Protein Quality

Since the pattern of the essential AA required by humans is not congruent with that of food proteins, proteins supplied exogenously are always of "lesser quality" (<1.00). It is irrelevant whether large amounts of several AA are found in the respective protein; the determining factor is that the body can only newly synthesize as much protein as the available amount of the most deficient AA permits. This synthesis-limiting AA is called the limiting amino acid. In the past, a number of approaches were used for purposes of practical evaluation of proteins. Net protein utilization (NPU) is based on N balance, usually determined in animal experiments. For this purpose, animals have to first be fed a protein-free diet in order to determine endogenous N production via mandatory N losses in feces and urine. Subsequently, the protein to be investigated is supplied at increasing levels until homeostasis of N balance is reached. With a protein of "high-value" this is achieved more quickly than with a protein of a lesser NPU. Biological value (BV) represents a refinement compared to NPU. The protein supplied is no longer assessed solely via its N content, but its digestibility is taken into account as well. Protein efficiency ratio (PER) used to be the prescribed standard procedure, especially in the U.S. For its determination, weight gain in young rats is related to their protein intake. Each of these methods has its disadvantages: the transferability of the animal experiments' results is not always insured, N balances do not capture all parameters, and they are costly and work-intensive. Additionally, each of these methods yields different results. Sometimes in the literature only one of these methods is mentioned, but values listed are derived by either of them, at times causing them to vary considerably.

In 1990, the FAO/WHO issued a report in order to introduce a method of worldwide validity for protein evaluation, specifying an exact method. The method is based on the definition of an ideal AA pattern (A), to which the AA contents of a given protein can be compared (all values in mg/g). The resulting amino acid score (AAS) is then corrected by the "true protein digestibility" that continues to be derived from animal experiments and has to be looked-up in the respective tables. Even this method still has some drawbacks: ideal AA patterns, as well as AA availability after heat and other treatments, remain points of contention. However, since AA analysis has become routine by now, it provides a fast and cost-effective method, particularly for developing countries.

Comparison of the resulting protein digestibility-corrected amino acid scores (PDCAAS) with commonly used BV values (B) show that animal products and soy achieve better values with PDCAAS, while other plant foods get lower scores. In plants, Lys tends to be the limiting AA. Requirements for Lys are set relatively high and will probably be adjusted downward for adults.

A. PDCAAS for Protein Evaluation

Pdcaas Value

AAS score

Limiting AA: tryptophan Actual protein digestibility for pinto bean in rats: 73 % Corrected AAS score: PDCAAS = 0.8 x 0.73 = 0.58

His Ile Leu dl Requirement (mg/g protein) dl Pinto bean (mg/g protein) I I AAS score

Lys Met + Cys Phe+Tyr Thr Try Val

Pinto bean Requirement

Eggs Milk Beef Wheat Soy Corn Rice Potatoes Beans

0.86

1.00

0.87

0.92

0.40

0.59

0.84

0.51

0.74

0.83

0.62

0.95

0.73

I I Protein Digestibility Corrected Amino Acid Score PDCAAS, according to FAO/WHO report, 1990

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