Amino acids are organic compounds that serve as the fundamental constituents of proteins. To understand their function, one can think of them as letters of an alphabet; just as letters combine to form words, amino acids link together in sequences to create a vast array of proteins. Each protein has a unique structure and performs a distinct job within the body. The human body requires 20 different amino acids to grow, repair tissues, and carry out numerous physiological processes. These compounds are integral to nearly every biological function.
The Three Types of Amino Acids
The 20 amino acids for human health are categorized into three groups based on the body’s ability to produce them. The first group is the essential amino acids, which cannot be synthesized by the body in sufficient quantities to meet physiological demands. Consequently, these nine amino acids—histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine—must be obtained from dietary sources.
A second category is nonessential amino acids. The body is capable of synthesizing these on its own, primarily in the liver. While they are just as important for building proteins, they are not required from the diet.
The final group is conditionally essential amino acids. Under normal circumstances, these are considered nonessential because the body can produce them. However, during periods of significant physiological stress, such as severe illness or injury, the body’s demand for these amino acids can outpace its production capacity. In these specific situations, they become essential and must be supplied through food.
Functions of Essential Amino Acids
Essential amino acids participate in a wide range of bodily processes beyond their role in forming proteins. A primary function is initiating muscle protein synthesis, which is necessary for the repair, maintenance, and growth of muscle tissue. The presence of these amino acids, particularly leucine, signals the body to begin building new proteins, which is important after exercise or injury.
Their influence extends to the nervous system, where certain essential amino acids act as precursors for neurotransmitters. For example, tryptophan is converted into serotonin, a chemical messenger that helps regulate mood, sleep, and appetite. Phenylalanine is a precursor to dopamine and norepinephrine, neurotransmitters that affect focus, alertness, and the body’s stress response.
These compounds also support the immune system by aiding in the production of antibodies and immune cells. They are used to create hormones and enzymes that regulate metabolism and other bodily functions. Lysine, for instance, is involved in calcium absorption and the production of collagen, a structural protein found in skin, bones, and connective tissues.
Dietary Sources of Essential Amino Acids
Obtaining all nine essential amino acids through diet is done by consuming protein-containing foods. Foods that provide all nine in adequate amounts are known as complete proteins. Animal-based products such as meat, fish, poultry, eggs, and dairy are prominent examples of complete protein sources. The body breaks down the protein into individual amino acids for various biological processes.
Some plant-based foods also qualify as complete proteins, most notably soy products like tofu and edamame, as well as quinoa. Most other plant foods, such as beans, lentils, nuts, and grains, are considered incomplete proteins because they are low in one or more of the essential amino acids. A diet that relies on these sources requires protein combining to ensure a full complement is consumed.
Protein combining involves pairing different incomplete protein sources over the course of a day to make up for any deficiencies. A classic example is the combination of rice and beans; rice is low in lysine, while beans are low in methionine. Eating them together, or within the same day, provides all nine essential amino acids, forming a complete protein profile.
Signs of Inadequate Intake
When dietary intake of essential amino acids is insufficient, the body’s ability to synthesize new proteins is hampered, leading to noticeable effects. One of the early signs can be a decline in immune function, as the production of antibodies and other immune components slows. This can result in increased susceptibility to infections.
A more visible consequence is the impact on tissues that have a high rate of protein turnover. This includes changes in the texture and strength of skin, hair, and nails, which may become brittle. For individuals who are physically active, inadequate intake can manifest as difficulty building or maintaining muscle mass and prolonged recovery times. Over time, a significant deficiency can lead to muscle wasting as the body breaks down its own muscle tissue for more immediate functions.