This guide is designed to give you a general broad overview of how metabolism works, you can obviously go much further in-depth from here, however, for the purpose of this guide, we will keep things as succinct as possible.
Our bodies are always using energy. We require energy to do everything from locomotion, thinking, cellular turnover, digesting food, etc. We are in a constant state of either digesting and storing "energy" or expending energy. Generally, our bodies will use readily-available energy in the form of recently digested food. If, however, you haven't eaten in a while, then your body must derive energy from energy that has been stored.
The following organs and glands regulate our energy usage and storage:
Pancreas - insulin, glucagon
Pituitary Gland - growth hormone, adrenocorticotropic hormone (ACTH)
Adrenal Gland - epinephrine (adrenaline)
Thyroid Gland - thyroid hormone
One of the primary sources of energy for our bodies is glucose. In fact, some cells, such as our brain cells, get all of their energy from glucose. The unfortunate part about this is that too much circulating glucose in our bloodstreams is toxic. This is where the pancreas, and more importantly, the hormone insulin comes into play. Insulin works as a transport to remove excess glucose from the blood and store it in our muscles and adipose (fat) cells.
So, if you are eating consistently, especially foods high in carbohydrates, your body is most likely matching its energy demands through food that is being actively digested. If, on the other hand, you are either not eating, or are burning more energy than what is currently being circulated through the blood (such as when you exercise), then your body must produce energy from storage. Producing energy from storage is regulated through glucagon. You can consider glucagon to be the polar opposite of insulin. The goal of insulin is to lower blood glucose levels when they are too high, glucagon's goal is to raise blood glucose levels if they become too low.
Energy is stored in the muscles and liver in the form of glycogen. Glycogen must be broken down into glucose through a process called glycogenolysis to produce glucose. Glycogen located in the muscles that is broken down into glucose remains in the muscles. Glycogen broken down in the liver is transported through the bloodstream in order to be utilized where needed.
Energy is also stored in adipose tissue (fat cells). Lipase is activated by hormones such as glucagon, growth hormone, and epinephrine to break down fat into glycerol and fatty acids. Glycerol and fatty acids are transported to the liver via the bloodstream and can be used to produce glucose.
I offer Nutritional Coaching here at the Athlete Institute for our CrossFit Combine and General Fitness members. Contact me for further information on how you can maximize your diet, and stay tuned for more blogs detailing the intricacies of nutritional science!
Tyler Robbins B.Sc. CSCS
Directors of Fitness
Head of CrossFit