Diabetes mellitus causes polydipsia, or excessive thirst, because of high blood sugar. According to the Mayo Clinic, the kidneys remove excess sugar from the blood, which results in higher urine production. As the body loses water through increased urination, or polyuria, this triggers thirst, and the diabetic drinks more water.
Diabetes mellitus is a disease characterized by abnormally high blood sugar. As explained by the U.S. National Library of Medicine's Medline Plus, high blood sugar results because the pancreas fails to produce insulin or because the body develops a resistance to insulin. Symptoms of diabetes include polydipsia, polyuria, fatigue, hunger, blurred vision and weight loss.
According to Diabetes.co.uk, polydipsia usually occurs together with polyuria. In diabetes mellitus, they are interrelated. The high blood sugar is the key. As explained by the Mayo Clinic, sugar draws water into the blood, and the kidneys try to remove the excess sugar in the blood by making more urine. The body's cells become dehydrated from this triggered loss of water, which causes thirst. Although the diabetic drinks more water to quench the thirst, the water that he drinks is quickly excreted as urine in order to expel the excess blood sugar. However, the cells are still lacking sufficient water, which again signals thirst. Therefore, the diabetic remains thirsty even after drinking plenty of water.