Glucose is a type of sugar your body uses for energy. It comes from the Greek word for “sweet.” Blood glucose or blood sugar travels through your bloodstream to your cells.
A hormone called insulin transports glucose from the blood into the cells for energy and storage. Diabetes patients have higher levels of glucose in their blood than normal. It’s either because they don’t have enough insulin or because their cells don’t respond well to insulin.
Having high blood glucose for an extended period of time can damage your kidneys, eyes, and other organs.
When glucose levels drop, glucose can be produced from several other compounds in a process called gluconeogenesis. Any compound that can be converted to pyruvate can be used to make glucose. Any compound that has been converted to acetyl CoA cannot be used to make glucose.
Triglycerides and Glucose Production
Triglycerides (the primary form of fat in the body) consist of three fatty acids and glycerol. Because fatty acids break down to acetyl CoA, they cannot be used to make glucose. The glycerol portion of a triglyceride, however, can be converted to pyruvate and thus can yield glucose.
Because glycerol represents only about 5 percent of the weight of a triglyceride molecule, about 95 percent of a triglyceride cannot be converted to glucose at all. Thus, fat is an inefficient source of glucose. The task of serving as a glucose source is left to amino acids, obtained by breaking down the body’s proteins.
Amino Acids and Glucose Production
The primary role of amino acids is to maintain supplies of needed body proteins. If amino acids are needed for energy or if they are consumed in excess, they first undergo deamination, a reaction in which they are stripped of their nitrogen. The nitrogen can be used to make other compounds, including the nonessential amino acids, or it can be excreted.
The principal nitrogen-excretion product of metabolism is urea. With nitrogen removed, most of the amino acids can be converted to pyruvate and can therefore provide glucose. Several of the amino acids can only be converted to acetyl CoA and therefore cannot supply glucose. Thus protein, unlike fat, is a fairly efficient source of glucose when carbohydrate is not available.
Regulation of Blood Glucose
If blood glucose falls below normal, a person may become dizzy and weak; if it rises substantially above normal, the person may become fatigued. Diabetes is a disorder characterized by elevated blood glucose. Left untreated, fluctuations to the extremes—either high or low—can be fatal. Blood glucose homeostasis is regulated primarily by two hormones: insulin, which moves glucose from the blood into the cells, and glucagon, which brings glucose out of storage when blood glucose falls (as occurs between meals).
After a meal, the pancreas is the first organ to respond as blood glucose rises. It releases the hormone insulin, which signals body tissues to take up surplus glucose. Muscle tissue responds to insulin by taking up excess blood glucose and using it to make glycogen. The liver takes up excess blood glucose, too, but it needs no help from insulin to do so. Instead, the liver cells respond to insulin by speeding up their glycogen production. Adipose (fat) tissue also responds to insulin by both taking up blood glucose and slowing its release of the fat stored within its cells. Simply put, insulin regulates blood glucose by:
- Facilitating blood glucose uptake by the muscles and adipose tissue.
- Stimulating glycogen synthesis in the liver.
The muscles hoard two-thirds of the body’s total glycogen to ensure that glucose, a critical fuel for physical activity, is available for muscular work. The brain stores a tiny fraction of the total, thought to provide an emergency glucose reserve sufficient to fuel the brain for an hour or two in severe glucose deprivation.
The liver stores the remainder and is generous with its glycogen, making it available as blood glucose for the brain or other tissues when the supply runs low. Without carbohydrate from food to replenish it, the liver glycogen stores can be depleted in less than a day.
Balanced meals and snacks, eaten on a regular schedule, help the body maintain blood glucose. Meals with starch and soluble fiber combined with some protein and a little fat slow digestion so that glucose enters the blood gradually at an ongoing, steady rate.
The Release of Glucose from Glycogen
The glycogen molecule is highly branched with hundreds of ends bristling from each molecule’s surface. When blood glucose starts to fall too low, the hormone glucagon is released into the bloodstream and triggers the breakdown of liver glycogen to single glucose molecules. Enzymes in liver cells respond to glucagon by attacking a multitude of glycogen ends simultaneously to release a surge of glucose into the blood for use by all the body’s cells. Thus, the highly branched structure of glycogen uniquely suits the purpose of releasing glucose on demand.
Glucose can be produced from compounds that can be converted to pyruvate, including glycerol and most amino acids. Fatty acids can be used only for energy and cannot make glucose.
Blood glucose concentrations are regulated primarily by two hormones: insulin, which moves glucose from the blood into the cells, and glucagon, which brings glucose out of storage when blood glucose falls.