Importance of Protein Quality

Consideration of protein quality is highly important when restricting an individual’s daily food intake. The diet must continue to provide the proper balance of amino acids and nitrogen essential for the body to satisfy protein synthesis for growth and maintenance. A high quality protein provides all of the essential amino acids in the proper balance. This is vital because a diet containing any limiting amino acids would have restricted protein synthesis since specific amino acid sequences of a protein could not be formed due to the deficiency of any one amino acid despite an excess of other amino acids.

For many years the standard classification of protein quality was by Protein Efficiency Ratio (PER). This value measured the weight gain of a growing animal relative to the protein intake. Following strict procedural control, laboratory animals were fed a diet with a test protein and the resulting weight gain determined. Some typical food product PER values are listed below.

Casein 2.5
Soy Protein 1.2
Whey Protein 3.2
Wheat 0.9
Egg 3.2
Rolled Oats 1.1

However, as more was learned about actual human amino acid needs PER was found to over estimate the value of some animal proteins and underestimate the value of some vegetable proteins. This occurred since the test animal had a more rapid growth rate and therefore increased need for certain essential amino acids compared to human growth rates and human amino acid needs. Due to these findings the method for evaluating protein quality has shifted from Protein Efficiency Ratio (PER) to Protein Digestibility-Corrected Amino Acid Scoring (PDCAAS). The 1993 Nutrition Labeling Regulations of the FDA use the PDCAAS method for the evaluation of protein quality. The PDCAAS of a food protein is used to determine the Percent Daily Value (%DV) when listed on the nutrition panel.

The PDCAAS method considers three parameters to determine protein quality:

1. The essential amino acid profile of the protein (determined analytically)

2. The digestibility of the protein/bioavailability of the amino acids (by animal studies)

3. The ability of the protein to supply the essential amino acids in the amounts
required by humans (determined by calculation vs. the WHO reference pattern)

For labeling purposes the amino acid content of the protein is compared to the World Health Organization (WHO) suggested amino acid pattern for children 2-5 years old since it is the most demanding pattern of any age group other than infants. The amino acid with the lowest percent vs. the reference pattern is the limiting amino acid and is termed the proteins Amino Acid Score. To derive the PDCAAS of the protein the amino acid score is multiplied by the percent digestibility. If the value is >1.0 the PDCAAS is considered to be 1.0 since the excess is not utilized as essential amino acids.

PDCAAS of Selected Protein Sources:

Casein 1.0
Soy Protein Isolate 1.0

Whey Protein 1.0
Wheat 0.40

Egg 1.0
Rolled Oats 0.57

Meat, Fish, Poultry 1.0

Daily food intake on a restricted diet will frequently utilize foods with high quality protein to assure the daily protein requirements are meet. Such proteins typically have a PDCAAS of 1.0 or a PER values of at least 2.5 thereby meeting or exceeding the World Health Organization’s reference protein amino acid content. High quality proteins are generally of dairy or animal origin such as milk and eggs or meat, fish and poultry. Foods with protein from unrefined vegetable or grain sources [other than soy] are typically considered less suitable as primary protein sources for individuals on a restricted diet, since such foods provide lower quality protein with several essential amino acids present at limiting levels.

Additional Information:

Protein Digestibility Values:

Casein 99%
Soy Protein 97%
Whey Protein 99%
Wheat, refined 96%
Egg 97%
Rice 75%

On a calorie restricted diet is important that there be no limiting amino acids because once that amino acid is depleted, protein synthesis in the body is restricted. Without sufficient proportions of essential amino acids available, specific amino acid sequence of a protein can not be synthesized despite an excess of other amino acids.

References:
Protein Quality Evaluation. Report of a Joint FAO/WHO Expert Consultation. Food and Agricultural Organization of the the United Nations, Rome, 1990.

Henley EC and Kuster JM. Protein Quality Evaluation by Protein Digestibility-Corrected Amino Acid Scoring. Food Technology April, 1994.

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