A protein is a complex, high molecular weight organic compound that consists of
amino acids joined by peptide bonds. Proteins are essential to the structure and
function of all living cells and viruses. Many proteins are enzymes or subunits of
enzymes. Other proteins play structural or mechanical roles, such as those that form
the struts and joints of the cytoskeleton. Still more functions filled by proteins
include immune response and the storage and transport of various ligands. In
nutrition, proteins serve as the source of amino acids
for organisms that do not synthesize those amino acids natively.
Proteins are one of the classes of bio-macromolecules, alongside polysaccharides and
nucleic acids, that make up the primary constituents of living things. They are
amongst the most actively studied molecule in biochemistry and were discovered by
Jöns Jacob Berzelius, in 1838.
In terms of human nutritional needs, proteins come in two forms: complete proteins
contain all eight of the amino acids (threonine, valine, tryptophan, isoleucine,
leucine, lysine, phenylalanine, and methionine) that humans cannot produce
themselves, while incomplete proteins lack or contain only a very small proportion
of one or more. Human bodies can make use of all the amino acids they extract from
food for synthesizing new proteins, but the inessential ones themselves need not be
supplied by the diet, because our cells can make them ourselves. When protein is
listed on a nutrition label it only refers to the amount of complete proteins in the
food, though the food may be very strong in a subset of the essential amino acids.
Animal-derived foods contain all of those amino acids, while plants are typically
stronger in some acids than others. Complete proteins can be made in an all vegan
diet by eating a sufficient variety of foods and by getting enough calories. It was
once thought that in order to get the complete proteins vegans needed to do protein
combining by getting all amino acids in the same meal (the most common example is
eating beans with rice) but nutritionists now know that the benefits of protein
combining can be achieved over a longer period of time. Ovo-lacto vegetarians
usually do not have this problem, since egg's white and cow's milk contain all
essential amino acids. Peanuts, soy milk, nuts, seeds, green peas, Legumes, the alga
spirulina and some grains are some of the richest sources of plant protein.
All eight essential amino acids must be part of one diet in order to
survive and are needed in a fixed ratio. A shortage on any one of these amino acids
will constrain the body's ability to make the proteins it needs to function.
Different foods contain different ratios of the essential amino acids. By mixing
foods that are rich in some amino acids with foods that are rich in others, one can
acquire all the needed amino acids in sufficient quantities. Omnivores typically eat
a sufficient variety of foods that this is not an issue, however, vegetarians and
especially vegans should be careful to eat appropriate combinations of foods (e.g.
nuts and green vegetables) so as to get all the essential amino acids in sufficient
quantities that the body may produce all the proteins that it needs.
Protein deficiency can lead to symptoms such as fatigue, insulin resistance, hair
loss, loss of hair pigment (hair that should be black becomes reddish), loss of
muscle mass (proteins repair muscle tissue), low body temperature, and hormonal
irregularities. Severe protein deficiency is fatal.
Excess protein can cause problems as well, such as causing the immune system to
overreact, liver dysfunction from increased toxic residues, possibly bone loss due
to increased acidity in the blood, foundering (foot problems) in horses, and can
also be linked to obesity.
Proteins can often figure in allergies and allergic reactions to certain foods. This
is because the structure of each form of protein is slightly different, and some may
trigger a response from the immune system while others are perfectly safe. Many
people are allergic to casein, the protein in milk; gluten, the protein in wheat and
other grains; the particular proteins found in peanuts; or those in shellfish or
other seafoods. It is extremely unusual for the same person to adversely react to
more than two different types of proteins.
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