A concept of dietary dipeptides: a step to resolve the problem of amino acid availability in the early life of vertebrates

The premise that a dietary dipeptide approach will improve the understanding of amino acid utilization in the fastest-growing vertebrate, the teleost fish, was tested by examining the muscle free amino acid (FAA) pool and enzyme activities, in concert with growth response, when dietary amino acids were provided in free, dipeptide or protein molecular forms. We present the first evidence in fish that, in response to a synthetic dipeptide diet, muscle FAA varies as a result of both growth rate and amino acid availability of specific peptides. We demonstrate significantly diminished muscle indispensable FAA (3-10-fold) in rainbow trout alevins fed a dipeptide-based diet compared with a protein-based diet. The dipeptide-based diet did not contain proline, resulting in 10-27-fold less muscle free proline and hydroxyproline in alevins. The response of alevins fed FAA-based or peptide-based diets can be indicative of collagen turnover (Hyp/Pro ratio) and showed significant differences between dietary treatments. Pyrroline-5-carboxylate (P5C) reductase activity was detected, suggesting that P5C may ameliorate proline deficiency, but synthesis from glutamate could not maintain free proline levels in muscle. This finding will provide an impetus to test whether proline is conditionally indispensable in young fish, as in mammals and birds. This study shows that amino acids given entirely as dipeptides can sustain fish growth, result in muscle FAA and enzyme responses in line with dietary levels and identify growth-limiting amino acids. The understanding of these factors necessitates a diet formulation that will improve the accuracy of determining amino acid requirements in the early life stages of vertebrates.