose and 1,6-anhydro-D-glucose, combined with other experimental findings described in detail in this paper, enabled us to certify unquestionably, that the rotation of hydroxymethyl group is the origin of gamma-relaxation in D-glucose as well as in the whole family of saccharides. Additionally, calculations of conformational changes with use of density functional theory (DFT) were performed to support our identification.