Why does polyuria occur with diabetes?

When a diabetic has very high levels of blood sugar, the kidneys become unable to filter and absorb the sugar adequately, according to Mayo Clinic. The body then excretes excess sugar and fluid to the urine, which causes frequent urination or polyuria.

The Merck Manual for Health Care Professionals defines polyuria as the excretion of more than 3 liters of urine per day. Uncontrolled diabetes mellitus is the most common cause of the polyuria in children and adults. If they experience polyuria lasting more than a few days, diabetics should see a physician, according to MedlinePlus. A physician may conduct a blood sugar test and an osmolality blood test.