Digestion and Bioavailability

Z A Bhutta, The Aga Khan University, Karachi,

Pakistan

© 2005 Elsevier Ltd. All rights reserved.

Proteins are the principal nitrogenous constituents of the protoplasm of all animal and plant tissue, and it is estimated that almost half of the dry weight of animal cells is composed of proteins. Proteins are crucial for the synthesis of body tissues and regulatory proteins, and it is also recognized that approximately 90% of all cellular proteins are present as enzymes.

The basic structural units of proteins are the amino acids, which are characterized by the presence of an amino NH3 component and an acid or carboxyl group. Nitrogen thus comprises approximately 16% of all proteins by weight. Most naturally occurring amino acids are of the l configuration. These amino acids are in turn linked together by peptide bonds. Units of 2 or 3 amino acids are called dipeptides or tripeptides, respectively, whereas by convention any protein structure of less than 100 amino acid residues is called a polypeptide. The primary structure of a protein refers to the chains of amino acids constituting it, whereas the secondary structure is formed by the linkages between close amino acids by hydroxyl or sulfide bonds. More complex proteins have a tertiary structure due to the amino acids being held together by strong interatomic forces. The quarternary structure of a protein refers to the manner of association or binding between different units.

Dietary proteins are the major sources of protein intake and constitute on average approximately 10-20% of daily energy intake. In addition, they are the main sources for the essential amino acids, which cannot be synthesized by humans. Despite wide variations in dietary composition, the average daily protein intakes in different populations of the world range from 50 to 70 g, although it must be recognized that the intake may be much lower in deprived populations, in both qualitative and quantitative terms. Almost half of the total protein entering the gastrointestinal tract daily is derived from endogenous sources, mainly intestinal secretions and cellular desquamation. Salivary, gastric, biliary, pancreatic, and intestinal secretions contribute approximately 20-30 g per day, whereas desquamated villus epithelial cells contribute an additional 30 g, and a relatively smaller amount (2g) is derived from plasma proteins leaking into the lumen.

An intricate and coordinated system of digestion ensures that under normal conditions, approximately 95% of ingested protein is digested and absorbed.

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