Insulin is a hormone released by the pancreas that has many functions in various metabolic processes, but is primarily involved in clearing excess glucose from the bloodstream. As discussed in Glucose 101,the body attempts to maintain blood glucose concentration within a tight range and when that limit is exceeded, insulin is released to clear the excess as rapidly as possible via multiple mechanisms:
Mechanism 1: Glucose uptake. Insulin binds to specific receptor zones (think of a lock and key) on tissue cells, signaling for the cell to bring glucose in, where it can be converted to energy. When insulin levels are low, most cells are unable to pull in glucose, forcing them to turn to other energy sources like fat. Note: Some liver and brain cells do not require insulin for glucose uptake.
Mechanism 2: Glycogen synthesis. When blood glucose is high, insulin signals for glucose to be converted into glycogen for storage in the liver and muscle. Since insulin enables glycogen production, it simultaneously inhibits glycogen breakdown into glucose; thus, while insulin levels are elevated, glycogen stores cannot be tapped into for energy.
Mechanism 3: Glucose accumulation as body fat. Another blood glucose clearance mechanism of insulin is the facilitation of glucose entry into adipose (fat) tissue cells. Within the adipose cell, glucose is broken into glycerol which is used to build triglycerides – a storage form of fat.
In summary, insulin pressures cells to use glucose for energy rather than fat, while contributing to the creation and storage of new body fat. Because of these features, insulin is often described as a “fat-sparing” hormone and is categorized as anabolic (involved in the building rather than breakdown of molecules). Under normal circumstances, as blood glucose levels fall, insulin concentrations will follow suit, enabling cells to switch over to glycogen or fat for energy.