Amino acids are building blocks for protein, so your body needs them for making and repairing structures and compounds such as muscle tissue, skin, fingernails, hair and enzymes. Most amino acids are produced in the body, but some are вЂњessentialвЂќ and can only be obtained from food. All the essential amino acids are found in grains, although in different quantities.
Essential Amino Acids
Adults need nine essential amino acids from food because their bodies cannot manufacture them. These amino acids include phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, leucine, lysine and histidine. To make protein in the body, certain amounts and ratios of essential amino acids are needed. Animal products such as meat, milk and eggs contain lots of all the essential amino acids, so they are classified as complete protein sources. On the other hand, most plant foods are deficient in at least one essential amino acid -- usually lysine -- so they are called incomplete sources. All grains contain at least some lysine and other essential amino acids, but quinoa is the only grain that contains enough to be considered a complete protein source.
Amino Acids in Grains
Commonly eaten grains include quinoa, oats, corn, millet, barley, wheat and rice. These grains contain varying amounts of amino acids and your body may be able to store some short-term for later use, although it's widely believed that storage doesn't occur. With the exception of quinoa, grains are deficient in lysine and relatively poor sources of threonine, leucine and histidine. On the other hand, all grains are rich in methionine and most are relatively good sources of phenylalanine, tryptophan, valine and isoleucine. Overall, quinoa and oats are the best grain sources of amino acids.
Lysine is the limiting amino acid in almost all grains. Without enough dietary lysine, your body cannot properly manufacture or repair protein-based structures and compounds. The recommended daily intake of lysine for adults up to the age of 59 is 38 milligrams per kilogram of body weight. To put that in perspective, a 140-pound adult needs to eat 5.5 cups of quinoa or 52 slices of whole-wheat bread to meet his daily lysine requirement.
People who eat animal products on a regular basis get plenty of lysine and other essential amino acids. On the other hand, vegetarians typically need to make more of a conscious effort to get enough essential amino acids. Grains are a good source of most, but vegetarians should combine them with lysine-rich foods such as soy beans, kidney beans or seeds.