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Protein Fundamentals - Part 3 Quality Determinants

by Donald G. Snyder, Ph.D.

This article is sponsored by Proper Nutrition, a proprietary company

    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 quality.

    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 of protein).

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 thus:

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 are used.

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 animal.

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 acid balance.

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.


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