Essential amino acid

Our bodies need food to grow. Some parts of food are very special. We call these parts building blocks.

Our bodies can make some building blocks. But we cannot make all of them. We must eat them in our food.

Nine special building blocks are very important. We must get them from what we eat. This helps our bodies stay healthy.

Some people need extra help. Babies and sick people might need more. They must eat certain foods to grow.

Eating many kinds of food is good. It helps us get every building block. This keeps us strong and well.

Your body needs building blocks to stay healthy. We call these small parts amino acids. There are 21 amino acids used by most life. Your body can make some of them on its own. These are called non-essential amino acids. But your body cannot make others fast enough. You must get these from the food you eat. We call these essential amino acids.

Humans need nine essential amino acids to stay well. These are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Some amino acids are conditionally essential. This means you might need them only during certain times. For example, babies or sick people may need more. This includes parts like arginine and cysteine.

One amino acid can limit your health. This is called the limiting amino acid. It is the one you are missing the most. If you do not eat enough of it, your body cannot use the other proteins well. Even if you eat plenty of protein, you still need every single one. Eating many different foods helps you get them all.

Your body is built from many small parts called amino acids. While there are 21 amino acids used by most life, humans cannot make all of them from scratch. We call the ones we must eat "essential amino acids." There are nine of these for humans: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. If you do not eat enough of even one, your body may struggle. This is because of a concept called the "limiting amino acid." This is the one amino acid that is furthest from meeting your body's needs. Even if you eat plenty of other proteins, the meal is limited by that one missing piece.

Some amino acids are called "conditionally essential." This means your body usually makes them, but sometimes it needs help. This might happen to newborn infants or people who are very sick. Six amino acids fall into this group: arginine, cysteine, glycine, glutamine, proline, and tyrosine. For example, an adult makes enough arginine through a process called the urea cycle. However, a growing child might need to get it from food instead. Other amino acids are "non-essential." This means your body can make them on its own using basic metabolic pathways. These include alanine, aspartic acid, asparagine, glutamic acid, serine, and selenocysteine.

Scientists learned how to find these building blocks by watching how bodies use nitrogen. Protein is the largest source of nitrogen in the body. Researchers at the University of Illinois used a clever method to test this. They fed students a special diet to keep a "positive nitrogen balance." This means the body was taking in more nitrogen than it was wasting. Then, they removed one amino acid at a time. If the students' nitrogen levels dropped, they knew that amino acid was essential. This helped experts understand exactly what humans need to stay healthy.

Today, we use very modern tools to study food. The USDA uses a technique called ion exchange chromatography to find amino acids in food. In 2018, they shared a huge database of 7,793 different foods with the public. This helps us know which foods are best for us. For example, the amount of protein you need depends on your weight. Experts like the WHO and FAO provide guidelines for how many milligrams of each amino acid you need per kilogram of body mass. These numbers can change as you grow from a baby to an adult.

Understanding these parts helps us choose the right foods every day. You might think eating a lot of broccoli is enough for protein. However, to get the same protein as an egg, you would have to eat a huge amount! Different foods have different amounts of these essential building blocks. Some foods might be low in one specific amino acid. This is why eating many different types of food is so helpful. By mixing different foods, you make sure your body gets every single essential part it needs to grow strong.

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.