Topic Overview

The body uses three main nutrients to function— carbohydrate, protein, and fat.

These nutrients are digested into simpler compounds. Carbohydrates are used for energy (glucose). Fats are used for energy after they are broken into fatty acids. Protein can also be used for energy, but the first job is to help with making hormones, muscle, and other proteins.

After a meal, the blood sugar (glucose) level rises as carbohydrate is digested. This signals the beta cells of the pancreas to release insulin into the bloodstream. Insulin helps glucose enter the body's cells to be used for energy. If all the glucose is not needed for energy, some of it is stored in fat cells and in the liver as glycogen. As sugar moves from the blood to the cells, the blood glucose level returns to a normal between-meal range.

Several hormones and processes help regulate the blood sugar level and keep it within a certain range (4.0 mmol/L to 7.0 mmol/L). When the blood sugar level falls below that range, which may happen between meals, the body has at least three ways of reacting:

• Cells in the pancreas can release glucagon, a hormone that signals the body to produce glucose from glycogen in the muscles and liver and release it into the blood.
• When glycogen is used up, muscle protein is broken down into amino acids. The liver uses amino acids to create glucose through biochemical reactions (gluconeogenesis).
• Fat stores can be used for energy, forming ketones.

Other hormones can raise the blood sugar level, including epinephrine (also called adrenaline) and cortisol released by the adrenal glands and growth hormone released by the pituitary gland.