Quality and Sources

B Torun, Center for Research and Teaching in Latin America (CIDAL), Guatemala City, Guatemala

© 2005 Elsevier Ltd. All rights reserved.

The amino acid composition of food proteins and the efficiency with which they are digested to allow amino acid absorption determine their capacity to provide nitrogen and essential amino acids for human growth and functions. This capacity, known as protein quality, influences dietary requirements: The lower the quality, the higher the required dietary protein intake. The nutritive value of food proteins is also influenced by protein concentration and the bioavailability of its amino acids. The latter can be affected by some forms of food storage and processing.

This article examines the ways of assessing the protein quality of foods and diets and the quality inherent to various protein sources. The following operational terms are used:

• Protein (or nitrogen) digestibility: The proportion of dietary nitrogen that is absorbed. 'True' protein digestibility is calculated correcting for endogenous or obligatory fecal nitrogen losses (i.e., nitrogen in epithelial cells, gastrointestinal secretions, and intestinal flora) (Table 1).

• Nitrogen balance (NB): The average amount of nitrogen that is retained or lost from the body. It is calculated from measurements of dietary, urinary, and fecal nitrogen and estimates of integu-mental (sweat, skin, nails, and hair) nitrogen losses (Table 1).

• Essential amino acids (EAA) (also called 'indispensable amino acids'): Amino acids that the diet must provide because humans cannot synthesize them from other components at a rate commensurate with normal bodily needs.

• Amino acid scoring pattern: Amino acid composition of a hypothetical reference protein that

Table 1 Calculation of operational definitionsa

Definition

Calculation

Table 1 Calculation of operational definitionsa

Definition

Calculation

Apparent digestibility

In - Fn In

True digestibility

In - (Fn - Fe) In

Nitrogen balance

In - Un - Fn - Integn

Amino acid score

mg of EAA in 1 g of food protein mg of EAA in 1 g of reference protein (or EAA scoring pattern)

Limiting amino acid

EAA with a score <1.00 (or <100%)

aDigestibility and amino acid scores can be expressed as fractional values (<1.00) or multiplied by 100 and expressed as percentages.

EAA, essential amino acid; FE, endogenous fecal nitrogen; FN, total fecal nitrogen; lN, nitrogen intake; IntegN, integumental nitrogen; UN, total urinary nitrogen.

aDigestibility and amino acid scores can be expressed as fractional values (<1.00) or multiplied by 100 and expressed as percentages.

EAA, essential amino acid; FE, endogenous fecal nitrogen; FN, total fecal nitrogen; lN, nitrogen intake; IntegN, integumental nitrogen; UN, total urinary nitrogen.

contains all EAAs in the amounts necessary to satisfy requirements.

Amino acid scoring procedure: Calculation of the proportion of each EAA in a protein or diet relative to the scoring pattern (Table 1). It can be expressed as percentage or as a fractional value. Limiting amino acids: EAAs in food proteins that are present in lower proportions than in the reference protein (i.e., with fractional value <1.00, relative to the reference protein) (Table 1). Amino acid score (or 'chemical score'): Value of the limiting amino acid with the lowest score in a protein (i.e., the 'most limiting amino acid'). A protein is assigned a percentage score of 100 (or a fractional score of 1.00) when none of its EAAs are limiting.

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