Exercise benefits all tissues in your body, including your heart, blood vessels, muscles, bones, ligaments, tendons, immune system, intestines, liver, pancreas and brain. That’s right, even tissues that have nothing to do with performing exercise receive the benefit of frequent activity.
Exercise is a Beneficial Stress
Exercise is considered a “stress” to many organ systems, but differs from the negative stress of everyday life in that it stimulates the breakdown, repair and growth of muscles, ligaments, tendons and bones in the process of creating a stronger and more resilient body.
During exercise, muscles must perform two main tasks:
“Burn” available fuel for energy AND Contract in response to a rush of electrical signals from the brain
Muscle Fuel During Exercise
Muscle is capable of burning multiple fuels during exercise, including glucose (from carbohydrates), fatty acids (from fat) and amino acids (from protein). The type of fuel that is burned for energy depends on the intensity and duration of exercise being performed.
In the same way that a car stores fuel in a fuel tank, muscles have evolved the ability to store glucose, fatty acids and amino acids on-board. All three fuels are burned for energy in the mitochondria, organelles within muscle cells that function much like a car engine.
Glucose is Stored as Glycogen
Glucose is stored within each muscle cell as glycogen. Glycogen is a quick-burning fuel used during high intensity exercise.
Fatty Acids are Stored as Triglyceride
Fatty acids are stored within muscle cells as triglycerides. Triglycerides provide a secondary fuel source for low intensity exercise.
Amino Acids are Stored as Muscle Protein
Finally, amino acids are stored within the muscle tissue as muscle protein itself. Unlike glucose and fatty acids, there is no storage tank for amino acids in the muscle tissue. The muscle itself is the amino acid storage tank.
The Choice of Fuel Depends on Exercise Intensity
As you can see in the graph below, as the intensity of exercise increases, the dependence on carbohydrate goes up and the dependence on fatty acids goes down. This means your muscles will use your glucose storage tank to fuel your workout so you can eat more carbs!
At low intensities, fatty acids are the main fuel source and only small amounts of glycogen are broken down. As the intensity of exercise increases, larger amounts of glycogen are broken down and burned for energy, making glucose the predominant fuel source.
Amino acids from protein are the lowest priority fuel, given amino acids are the infrastructure of the muscle tissue itself. In order to preserve muscle mass, the muscle will burn glucose and fatty acids before resorting to amino acids. Brilliant design.
Even though amino acids from muscle protein are the last choice for fuel during exercise, microscopic tears result from repeated muscle contractions, called microtrauma. These microscopic tears are one of the signals that the muscle requires in order to repair during rest.
Think of microtrauma as the repeated wear-and-tear that your car experiences from driving long distances. In the same way that you replace damaged engine parts with newer and more efficient technology, microtrauma requires repair work immediately following exercise.
Recovery is a vital, often overlooked aspect of your workout regimen. It’s very important to let your muscles rest – and replace your “fuel” with the right foods.