BioChemistry, Nutrition

Amino Acids

Amino Acids

Amino acids are essential to the body in the roles they play as both the building blocks of proteins and as intermediaries essential for metabolism. There are 20 amino acids present in the structure of proteins. Each amino acid varies in the chemical effect it has on the metabolic processes within the body. Each protein has a unique characterization and chemical property, which is determined by the sequence of amino acids. The specificity of the amino acid content and the sequence is determined by the gene that encodes that particular protein.

The 10 amino acids produced by the body are: alanine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine and tyrosine. The other 10 amino acids are essential amino acids: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. These amino acids must be ingested because the body does not possess the required enzymes for their biosynthesis.

Branched-chain Amino Acids

There are three branched-chain amino acids (BCAAs), isoleucine, leucine, and valine. These essential amino acids contribute to roughly 35 percent of the amino acids found in muscle tissue. During exercise, these BCAAs can be metabolized in the muscle tissue as a source of fuel.

Nitrogen Balance-The BCAAs are very important for nitrogen balance. When the body undergoes any degree of stress, whether it be from exercise or injury, it relies more heavily on these specific amino acids, as they help maintain sufficient proportions of nitrogen within the muscle cells. When the amount of nitrogen in the muscle tissue is sufficient, the muscle is in a positive environment and in an ideal situation for growth. On the other hand, when the amount of nitrogen in the muscle is inadequate, the muscle tissue is in a negative environment. The possibility for growth becomes nonexistent and the likelihood of muscle cannibalism increases. Amino acids contain a structural component of nitrogen. Measuring the amount of nitrogen consumed and comparing it to the amount of nitrogen excreted, enables one to determine nitrogen balance. When the amount of nitrogen ingested is equal to the amount of nitrogen excreted, nitrogen balance is obtained. A balanced system indicates a sufficient supply of protein, available for growth and repair. Ample quantities of protein are essential for muscle growth and repair, as well as, maintenance.

Athletes and bodybuilders have found increased muscle growth and strength when supplementing their diets with BCAAs. It has been found that during low-intensity, high-duration exercise, the body utilizes roughly 90 percent of its total daily leucine content as a source of energy. In addition, it is recommended that the macronutrient ratio for protein be increased during training.

Essential Amino Acids

The human body has the capability to produce only 10 of the 20 amino acids found in proteins. The remaining 10 amino acids must be ingested.  Each essential amino acid must be present in the diet, and is necessary, to avoid any degradation of the body’s proteins. If one amino acid is absent, the body must compensate by cannibalizing its own muscle to produce that particular amino acid. Unlike fat or starch, the human body does not store excess amino acids for later use. Essential amino acids must be consumed on a daily basis.


The most prudent role of arginine is in it’s ability to stimulate the release of growth hormone. It has been found that an increased level of GH can assist the body in fat reduction, increase muscle mass and improve recovery and healing. Arginine also plays an important role in creatine synthesis. In that role, arginine acts as a precursor to creatine. Creatine is valuable in its energy role during anaerobic activities, such as weight lifting and sprinting. 

Lastly, arginine aids the body in liver detoxification.  It functions as an enzyme in the urea cycle, by which it converts ammonia to urea to be excreted in urine. Because ammonia is toxic, the more one exercises, the more ammonia becomes an issue because exercise increases ammonia levels within the body. 


Histidine, like arginine, is a conditionally essential amino acid. It is essential for infants and athletes, but not for most adults.  Its major function is in the growth and repair of tissue.  It has a major role in digestion and the production of gastric juices. It is also important in the formation of red blood cells and white blood cells. 


Isoleucine is an integral part of muscle tissue. As a source of energy, it is metabolized within the muscle tissue during exercise. It is essential in the formation of hemoglobin and is involved in the process of regulating blood sugar levels.


Leucine is the primary amino acid utilized for energy. Leucine is also found in higher concentrations in muscle, during periods of rest. Studies suggest that roughly 90 percent of ingested leucine is metabolized for energy during exercise, making it particularly important to endurance athletes. Some studies indicate leucine may stimulate the release of insulin and may, indirectly, be involved in protein synthesis.


Lysine is  found in high concentrations within the muscle tissue and is required for proper growth and development. It is also needed for the repair of connective tissue. In cases where the amount of lysine in the muscle tissue is inadequate, protein synthesis, growth and repair become restricted. Lysine plays a necessary role in the proper development of bone tissue, and is needed for the formation of collagen and certain enzymes.


Methionine, a sulfur-containing essential amino acid, is necessary for proper growth and development. It is involved in the process of transmethylation, whereby enzymatic compounds (required in muscle activation and contraction) are formed. It is also important in the removal of waste products from the liver. It assists in the breakdown of fats, and prevents build-up of fat in the liver and arteries. It functions in the digestive system and aids the body during muscle weakness. 

Methionine is also essential for the synthesis of choline (the brain food) and lecithin. Because the body cannot produce methionine, it is recommended that a diet be supplemented with choline or lecithin. This additional supplementation helps preserve methionine.


Phenylalanine is responsible for a variety of different  functions within the body. It is highly involved in appetite control, whereby it inhibits the body’s desire for food by increasing the brain’s production of norepinephrine and cholecystokinin (CCK). CCK is believed to “trigger” the brain when adequate food has been ingested. Without phenylalanine, the body cannot produce the neurotransmitters epinephrine and norepinephrine, which are vital for memory. These neurotransmitters are also important for tissue growth and repair, making phenylalanine an important amino acid for athletes. In addition to being a precursor to the neurotransmitters epinephrine and norepinephrine, other essential metabolites are derived from this amino acid as well. Lastly, phenylalanine is a precursor to tyrosine, one of the non-essential amino acids. 


Threonine plays an important role in the prevention of fatty acid build-up. Its primary function is in the formation of collagen and elastin.


The primary function of tryptophan is its supporting role of B vitamins. For athletes, tryptophan can be beneficial in its involvement in increasing GH levels. It has been found that when ingested 24 hours prior to exercise, cardiovascular capacity and endurance can be increased. Athletes have also found greater benefits of L-Tryptophan when combined with vitamins B3, B6 and magnesium.  

In 1989, the FDA placed a ban on tryptophan when it was believed to be linked to a rare blood disorder, Eosinophilia Myalgia Syndrome. Unfortunately, the ban was premature. The FDA has now partially unblocked its stance on the prohibition of the sale of L-Tryptophan. L-Tryptophan can now be sold legally as a supplement. However, a ban on imported L-Tryptophan still exists.


Valine is one of the three branched-chain amino acids found in proteins. It is an integral part of muscle tissue and is metabolized as an energy source during exercise. During post exercise and recovery, valine is needed for tissue repair. It is also important for nitrogen balance and muscle metabolism. 

Non-essential Amino Acids


Alanine can be produced within the body. It is found in high concentrations in muscle tissue and found to be very important during exercise. It is involved in the process that converts glycogen and pyruvate into a source of energy. This process, known as the Glucose-Alanine Cycle, is complicated. Glucose and pyruvate are used directly for energy. Any unused pyruvate is converted into alanine and transported to the liver, where it is converted into glucose. The glucose is then returned to the muscle tissue to be used for energy.  This process is believed to be a means of conserving the limited amounts of glycogen in the muscle tissue, thereby maintaining the glucose level during exercise. Supplementing a diet with L-Alanine would be helpful in the prevention of muscle breakdown, as well as, minimizing glycogen depletion.


Asparagine aids the central nervous system because it helps the body maintain a balance that prevents large fluctuations in mood. It prevents the system from extreme nervousness and extreme calmness.

Aspartic acid

Aspartic acid is metabolized from glutamic acid. It is involved in the Krebs cycle and the urea cycle. Aspartic acid is useful to athletes, as it assists the body in detoxifying the liver by reducing the blood ammonia level after exercise. This is important because ammonia is a toxic substance. In addition, aspartic acid helps increase stamina and reduce fatigue.


Cysteine is one of the sulfur-containing amino acids found in proteins.  It is formed in the body from L-methionine, in the presence of vitamin B6. If the body is deficient of this vitamin, L-cysteine cannot be produced.  It is a precursor to glutathione. Cysteine is valuable in helping rid the body of harmful toxins and thus preserving cells. It also assists the body in destroying free radicals. In addition, cysteine can aid in weight loss. It helps the body burn fat and also promotes muscle growth. It plays a role in energy production and it can be converted into glucose and used as energy.

Glutamic acid

Glutamic acid (or glutamate) is involved in the Krebs cycle, during which, carbohydrates are metabolized to produce ATP. This amino acid assists the body in removing ammonia from muscle cells. The presence of glutamic acid is needed when energy is produced from BCAAs. When amounts of glutamic acid are limited, energy production decreases. Glutamic acid is also an important source of brain fuel. Besides glucose, this is the only component that is used as food for the brain. All other components are not able to pass through the blood-brain barrier. 

Glutamic acid is a conditionally essential amino acid, and is especially important during power exercises and intense weight training.  It is essential in counter balancing the catabolic effects of cortisol.


Glutamine is essential for proper functioning of the brain. It acts as a neurotransmitter. It can convert back to glutamic acid, so that is can pass across the blood brain barrier to provide fuel to the brain. It is also essential to the immune system and plays a vital role in cellular replication.


Glycine is manufactured in the body from serine and folate. Its most important role is as a precursor to numerous vital substances the body needs for proper function. They are, but not limited to, proteins, DNA, phospholipids, and creatine. Its role in creatine production is also important, as well as, its role in the prevention of muscle breakdown. In this regard, it is a very valuable amino acid to bodybuilders and athletes. Glycine is involved in the detoxification of the liver, by helping eliminate phenols. It is also essential in the proper functioning of the central nervous system. It can act as a neurotransmitter with an inhibitory response. 



Proline is synthesized within the body from ornithine or glutamic acid.  It is most known for its role in collagen, where it is found in very high concentrations in the form of hydroproline. Hydrproline is formed from proline. It aids the body in the healing of collagen and cartilage. It is believed to strengthen joints, tendons, as well as, the heart muscle.


Serine is found in proteins and derived from glycine. Like glycine, it is a precursor to many substances that the body uses, such as choline and phospholipids. Serine is needed for the metabolism of fats and fatty acids. It aids in muscle growth, and helps promote a healthy immune system. It also aids in the production of immunoglobulins and antibodies. 



Tyrosine is found in proteins and synthesized in the body from phenylalanine. Tyrosine is important in its role as a precursor to neurotransmitters, such dopamine and norepinephrine. It helps to control appetite and is also involved in melatonin production. 


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