Since diabetes is a disease that affects your body's ability to use glucose, let's start by looking at what glucose is and how your body controls it. Glucose is a simple sugar that provides energy to all of the cells in your body. The cells take in glucose from the blood and break it down for energy (some cells, like brain cells and red blood cells, rely solely on glucose for fuel). The glucose in the blood comes from the food that you eat.
When you eat food, glucose gets absorbed from your intestines and distributed by the bloodstream to all of the cells in your body. Your body tries to keep a constant supply of glucose for your cells by maintaining a constant glucose concentration in your blood -- otherwise, your cells would have more than enough glucose right after a meal and starve in between meals and overnight. So, when you have an oversupply of glucose, your body stores the excess in the liver and muscles by making glycogen, long chains of glucose. When glucose is in short supply, your body mobilizes glucose from stored glycogen and/or stimulates you to eat food. The key is to maintain a constant blood-glucose level.
To maintain a constant blood-glucose level, your body relies on two hormones produced in the pancreas that have opposite actions: insulin and glucagon.
Insulin is made and secreted by the beta cells of the pancreatic islets, small islands of endocrine cells in the pancreas. Insulin is a protein hormone that contains 51 amino acids. Insulin is required by almost all of the body's cells, but its major targets are liver cells, fat cells and muscle cells. For these cells, insulin does the following:
- Stimulates liver and muscle cells to store glucose in glycogen
- Stimulates fat cells to form fats from fatty acids and glycerol
- Stimulates liver and muscle cells to make proteins from amino acids
- Inhibits the liver and kidney cells from making glucose from intermediate compounds of metabolic pathways (gluconeogenesis)
As such, insulin stores nutrients right after a meal by reducing the concentrations of glucose, fatty acids and amino acids in the bloodstream.
See the next page to learn about glucagon.