Essential amino acid

Amino acids are the fundamental building blocks of proteins in all living things. While there are 21 amino acids common to most life forms, humans cannot produce every type from scratch. When an organism cannot synthesize an amino acid fast enough to meet its biological demands, that amino acid becomes essential. This means it must be obtained through the diet. Understanding these molecules is vital for nutrition and medicine. It helps us understand how the body maintains its health and builds new tissues.

Humans categorize amino acids into three main groups based on how the body handles them. The first group is essential amino acids. There are nine of these: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. The second group is non-essential, or dispensable, amino acids. These include alanine, aspartic acid, asparagine, glutamic acid, serine, and selenocysteine. The third group is conditionally essential. These six amino acids—arginine, cysteine, glycine, glutamine, proline, and tyrosine—are usually made by the body. However, synthesis can become limited during specific pathophysiological conditions.

Conditionally essential amino acids are required when the body is under stress. For example, newborn infants may need to get these from their diet. People experiencing severe catabolic distress also fall into this category. A person with a diseased liver might be unable to synthesize cysteine. In adults, the urea cycle typically produces enough arginine to meet needs. However, a growing child might not produce enough on their own. This distinction shows that nutritional needs change based on age and health status.

Biological processes allow the body to create non-essential amino acids from other sources. This happens through basic metabolic pathways. For instance, glutamate dehydrogenase catalyzes the reductive amination of α-ketoglutarate to create glutamate. Many amino acids are formed through a process called transamination. During this step, the chirality, or spatial arrangement, of the amino acid is established. Alanine and aspartate are created by the transamination of pyruvate and oxaloacetate. Other amino acids come from different precursors, such as serine, which is formed from 3-phosphoglycerate.

Determining these requirements was a major scientific challenge. Historically, researchers calculated requirements by measuring nitrogen balance. Since proteins are the body's largest nitrogen source, nitrogen levels indicate protein status. A positive nitrogen balance means the body is consuming more nitrogen than it excretes. This suggests the body is using nitrogen to build protein. A negative balance means more nitrogen is excreted than consumed. This indicates insufficient intake for maintaining health.

In famous studies at the University of Illinois, graduate students helped define these needs. They were fed an artificial diet to maintain a slightly positive nitrogen balance. Researchers then omitted one amino acid at a time. If a negative nitrogen balance occurred, that amino acid was deemed essential. This method helped identify the specific needs of humans. Scientists also used these methods to determine the protein content of various foods. They calculated net protein utilization and biological value to measure how well the body uses protein.

Modern nutrition relies on exact data and the concept of the limiting amino acid. The limiting amino acid is the essential one that is furthest from meeting nutritional requirements. Even if a meal has enough total protein, it is considered nutritionally limited if one essential amino acid is missing. Today, the USDA uses ion exchange chromatography to find exact amino acid contents. In 2018, they published a database covering 7,793 foods. This data helps us understand that food quality depends on more than just total protein. For example, an egg provides more protein per calorie than broccoli. To get the same protein profile as an egg, one would need to eat over 3 kg of broccoli.