Amino Acids
Better Nutrition|September 2020
What they are and where to find them.
By Lisa Turner

You’ve heard a lot about amino acids and how important they are for building muscle. But these building blocks of protein are responsible for many other critical systems and functions in the body, including neurotransmitter and hormone production, immune health, nervous system function, tissue repair, digestion, and reproduction.

When you eat foods that are high in protein, the body breaks them down into amino acids. Your body needs 20 different amino acids, which are categorized as essential, conditionally essential, or non-essential:

Essential amino acids are considered “essential” because your body can’t make them—you have to get them from your diet. There are nine of them: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

Non-essential amino acids are synthesized by the body, even if they’re not consumed in the diet. The eleven non-essential amino acids are alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine.

Conditionally essential amino acids, also called “conditional amino acids,” include some non-essential amino acids whose synthesis may be limited under certain conditions, including serious illness, injury, surgery, or extreme trauma or stress. For instance, tyrosine is considered an essential amino acid for people with phenylketonuria (PKU), a condition in which the body can’t synthesize tyrosine. Other conditional amino acids include arginine, cysteine, glutamine, glycine, proline, and serine.

You’ll find amino acids in a variety of foods, but there’s a catch: meat, fish, dairy, eggs, and other animal foods contain all nine essential amino acids and are considered complete proteins. Some plant foods—including soy and quinoa—contain all nine essential amino acids, but there’s some debate over whether they contain adequate quantities to be considered complete proteins. Beans, grains, and nuts are also rich in certain amino acids, but are low or lacking in others—called the limiting amino acid. For example, beans are low in tryptophan and methionine, and grains, nuts, and seeds lack lysine.

If you eat a variety of plant-based proteins, it’s easy enough to compensate for limiting amino acids and get all nine essentials—and you don’t have to eat them all at the same meal. Here’s a guide to the best food sources of amino acids, and ways to add them to your diet.

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