Provided is a means for reducing the adverse effects of high glucose concentration on the living body. An agent containing N-acetyl-D-mannosamine for protecting a glucose-sensitive cell from glucose toxicity due to high glucose concentration； a pharmaceutical composition for protecting a glucose-sensitive cell from glucose toxicity due to high glucose concentration, containing an effective amount of N-acetyl-D-mannosamine and a pharmaceutically acceptable carrier； use of N-acetyl-D-mannosamine in producing a medicament for protecting a glucose-sensitive cell from glucose toxicity due to high glucose concentration； and a method of protecting a glucose-sensitive cell from glucose toxicity due to high glucose concentration, including administering an effective amount of N-acetyl-D-mannosamine to a subject in need thereof. The glucose-sensitive cell is selected from the group consisting of a nerve cell, a renal cell and a vascular endothelial cell.
