Back to the Basics: An Overview Of Amino Acids
Amino acids are the foundation of life. They are involved in everything from muscle growth and the maintenance of connective tissues to making the chemicals necessary for our brain and vital organs to function. Beyond serving as the building blocks for all-important proteins, amino acids are in and of themselves important signaling factors and intermediaries in many metabolic pathways.
The average human body is 60-70% water while protein constitutes two-thirds of the non-water component of the body. We can all appreciate how important it is to drink plenty of fluids in order to balance the water lost through sweat, urine, and respiration. But we also lose protein on a daily basis as muscles and tissues degrade and amino acids are oxidized. Just as it’s important to rehydrate with water, a significant dietary input of amino acids is required to maintain this body pool at an optimal level. These requirements are even greater under conditions that affect our overall diet quality and subject us to a variety of physiological stresses.
Here is an overview of basic information about amino acids and how and why they are perfectly suited for, and in fact essential to, the dietary goals of people committed to health and their own well-being.
What Are Amino Acids?
Amino acids (20 in total for biological purposes) are important organic compounds that exist in all protein-containing food sources. They are the building blocks of proteins.
In chemical terms, an amino acid is an organic compound containing amine (-NH2) and carboxylic acid (-COOH) functional groups. In addition, each amino acid contains a unique side chain (R group) that features an element or chemical structure that imparts a specific characteristic or function to that amino acid.
While at least 500 naturally occurring amino acids have been identified, in this article we will concern ourselves only with those that are referred to as “proteinogenic,” which means they are used in the making of all biological proteins. You may already be familiar with some of these amino acids since an increasing number are available as nutritional supplements (for example, the ever-popular branched-chain amino acids).
Currently, there is a great deal of interest in identifying functional benefits of different amino acids beyond their role as constituents of muscle protein. As we discuss each amino acid, keep in mind that these functions occur in an environment in which all amino acids are present and maintained in a specific balance and physiological concentration.
Eleven of the standard amino acids found in the human genetic code are considered dispensable or nonessential, meaning the body has the ability to make or synthesize these amino acids, and so it is not necessary to get them from the diet.
Nine amino acids must be obtained from food sources or supplements since humans do not have the ability to make them, and these are deemed indispensable or essential. There is a recommended dietary allowance (RDA) for the essential amino acids, while there is no required dietary intake defined for the nonessentials.
There are also some conditionally essential amino acids. In healthy people, these amino acids can be synthesized, but in certain disease states, some amino acids must be obtained from diet since the amount produced by the body is inadequate to meet demands. This occurs, for instance, when the liver functions less than optimally. Liver damage impairs the conversion of some essential amino acids to nonessential amino acids, and these nonessentials then become classified as conditionally essential amino acids. Overall good nutrition is important as well, since sub-optimal vitamin and mineral status can interfere with the synthesis of nonessential amino acids by impairing enzymes or other cofactors involved in the reaction.
Essential vs. Nonessential Amino Acids
The essential amino acids include:
Histidine was thought to be nonessential for adults since it appeared that only infants could not synthesize it, but more extensive studies suggest that adults, too, rely upon dietary sources of this amino acid.
The nonessential amino acids include:
- Aspartic Acid
- Glutamic Acid
While in theory, we could function perfectly well without any dietary intake of nonessential amino acids, we are actually much more metabolically efficient if we have some percentage of these common amino acids supplied by the diet. In the absence of a dietary source, the body has to divert energy and resources to making the particular amino acid that is in short supply. Depending upon the circumstances and the desired outcome, this may or may not be a good thing.
For example, with an illness or a catabolic disease, having an abundance of all amino acids facilitates immune system function by ensuring all the amino acid components are available and energy does not need to be devoted to synthesizing nonessential amino acids.
On the other hand, efforts to lose weight for health can be helped along by the added calorie cost of processing protein and making nonessential amino acids. Bodybuilders who eat very high-protein diets or individuals who need to reduce urea production (for example dialysis patients or people with kidney impairments), can benefit from supplementing with essential amino acids, which effectively reduces the breakdown of nonessential amino acids (and the consequent release of nitrogen). On average, a high-quality dietary protein contains a ratio of essential to nonessential amino acids of approximately 55:45 which is similar to the makeup of human muscle.
Amino Acids in Balance: The Whole Is Greater Than the Sum of the Parts
Every amino acid can serve as a structural component of protein, skeletal muscle being the main reservoir in the human body. Individual amino acids have unique functions, such as L-arginine’s role in the production of nitric oxide, L-leucine’s ability to initiate the process of protein synthesis, or L-lysine's beneficial effect on blood sugar regulation. However, it is an oversimplification to expect a specific outcome in response to taking a single amino acid supplement.
When a specific action is attributed to an amino acid, it is in the context of all the chemical reactions occurring throughout the human body, which involve all the amino acids working together. For example, it seems reasonable to consume arginine to stimulate nitric oxide production since that is what arginine does in the body. Yet arginine supplements do not cause a large increase in arginine concentrations in the blood and tissues because much of the orally ingested arginine is taken up and metabolized in the liver. It is more effective to take a citrulline supplement which is converted to arginine in the kidneys, thus making arginine available to circulate to endothelial cells and other tissues.
Just like the way in which many vitamins function, taking more of a single amino acid does not necessarily create more of an effect, and often it is only with a deficiency of the nutrient that a problem arises. Isolated amino acid deficiencies are very rare and usually attributable to metabolic defects.
Amino acids function as a team in a physiological system with many other compounds and factors that affect how it operates. To keep amino acids in balance, eat a varied protein- and antioxidant-rich diet and take a well-formulated essential amino acid supplement as needed.