D-amino acid content of feed

A review

Abstract

The most important source of D-amino acids in nutritional protein is the processing that some foods undergo, either in cooking or as part of the manufacturing process used to prepare commercial food products. Supermarkets contain increasing quantities of processed food products, including breakfast cereals, fried potato and corn chips, liquid and powdered infant formulas, and meat substitutes. Such products probably contain significant quantities of D-amino acids, coupled with the evidence that these D-amino acids most likely have deleterious or negative nutritional effects. Alkali treatment of proteins catalyses racemization of optically active amino acids. The racemization rates vary among proteins but it is observed that, within each protein studied, the relative order is similar. Factors which influence racemization include pH, temperature, time of exposure to alkali, and the inductive nature of amino acid side chains. Protein-bound D-amino acids formed during alkali and heat treatment of food proteins may adversely affect the nutritional quality and safety of processed foods. D-amino acids in dietary proteins reduce the digestibility as well as the availability of the component amino acids. This may be the result of decreased amounts of essential amino acid L-enantiomers, decreased digestibility through peptide bonds not susceptible to normal peptidase cleavage, specific toxicity of certain D-isomers, and/or modification of the biological effects of lysinoalanine or other unnatural amino acids. On the other hand, certain D-amino acids may be beneficial. The limited digestibility of D-amino acids in dietary proteins can be utilised in nutrition, for example in weight management or chronic pain control.