Introduction
Being
vegetarian does not mean your diet will be lacking in protein. Most
plant foods contain protein and in fact it would be very
difficult to
design a vegetarian diet that is short on protein. Excess dietary protein
may lead to health problems. It is now thought that one of the benefits
of a vegetarian diet is that it contains adequate but not excessive
protein.
Proteins are made up of smaller units called amino acids. There are about
20 different amino acids, eight of which must be present in the diet. These
are the essential amino acids. Unlike animal proteins, plant proteins may
not contain all the essential amino acids in the necessary proportions.
However, a varied vegetarian diet means a mixture of proteins are consumed,
the amino acids in one protein compensating for the deficiencies of another.
Structure & Functions
Proteins are highly complex molecules comprised of linked amino acids.
Amino acids are simple compounds containing carbon, hydrogen, oxygen, nitrogen
and occasionally sulphur. There are about 20 different amino acids commonly
found in plant and animal proteins. Amino acids link together to form chains
called peptides. A typical protein may contain 500 or more amino acids. Each
protein has it's own unique number and sequence of amino acids which determines
it's particular structure and function.
Proteins are broken down into their constituent amino acids during digestion
which are then absorbed and used to make new proteins in the body. Certain
amino acids can be made by the human body. However, the essential amino
acids cannot be made and so they must be supplied in the diet. The eight
essential amino acids required by humans are: leucine, isoleucine, valine,
threonine, methionine, phenylalanine, tryptophan, and lysine. For children,
histidine is also considered to be an essential amino acid.
Proteins
are essential for growth and repair. They play a crucial role in virtually
all biological processes in the body. All enzymes are proteins
and are vital for the body's metabolism. Muscle contraction, immune
protection, and the transmission of nerve impulses are all dependent
on proteins. Proteins
in skin and bone provide structural support. Many hormones are proteins.
Protein can also provide a source of energy. Generally the body uses
carbohydrate and fat for energy but when there is excess dietary protein
or inadequate
dietary fat and carbohydrate, protein is used. Excess protein may also
be converted to fat and stored.
Dietary Sources
Most foods contain at least some protein. Good sources of
protein for vegetarians include nuts and seeds, pulses, soya products (tofu,
soya milk
and textured soya protein such as soya mince), cereals (wheat, oats, and
rice), free-range eggs and some dairy products (milk, cheese and yoghurt).
Different foods contain different proteins, each with their own unique
amino acid composition. The proportions of essential amino acids in foods
may differ from the proportions needed by the body to make proteins. The
proportion of each of the essential amino acids in foods containing protein
determines the quality of that protein. Dietary proteins with all the essential
amino acids in the proportions required by the body are said to be a high
quality protein. If the protein is low in one or more of the essential
amino acids the protein is of a lower quality. The amino acid that is in
shortest supply is called the limiting amino acid.
Protein quality is usually defined according to the amino acid pattern
of egg protein, which is regarded as the ideal. As such, it is not surprising
that animal proteins, such as meat, milk and cheese tend to be of a higher
protein quality than plant proteins. This is why plant proteins are sometimes
referred to as low quality proteins. Many plant proteins are low in one
of the essential amino acids. For instance, grains tend to be short of
lysine whilst pulses are short of methionine. This does not mean that vegetarians
or vegans go short on essential amino acids. Combining plant proteins,
such as a grain with a pulse, leads to a high quality protein which is
just as good, and in some cases better, than protein from animal foods.
Soya is a high quality protein on its own which can be regarded as equal
to meat protein.
The limiting amino acid tends to be different in different proteins.
This means when two different foods are combined, the amino acids in one
protein can compensate for the one lacking in the other. This is known
as protein complementing. Vegetarians and vegans eating a well-balanced
diet based on grains, pulses, seeds, nuts and vegetables will be consuming
a mixture of proteins that complement one another naturally without requiring
any planning. Beans on toast, cheese or peanut butter sandwich, muesli
with milk (soya or cow's), and rice with peas or beans are all common examples
of protein complementing.
Previously, it has been thought that protein complementing needed to
occur within a single meal. However, it is now known that this is not necessary
as the body keeps a short-term store of the essential amino acids. A well-balanced
vegetarian or vegan will easily supply all the protein and essential amino
acids needed by the body.
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