In the first article of this series,
we examined both the essential and non-essential amino acid components
of protein and showed how each protein contains one particular essential
amino acid that limits its value to the diet. In the second article,
we acknowledged how the digestibility of a protein (the ease or difficulty
of its being digested in the GI tract) contributes to the quality of
that protein and examined how we can measure this aspect of protein
In this article we recognize that
the quality of a protein is also based on its content and balance of
amino acids (the building blocks of protein) and on how well the body
utilizes absorbed protein. We'll examine in some detail how we measure
quality of a protein under these circumstances.
Testing to determine quality of protein
involves in vitro procedures (without the use of a living organism)
and in vivo procedures (using a living organism). Usually in vitro testing
is easier and quicker but gives only approximations. In vivo testing
is more difficult and takes longer, but gives real answers.
In Vitro Amino Acid Tests for Protein Quality
Two procedures are usually followed
in testing for amino acids as a measure of the quality of protein: the
amino acid profile and the amino acid score.
Amino Acid Profile
The listed contents of the amino
acids contained in any given protein is called the amino acid profile
for that protein. These levels, we must recognize, are determined by
chemical procedures in the laboratory without considering either the
digestibility of the protein or its utilization by the body. As such,
the amino acid profile does not necessarily reflect the true availability
of the protein to the body. The amino acid profile is, nevertheless,
an important test because it tells us what essential amino acids are
in the protein and thus are theoretically available for utilization.
Amino Acid Score
Scientists have come up with a way
of handling the problem of identifying limiting amino acids in reference
to protein quality which is extremely useful in comparing proteins.
It is called amino acid score.
An amino acid score for any particular
protein is calculated by comparing its amino acid levels to those of
a reference protein, which is a theoretically ideal or an actual high
quality protein. The amino acid score makes an adjustment for amino
acid composition in evaluating quality of protein and gives us a score
which dramatically points out the most limiting amino acid: that is,
that essential amino acid in greatest deficiency in that protein. The
formula for amino acid score is:
Amino Acid Score = Content of individual essential amino
acid in food protein (mg/gr of protein)
Content of same amino acid in a reference pattern (mg/gr
Amino acid scoring patterns for all the essential amino
acids are important for the formulation of special-purpose diets in
clinical practice. If a protein is of good quality and highly digestible,
the amino acid score approaches the true value of the protein to the
body. But the amino acid score, like the amino acid profile, does not
consider digestibility or utilization of the protein. To consider these
aspects requires biological (animal) testing procedures.
In Vivo (Biological) Tests for Quality
Biological evaluations to determine protein quality
are tedious and time-consuming and must be performed on living organisms.
The four noted below are those most commonly encountered.
Biological Value (BV)
One test to biologically determine protein quality includes
a consideration of protein losses in the urine as well as the losses
in feces. Test results are referred to as the Biological Value (BV)
of a test protein. The formula is:
BV of test protein = I - (F - Fo) - (U - Uo) x 100
I - (F - Fo)
where I is intake of nitrogen; F is fecal nitrogen;
Fo is obligatory fecal nitrogen; U is urinary nitrogen; and Uo is obligatory
urinary nitrogen. Obligatory nitrogen values for feces and urine are
calculated when subjects are on a nitrogen-free diet. It is a measure
of the loss of nitrogen from the normal ongoing breakdown of tissue
constituents and not those losses from dietary sources. By subtracting
obligatory losses from total values obtained, we theoretically get those
losses from just the dietary.
BV is a measure of dietary nitrogen retained for the
body's use and is expressed as a percentage of nitrogen utilized.
Proteins exhibit a higher BV when fed at levels below
the amount necessary for nitrogen equilibrium; that is, when the amount
of nitrogen being eaten balances that amount lost. We'll discuss various
concepts involved with this idea of nitrogen equilibrium in a future
article in this series. We note here, however, that equilibrium testing
is the means by which protein requirements in humans are determined.
Net Protein Utilization
Another biological measure of protein quality that we
often see, which includes an evaluation of protein digestibility as
well as the content of essential amino acids, is net protein utilization
(NPU). NPU measures the retention of absorbed nitrogen and is calculated
NPU = I - (F - F0) - (U - U0)
Protein Efficiency Ratio
The PER or protein efficiency ratio, represents the
gain in body weight of a growing animal fed a test protein divided by
the grams of protein consumed. Usually, if not always, growing rats
PER = grams gain in body weight
grams of protein consumed
Two groups of young growing rats are selected for testing.
Each group is fed a complete diet that contains 10% protein. The experimental
group's protein source is the test protein, and the control group is
fed casein (a milk protein) as its protein source.
The PER result for casein used as the control is arbitrarily
adjusted to a value of 2.5. The test protein results are then similarly
adjusted as was needed to adjust the casein value to 2.5. Standardization
thus permits comparison among different laboratories conducting PER
tests on protein.
Despite comparability of data, please note that PER
is mostly a test of the protein's value for the growth of a healthy
Amino Acid Availability
Unfortunately, as noted in the previous article in this
series, measurements of apparent protein availability by tests such
as the NPU don't provide data on the differences in availability of
specific amino acids in food proteins - in total sometimes called amino
A chick test is commonly used to determine amino acid
availability. In this test, we measure chemically over a specified time
the total amino acids (rather than protein) eaten (% of sample) and
the total amino acids that have been excreted (% excreted). The chicks
are fed a nutritionally balanced diet with the test protein as the sole
source of protein in the diet. The so-called true biological value of
each amino acid is then determined by measuring the % eaten minus the
% excreted over time. The digestibility of the amino acids thus determined
is referred to as the percent availability of that amino acid in the
protein - in this case, for chickens. The test, of course, could be
performed using humans as test animals.
Actually, there isn't a lot of data on the availability
of specific amino acids in food protein, especially those products processed
or concentrated to serve as protein supplements.
Questions About Your Protein Supplement
In the last article on digestibility of protein, we
suggested that it is appropriate to ask what method of drying was used
to concentrate your dietary protein supplement and what raw material
was used and where it came from. Information should also be made available
on the digestibility of the protein concentrate - in the bottle, not
of the raw material used to prepare the concentrate. Similar data on
handling and processing should be made available on all protein supplements
in the marketplace if an inquiry is made.
How about protein quality? Most protein products are
accompanied by information on the amino acid profile - sometimes we
even see data on PER. But how about data on utilization of the protein
provided by tests such as Biological Value or Net Protein Utilization
or Amino Acid Balance?
Serious, responsible processors should gladly make available
this kind of information. Unfortunately, it is often quite difficult
to get. Information of this nature about the protein supplement Seacure¨
is freely available by calling Proper Nutrition, Inc., at 1-800-555-8868.
Next time we'll discuss protein requirements: what they
are and just how we obtain them.