Glucose is a carbohydrate found in most foods that your body uses for energy. Its levels in your blood are tightly regulated by hormones, including insulin and glucagon. All of your glucose is eventually filtered by your kidneys and reabsorbed into your blood by glucose-specific transporters. If you have too much glucose in your blood, it may not be reabsorbed effectively. This process occurs in uncontrolled diabetes mellitus, and it can have serious short-term and long-term effects.
What is Glucose?
Glucose is a simple carbohydrate that most of your body uses as its main source of energy, and your brain uses as its only source of energy. It is found in almost all of the foods you eat, whether as glucose itself, or as a more complex carbohydrate that is broken down into glucose in your intestines during the process of digestion and absorption.
Glucose in your blood is maintained within a very narrow range. Too little is known as hypoglycemia, a condition that can lead to coma and death. Too much, for too long a time, is known as diabetes mellitus, a condition that can cause damage to small blood vessels in your body, leading to heart, kidney, nervous system, and eye dysfunction. When too much glucose is in your blood, your body releases insulin, which causes glucose to be taken up and stored in your liver and muscle cells. When too little glucose is in your blood, your body releases glucagon, which causes your liver to release stored glucose, and make new glucose for release into your blood.
Kidney Filtration and Reabsorption
All of your blood is filtered by your kidneys to make urine. As a result of this filtration, most smaller molecules in your blood become part of the urine for a short while, and then, if they are necessary molecules, they are reabsorbed by your kidneys and returned to your blood. Glucose is one of these molecules -- all of the glucose in your blood is filtered by your kidney into a presumptive "urine" pool, but is then reabsorbed into your blood, so that your urine typically does not contain any glucose.
Glucose is reabsorbed in the kidney along with sodium in a type of transporter known as a "sodium-dependent glucose transporter." However, because there is a physical transporter required for glucose to be reabsorbed, it shouldn't be surprising that too much glucose can overwhelm the ability of this transporter to reabsorb glucose effectively. Practically, this usually happens when glucose concentrations in your blood are above about 300 milligrams of glucose per deciliter, at which point glucose starts appearing in your urine.
What if Glucose is in Your Urine?
The short-term effects of having glucose in your urine -- or glycosuria -- have to do with water balance. Glucose is able to pull free water with it into the urine. As a result, excess glucose in your urine can deplete the amount of fluid you've got in your blood vessels via a process called osmotic diuresis. This can cause fluid and electrolyte disturbances that may require hospital admission to correct. The long-term effect of having glucose in your urine is that glycosuria typically means you've got diabetes mellitus, a condition that can cause a significant decrease in life-expectancy if left untreated.