Protein Metabolism: The Demands of Exercise

Protein plays a key role in metabolism, nutrition, and muscle building. For this reason, individuals all across the fitness spectrum are looking into protein supplements like powders, bars, and even ice creams.

It seems that protein supplements and protein-rich foods are trending. Most of us would associate them with lean muscle or fitness. Vegans and vegetarians are also major contributors to this trend as their diets are often low in protein.

Whatever the reason may be, protein has a clear and undisputed role in human nutrition. As one of the four macromolecules of biology, protein is a major building block. It contributes not just to muscles, but cells, organs, and organ systems of all kinds. The immune system, for example, relies heavily on protein markers in the blood to detect antigens or foreign substances in the body.

Protein Metabolism

When consumed, protein is broken down into amino acids. Long chains of amino acids and amino acids complexes (known as beta-sheets or alpha-helices) make up what we refer to as proteins.

There are twenty common amino acids, nine of which are essential for human nutrition. Essential amino acids are named so because they are not synthesized in the body and can only be acquired through dietary means.

A danger of diets restrictive in animal products is malnutrition due to an amino acid deficiency. This is because most animal products represent complete sources of protein, that is, food containing all essential amino acids. Plant-based protein is an incomplete protein source. It therefore must be mixed with a variety of other food types to ensure that all essential aminos are being ingested. Together protein and amino acids represent the major building blocks for cells, tissue, and organ structures.

Protein as Fuel

At rest and during exercise carbohydrates and fats are the major source of energy for muscle contraction. Protein is not the preferred fuel of working muscles and the body will attempt to spare as much protein as possible. The role of protein is more defined during periods after a bout of exercise, as this will have the role of generating new muscle.

There are cases, however, in which the body will burn protein for energy. These cases are characterized by a lack of carbohydrates and/or fats for energy production and protein is burned. Ketogenic diets, for example, are based on a restriction of carbohydrates to induce a process known as ketosis. During ketosis, the body breaks fatty acids down into what is known as “ketone bodies” which can compensate for the absence of sugar, which has been depleted. In such a case, the protein may also be consumed to compensate for the lack of carbohydrates as a fuel source.

Muscle Protein (Nitrogen) Balance

To build lean muscle, one must be aware of muscle protein balance, or nitrogen balance. Muscle protein balance refers to the amount of protein being synthesized versus the amount broken down and consumed in skeletal muscle. One is able to measure this protein balance by comparing body nitrogen input versus nitrogen output since nitrogen is part of all amino acids.

Nitrogen Balance:

Nitrogen input equals output, most healthy adults are at nitrogen balance.

Positive Nitrogen Balance:

Nitrogen input exceeds nitrogen output. This points to nitrogen being used for protein synthesis and growth. Children and pregnant women are at positive nitrogen balance.

Negative Nitrogen Balance:

Nitrogen input is lower than output. This means after breakdown, the body uses protein as fuel. This can happen in the absence of carbohydrates and fats for fuel, like in starvation.

Resistance exercise can aid muscle protein balance to build more muscle. However, this is not possible in the absence of food (Tipton, 2001). During a time period of 24-48 hours after exercise, food consumed will determine protein balance. In the absence of food, it stays negative, suggesting more protein degradation than synthesis.

It is important to supplement with protein during the sensitive time period of protein balance to build muscle. With protein and carbohydrates present there is a greater stimulation of protein synthesis and a decrease in protein degradation in the muscle. Insulin released in response to carbohydrates halts the increase in the protein degrading, contributing to an increase in protein balance.

If you would like help figuring out what are the best foods and exercises to build lean muscle and maintain a healthy nitrogen balance, visit our team of Nutritionists and Personal Trainers at Santa Cruz Core Fitness + Rehab.

Sources:

Tipton, KD., RR Wolfe. Exercise, protein metabolism, and muscle growth. Int. J. Sport Nutr. Exerc. Metab. 11(1):109-132, 2001. https://www.ncbi.nlm.nih.gov/pubmed/11255140