How Much is Enough? Synthesis of Muscle Protein and Nitrogen Balance as an Indicator of Adequate Consumption

When you consume protein, you are supplying your body with the amino acids and nitrogen to create structural cells, enzymes to carry out essential chemical reactions, hemoglobin to transport oxygen through the blood, hormones to regulate reactions, and non-protein nitrogen compounds needed for growth and maintenance of the body (nitrogen and amino groups in those amino acids are also used to make molecules such as nucleic acids, hormones, ATP, and

other bio-molecules). Protein contributes to the maintenance of the human body in a vast number of ways, and it is essential for many critical metabolic processes in the body. Because protein synthesis is necessary to carry out fundamental bodily processes, building muscle is among the least prioritized functions of protein metabolism. Consequently, it is necessary to induce a protein surplus in order to produce conditions where muscular growth is possible.

Nitrogen balance is the measure of nitrogen input minus nitrogen output. Unlike other macronutrients (fats and carbohydrates), proteins contain nitrogen in addition to carbon, hydrogen, and oxygen. Nitrogen typically enters the body via proteins, and leaves it mainly in the form of urea. So since most of our nitrogen intake comes from protein, measuring the loss of nitrogen (through tracers containing the rare but stable isotope nitrogen-15) can tell something about protein metabolism and synthesis. Because nitrogen is practically exclusive to proteins (In fact about 95% of bodily nitrogen can be linked to protein), nitrogen content in the body serves as a general indicator of adequate protein consumption:

Nitrogen Balance

  • A negative nitrogen balance indicates a dietary protein deficiency. When nitrogen excretion exceeds nitrogen consumption, there is implied predominance of catabolic processes in the body — your body is breaking down proteins in the body to meet its metabolic requirements, thereby releasing the extra nitrogen which is then excreted. Your body has to break down the proteins it already has because it is not receiving enough dietary protein to carry out metabolic processes. In this case the body would be diminished because it is losing protein to carry out its metabolic processes.

  • An equilibrium in nitrogen balance indicates sufficient protein consumption to maintain the body’s current status. When net nitrogen consumption is equal to net nitrogen excretion, anabolic and catabolic processes are occurring at roughly the same rate. The net amount of protein synthesized is for structural and functional use is equal to the amount of protein broken down. In this case the body would not grow or diminish, it would remain the same.

  • A positive nitrogen balance indicates that there is a protein surplus in the body. A nitrogen surplus indicates that your body is retaining more protein than it is excreting, and that there is excess protein after all metabolic requirements. This surplus is a prerequisite (but not necessarily a guarantee) for a predominance of anabolic processes in the body. A positive nitrogen balance, therefore is necessary for growth — the synthesis of more proteins than necessary to sustain the body at its current state.

  • For more information, check out this informative video

A positive nitrogen balance is an important consideration for anyone looking to build muscle mass because it is a strong indicator of protein surplus, which is a prerequisite for muscular growth. However, the issue of protein synthesis for muscle growth (or any type of protein) is dependent on having all of the proper amino acid precursors and specific enzymes to synthesize that specific protein. A study published by The Journal of Physiology states that the rate of muscle protein synthesis is directly increased through large extracellular doses of essential amino acids (those which your body can not produce) whereas they are not affected by doses of non-essential amino acids, although all 20 amino acids are required for protein synthesis. How does the amount of available amino acids affect the rate or extent of protein synthesis?

Amino Acid Concentration and Rate of Muscle Protein Synthesis

One contributing factor is the concentration of amino acids and other precursors to protein synthesis. Although complex, protein synthesis is still a chemical reaction which can be analyzed using kinetics. This study suggests that the rate of the amino acid utilization in muscle protein synthesis increases as their intracellular concentrations increase. This study demonstrates that the rate of protein synthesis can be described as a result of varying concentrations of precursors to the reaction. An increase in reactant concentration yields and increase in the rate of the reaction. However, it would be misleading, and a drastic oversimplification to assume that kinetic considerations are the only (or primary factor) that guide the rate of protein synthesis.

Protein synthesis is catalyzed by enzymes, which greatly increase the rate of protein synthesis reactions by reducing the necessary activation energy of the reaction. The body is a complex chemical system that has various responses to various stimulus. BCAAs (Branched Chain Amino Acids), a specific categorization of amino acids, are shown increase anabolism of protein synthesis by both increasing the rate of protein synthesis, but also decreasing the rate of protein degradation. A study published by the Journal of Nutrition states that BCAAs have a significant impact on protein synthesis by activating key enzymes, but also attributes increased muscular growth to other external stimulus, specifically intramuscular signalling, and exercise. This study suggests that the rate of protein synthesis is affected by numerous factors. For more information you can read the study here.

Protein is very crucial to many metabolic processes in the body, and contributes heavily to the growth of muscle. The amount of protein necessary to induce muscle growth can be considered to be enough so that a positive nitrogen balance is produced in the body. As proven in various studies, the rate of protein synthesis increases as the concentration of amino acids increases. This follows kinetic chemistry, as an increased concentration of reactants yields a higher concentration of products. However, protein synthesis is a very complex process involving a lot of parts, and ultimately can not be simplified to such basic terms. In order to synthesize muscle proteins it is imperative that all necessary precursors are available in the body, and the rate of synthesis is heavily influenced by enzymatic activity as well as other factors.


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