What is an essential amino acid?

An essential amino acid is defined as one that must be obtained from the diet because the body cannot synthesize it in sufficient quantities to meet its needs. There are nine essential amino acids for humans, which include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. These amino acids are crucial for various bodily functions, including protein synthesis, hormone production, and enzymatic reactions.

Since the human body lacks the means to produce these amino acids, it is imperative that they are consumed through food sources, such as meat, dairy, eggs, and certain plant-based proteins. This necessity highlights the importance of a balanced diet, particularly for individuals following vegetarian or vegan diets who need to ensure they are obtaining all essential amino acids from plant sources.

Other options may describe amino acids in various contexts, such as non-essential amino acids that the body can synthesize, or suggest dietary sources that do not encapsulate the full definition of what makes an amino acid essential. However, the key characteristic of essential amino acids is their requirement for dietary intake.