Protein, a macronutrient, is not a favorable source of energy for the body during exercise. While protein, fat and carbohydrates can all be used as sources of energy for the body, it is never a single macronutrient contributing one-hundred percent of the energy; but rather a percentage of each are being used all of the time and these percentages change according to activity type, intensity and even duration.
The range of total energy expenditure contribution of protein during exercise is about 5% (strenuous weight training) or up to 15% (during prolonged endurance exercise). Increased breakdown of protein during exercise is observed usually only when muscle glycogen has been depleted by 33-55%. The purpose of this article will be to discuss the actual breakdown and digestion of protein for absorption and come to some conclusions about the consumption of proteins and different types of amino acids in the bodybuilding community.
To better understand the relevance of protein and amino acid intake, it will first be important to discuss the general digestion and absorption of protein itself. Digestion of all macronutrients, protein, carbohydrates and fats, all begin in the mouth with at least mastication. For protein, this mastication breaks down the quaternary structure (a larger arrangement of molecules) of that protein. Unlike carbohydrates, nothing happens enzymatically for protein in the mouth.
The protein then travels through the esophagus and then enters the stomach. Hydrochloric acid then begins to breakdown and unfold the 
polypeptide chains, thus breaking down the protein’s tertiary structure. While still in the stomach, pepsin, an enzyme, then breaks down the polypeptide chains into shorter chains, which then enter the small intestine. The enzyme protease then begins to once again break down these polypeptide chains into smaller chains. Peptidase, the final enzyme that acts on the chains, breaks them down into di & tri peptides and single amino acids. These amino acids can then be absorbed via active transport (which requires additional energy and sodium) or facilitated diffusion of the villi in the small intestine.
It is important to note that it is the single amino acids that are ultimately absorbed and used as the “functional unit” of protein. That being said, ingestion of single amino acids can bypass these enzymatic processes and breakdown steps and skip directly to absorption.
Since protein is a hotly debated topic within the bodybuilding community, whether it be about the type or amount of protein, it will be relevant to mention the supplementation of branched-chain amino acids (BCAA) as well as single amino acids.
This seems to be a highly controversial topic among “gurus”, trainers and coaches. The idea behind BCAA supplementation for bodybuilders is to minimize the catabolism of protein or muscle tissue, usually, during the workout, whether it be a weight training workout or cardio session (morning or post-workout). The important factor here will be the type and intensity of the training at hand.
Protein catabolism and oxidation (usage) of branched-chain amino acids really only increases significantly during prolonged endurance exercise due to the body’s attempt to spare carbohydrates, its favorite source of energy. While the use of BCAA’s intra-workout can arguably aid in the recovery aspect of the muscle tissue it is likely to be minimally or no more effective than the ingestion of quality protein and carbohydrates. In argument to the contrary, even if glycogen stores were depleted to the degree previously mentioned, thus influencing an increase in protein catabolism (or muscle tissue), it would then make sense to supplement with carbohydrate intra-workout or a combination of BCAA and carbohydrate if that is preferred.
However, with the ingestion of carbohydrates (the body’s more favorable source of energy unit), the amount of fat oxidation is likely to decrease. Therefore, the major point to consider with BCAA and carbohydrate supplementation is not only type and intensity of training but also the goals specific to yourself or your client, whether
they be fat loss or muscle gain oriented. As you can see here the beginning of the conflict of muscle gain and fat loss simultaneously is rather impossible.
Another interesting point of relevance concerning BCAA supplementation that I have heard, is consuming BCAA prior to morning fasted cardio. While the idea of morning, fasted cardio can be an entire debate itself, here we are concerned with the relevance of consuming BCAA prior to, or during this cardio session. The idea here is to, once again, preserve muscle tissue during the fasted session. It is important for arguments sake to say that the entire reason for doing the morning fasted cardio session in the first place is to maximize fat oxidation or fat burn.
By ingesting a BCAA supplement prior to or during this bout of exercise you can potentially “adjust” the substrate utilization of the exercise session and what was originally intended as your “fat burn” session of the morning has now just turned into a more minimal fat burn and usage of the BCAA’s for energy that you just ingested. Once again it should be made known that intensity, type and duration are all still factors here and this is not to say that one cannot benefit from BCAA supplementation.
As you can see there are many factors that play a role in substrate utilization during exercise and that the digestion and absorption of protein is simple yet complicated simultaneously. The major points to consider here are the intended goals of you or your client, whether they be fat loss or muscle gain. While the digestion and absorption of protein will never stray from the relative process outlined above, there are a wide variety of controversies that surround protein itself as a macronutrient including, but not limited to the topics discussed here.
Sources and Photo Credits
Williams, M. H., Anderson, D. E., & Rawson, E. S. (2013). Nutrition for health, fitness & sport. (Tenth Edition ed.). New York: McGraw-Hill.
http://en.wikipedia.org/wiki/File:Main_protein_structure_levels_en.svg
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