In diabetes, excess glucose is metabolized via the polyol pathway, aldose reductase (AR) reduces glucose to sorbitol with the help of nicotinamide adenine dinucleotide phosphate (NADPH), and then sorbitol is oxidized to fructose by sorbitol dehydrogenase in a process involving nicotinamide adenine dinucleotide (NAD+). This evidence concerns the gene AR and diabetes mellitus.