Glucose in excess in the bloodstream acts as a poison. One way by which it acts as a poison is when it begins to attach to the protein molecules of tissues in greater amounts. As an example, we find that glucose in normal amounts in the bloodstream, i.e. 80-140 mg/DL, is attached to 6% of hemoglobin (a protein of the red blood cell).
When glucose is in excessive amounts, i.e. greater than 200mb/DL, as is commonly seen in diabetes mellitus, a greater percentage (e.g. 10%) of glucose is attached to hemoglobin. This is commonly measured as HbA1. The attachment of glucose to protein molecules of tissue cells alters its function (metabolism). The cells become sick and it cannot readily release the oxygen that it carries to serve the needs of other tissues, for example, the retina of the eye. This relative lack of oxygen supply to the all important reflecting surface of the eye results in an overgrowth of new blood vessels that are prone to rupture and bleed. This is the common cause of blindness in the diabetic patient.
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