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Sports Nutrition
Lesson 30
Protein: Metabolism and Function
WHAT HAPPENS TO PROTEIN IN THE HUMAN BODY?
Dietary protein consists of long chains of amino acids. In the digestive process,
enzymes in the stomach and small intestine break the complex protein down into
polypeptides and then into individual amino acids.
The amino acids are absorbed through the wall of the small intestine, pass into the
blood and then to the liver via the portal vein.
Protein: Metabolism and Function
The digestion of protein takes several hours, but once the amino acids enter the
blood they are cleared within 5-10 minutes.
The liver is a critical center in the amino acid metabolism. It is continually
synthesizing a balanced amino acid mixture for the variety of protein requirements
of the body.
These amino acids are secreted into the blood and carried
as free amino acids or as plasma proteins such as albumin.
Protein: Metabolism and Function
The body cells will use only the amount of amino acids necessary to meet their
protein needs. They cannot store excess amino acids.
Because the human body does not have a mechanism to store excess nitrogen, it
cannot store amino acids.
Through the process of deamination , the amino group NH2 containing the
nitrogen is removed from the amino acid, leaving a carbon substrate known as
alpha-ketoacid. The excess nitrogen must be excreted from the body.
Protein: Metabolism and Function
The liver forms ammonia from the excess NH2 and passes it into the blood and it
is eventually eliminated by the kidneys into the urine.
The alpha-keto acid that remains after the NH2 is removed can do several things.
1. It can be oxidized for the release of energy.
2. It can accept another NH2 and be reconstituted back into an amino acid
3. It may be channeled into the metabolic pathways of CHO and fat
Protein: Metabolism and Function
Some of the amino acid are glucogenic amino acids - glucose forming in the
process called glucogenisis.
The ketogenic amino acids are metabolized in the liver to acetyl CoA which may
be used for energy production in the Krebs Cycle or converted to fat.
The glucose and fat produced may be transported to other parts of the body to be
used.
Although excess protein cannot be stored in the body, the energy content is not
wasted.
Protein: Metabolism and Function
CAN PROTEIN BE FORMED FROM CARBOHYDRATES AND FATS?
Yes, but with some limitations.
CHO and fat do not contain nitrogen. However if the body has an excess of
amino acids, the liver may be able to use the nitrogen containing amino groups
from these excess amino acids and combine them with alpha-keto acids derived
from either CHO or fat metabolism.
Protein: Metabolism and Function
A key alpha-ketoacid from CHO is pyruvic acid.
Fat yields acetic acid.
The result is the formation in the body of some of the non essential amino acids
using CHO and fats as part of the building material. Remember that this cannot
happen without the presence of Nitrogen from dietary sources.
Protein: Metabolism and Function
WHAT ARE THE MAJOR FUNCTIONS OF PROTEIN IN HUMAN NUTRITION?
In one way or another protein is involved in almost all body functions.
3 of its main roles are
1. Serves as the structural basis for the vast majority of body tissues
2. Essential for regulating metabolism
3. Used as an energy source
The handout containing Table 6.5 summarizes the function of protein.
Protein: Metabolism and Function
Although protein is not a major energy source for humans at rest, it can serve
that function under several conditions. The primary function of protein is to
promote synthesis of body proteins essential for optimal structure and function.
During periods of starvation or semi-starvation, both dietary protein and body
protein stores are used for energy purposes. Energy production takes
precedence over tissue building.
If the active individual wants to retain lean muscle mass, it is essential to consume
sufficient CHO calories in the diet to provide a protein sparing effect.