Never heard of aldose reductase? Here’s a primer: Aldose reductase is the enzyme that converts glucose to sorbitol. Aldose reductase activity increases in the body as the glucose concentration rises, so in diabetics with high blood sugar levels, more and more sorbitol gets produced. Sorbitol, while good in small amounts, is bad when overproduced and contributes to many of the complications of diabetes, especially kidney disease (nephropathy), nerve pain (neuropathy), and loss of vision (retinopathy).