Protein Energy Yield and Muscle Fiber Mechanics

Protein as an Energy Source

Caloric Yield: Pound for pound, protein contains as much potential energy as a carbohydrate! (Actually, even a bit more: 4.3 vs. 4.1 kcal/g). Then why don’t we consume proteins for energy? Unlike carbohydrates and fats, the body has no protein reserves. All of the proteins in the body are part of pre-existing body tissue, such as skeletal muscle and organs. Proteins are composed of long chains of repeating subunits called amino acids.

Amino Acid Classification

Twenty different amino acids exist, and they are used in combination to create a variety of body tissues. These are categorized as follows:

• 9 Essential Amino Acids: These cannot be synthesized by the human body and must be obtained through diet.
• 11 Non-Essential Amino Acids: These can be produced by the body.

The Energy Backup Plan

In order for protein to be used as an energy source, it must first be broken down into its separate amino acid subunits. Alanine is the main amino acid used in the production of energy. In the liver, alanine is converted into glycogen and then sent through the bloodstream to working muscles as glucose. This is typically only used as an energy source during endurance events or when glycogen stores are severely diminished, acting as an energy backup plan.

Muscle Fiber Types and Recruitment

The body utilizes two to three different systems for producing energy depending on the type of activity and available nutrients. Muscles can be further subdivided based on their twitch (contraction) speed, and different fibers will be recruited based on the type of activity being performed.

Slow Twitch Muscle Fibers (Type I)

• Color: Red or dark.
• Features: These fibers slowly generate and relax tension. They can maintain a low level of tension during contraction for a long period of time.
• Enzymatic Profile: They have low myosin ATPase (the enzyme located in muscles that provides instant energy for muscle contraction). They also have low glycolytic enzymes, which permit the release of glycogen within muscles, but contain high levels of oxidative enzymes.
• Activity: Active during long-distance activities, such as swimming, cycling, and running.

Fast Twitch Muscle Fibers (Type II)

• Color: Pale.
• Features: These fibers quickly generate and relax tension. They can generate high amounts of tension with relatively low endurance levels (short periods of time).
• Enzymatic Profile: They have high levels of myosin ATPase and high glycolytic enzymes.
• Activity: They are active 3-5x faster than slow twitch fibers. These are utilized during fast, powerful muscle fiber contractions such as short sprints, powerlifting, and explosive jumping.

The Importance of Myoglobin

The main difference seen in muscle fiber types is primarily due to their reliance on oxygen for energy production. The more a muscle utilizes aerobic processes for energy production, the more it is able to sustain longer-term activity. Myoglobin is a red, iron-containing protein that delivers oxygen to working muscle cells, similar to hemoglobin. High levels of myoglobin allow for the sustenance of ATP production over long periods of time.

• Slow Twitch Muscles: Possess high levels of myoglobin.
• Fast Twitch Muscles: Possess low levels of myoglobin.