Proteins, Nutrition, and the Essential Amino Acids
Protein in the diet
Each day, your body requires 20 to 30 grams of protein to build important
molecules, and any excess proteins you eat are broken down into their amino
acids and transported to cells via the bloodstream. Protein breakdown
produces the waste urea, which is filtered from the blood by the kidneys. Urea
is what gives your urine its yellow color.
Why protein is needed
Dietary protein is used mostly for a source of amino acids to make new
molecules. Amino acids are used to synthesize certain hormones, other amino
acids, some neurotransmitters (chemicals used in communication between
nerve cells), and new proteins. These proteins have diverse roles in the body,
acting as enzymes, receptors on cell membranes, oxygen transport molecules
(hemoglobin), structural components (hair and nails), hormones, antibodies, and
muscle proteins.
Amino acids
Table 1. The Essential Amino Acids and Their Plant
Sources
In the digestive system,
dietary protein is broken down Essential amino
Beans and other Corn and other
into its amino acid subunits.
acids
legumes
grains
Isoleucine
Then, in the body cells, the
X
Lysine
amino acids are linked in
X
Valine
specific sequences to form
X
X
Histidine
new proteins. People can
X
X
Threonine
synthesize 11 of the 20
X
X
Phenylalanine
different amino acids by
X
X
Leucine
converting one type of amino
X
X
acid into another. There are 9 Tryptophan
X
Methionine
amino acids that we cannot
X
synthesize, and they are
called essential amino acids because it is essential that we have these in our
diet. Animal sources of protein such as meat, milk, or eggs contain all the
essential amino acids, and combinations of plant sources such as beans, corn,
and soybeans can supply the essential amino acids. Because many plant
proteins do not have all of the essential amino acids, individuals on a vegetarian
diet must include foods from a variety of plant sources whose proteins together
will provide all nine of the essential amino acids. If not, they risk protein
deficiency- malnutrition caused by not eating enough protein.
Text and table adapted from: Audesirk T., et al, Biology Life on Earth Sixth Edition, pages
585-586.