A healthy person should have a fasting blood sugar level below 100 milligrams per deciliter when fasting, and below 140 milligrams per deciliter two hours after eating, according to WebMD. If fasting blood sugar level is above 126 milligrams per deciliter, a patient is said to have diabetes.
Fasting blood sugar levels are usually measured six to eight hours after a person's last meal, according to Edward S. Horton, MD. The normal range for this measurement is between 70 and 100 milligrams per deciliter. After eating, blood sugar rises, but usually not above 140 milligrams per deciliter in a healthy patient. When blood sugar level rises above 180 or 200, the kidneys are unable to reabsorb sugar quickly enough and the patient begins excreting it in the urine. At very high blood sugar levels above 400 or 500, patients experience diminished mental function.
It is rare for blood sugar levels to be too low if a person is not taking diabetes medications, states WebMD. If blood sugar drops below 60, many patients experience symptoms such as shakiness, feelings of hunger and a racing heart, explains Edward S. Horton, MD. Eating or drinking causes the blood sugar to rise, and these symptoms to disappear quite promptly. At blood sugar levels below 50 milligrams per deciliter, patients may progressively lose mental function and experience seizures.