Blood sugar (i.e., glucose) regulation is an example of the constant exquisite coordination that produces everything from basic physiologic functions to elite athletic performance.
The human body is fantastically complex. Numerous body parts, organs, structures and even individual cells coordinate with each other to keep us alive. Each system strives to maintain an optimal range (i.e., homeostasis) despite varying internal and external signals. Blood sugar (i.e., glucose) regulation is an example of the constant exquisite coordination that produces everything from basic physiologic functions to elite athletic performance.
Every cell in the human body can use glucose as a direct fuel source to generate adenosine triphosphate (ATP), a high-energy molecule.
Some cells convert glucose into other energy forms, such as:
- Glycogen (many glucose molecules linked together), which is primarily made and stored in the liver and muscles.
- Fatty acids and ketone bodies. Both can be made in the liver. The adipose tissue makes fatty acids. Ketone bodies are released directly into the blood. Fatty acids, as lipids, can either be stored where they are made or released into the blood.
The protein hormone insulin regulates glucose homeostasis. Insulin is secreted from the beta cells of the pancreas into the blood in response to elevated blood glucose levels. The release generally results from eating and is most responsive to the ingestion of carbohydrate. For example, a meal composed only of carbohydrate can increase insulin levels from a low baseline to levels 10 times higher; a meal that is composed of only fat and protein might cause up to a twofold or threefold increase (2).
When the increased insulin is detected by insulin receptors on the surface of cells, it signals those cells to absorb glucose from the blood. As a result, the internal cell concentration of glucose is increased, and the glucose concentration in the blood is reduced. Within the cells, various responses can take place such as:
- Increased glycogen synthesis.
- Increased fatty acid synthesis.
- Increased protein synthesis.
- Increased cellular growth.
- Up-regulated glycolysis (the process of burning glucose anaerobically to make ATP).
- Decreased net breakdown of proteins, fatty acids and glycogen.
Insulin is one of the most powerful central regulators of metabolism. Its presence is a signal of energy sufficiency that drives energy storage (e.g., as glycogen or fatty acids) or begins energy-consuming processes such as cell division. The liver, adipose tissue and skeletal muscle are the predominant targets of insulin action (3). Bulk glucose uptake by the brain happens without insulin, although research suggests insulin affects the brain’s control of energy and glucose homeostasis (4,5).
When insulin is low (e.g., several hours without food), the opposite responses take place, such as the breakdown of glycogen and fatty acids. Other hormones—such as glucagon, somatostatin, cortisol, growth hormone and adrenaline—might inhibit insulin’s effects in various conditions. Insulin can therefore be considered a comprehensive fuel-selector switch: Its presence shifts the body toward carbohydrate use, and its absence shifts the body toward fat use.