What is a protein?
Proteins are organic compounds which provide the body with nitrogen for a variety of uses.
Protein are used for tissue repair, as well as the synthesis of some hormones and enzymes.
Proteins are made up of sub-units called amino acids (AAs). There are 20 AAs which occur in
food, although more are present in the body. Of these, 8 are referred to as indispensable
indicating that they must be obtained from the diet. The remaining 12 are considered dispensable
in that they can be synthesized in the body.
With few exceptions (i.e. gelatin), every dietary protein contains all of the AAs in varying
amounts. This means that that concept of ‘complete’ and ‘incomplete’ proteins is inaccurate. All
proteins are complete, in that they contain all AA. It is more accurate to say that proteins have
a limiting AA, which is the indispensable AA occurring in the lowest quantity relative to what is
needed. Consuming a variety of protein sources should ensure adequate amounts of all amino
acids.
The ratios of AAs determine, to a great degree, how well the human body can use these
proteins. This is sometimes referred to as the biological value (BV) or protein efficiency ratio
(PER).
Digestion of protein
As they are digested, proteins are broken down in the stomach into smaller chains of AAs.
These chains include single AAs (peptides), chains of two AAs (dipeptides), and chains of three
AAs (tripeptides). Once AAs enter the bloodstream, they are treated identically in the body.
This means that, for all practical purposes, the protein from an egg is treated no differently than
from an amino acid capsule. The only real difference in quality between proteins are in the
relative ratios of AAs.
As described in the chapter 5, dietary protein is converted to glucose with 58% efficiency
(8). This reflects the fact that over half of the AAs can be readily converted to glucose. While
some AAs can be converted to ketones in the liver, this is not thought to contribute significantly
to ketosis.
How much protein per day is needed to sustain the body?
Unlike carbohydrate and fat, the body does not have any way to store protein except as
muscle tissue and a small pool of free AAs in the bloodstream, muscle and liver. The utilization of
any body protein for uses other than tissue synthesis should be interpreted as a loss of skeletal
muscle tissue. Additionally, periods of severe starvation can cause the loss of cardiac, smooth or
organ protein. Although recent research differentiates between the loss of essential and non-
essential lean body mass (LBM), any loss of LBM should generally be avoided as there is no way
to ensure that only non-essential LBM is being lost (9).
Under all dietary conditions, the body has a certain minimum protein requirement needed
for basic tissue repair, enzyme and hormone synthesis synthesis. This is represented by the