Influence of dietary level of dispensable amino acids on nitrogen balance and whole-body protein turnover in growing pigs

A nitrogen balance and a whole-body protein turnover experiment was carried out with 12 growing female crossbred pigs, 45 kg bodyweight (BW) to study the effect of increasing dietary levels of dispensable amino acids (DAA, 5%, 9% and 14%) at constant contents of indispensable amino acids (7%). Whole-body protein turnover was determined using the end-product method after a single oral dose of ¹⁵N-glycine to a semisynthetic diet. Daily N intake was 1.45, 1.96 and 2.48g/kg BW⁷⁵ for low, medium and high DAA levels. The dietary protein was absorbed completely irrespective of treatment. Raising dietary DAA increased urinary N excretion and N retention (urinary N, 0.16, 0.44 and 0.80g/kg BW^0.75/day; N retention, 1.20, 1.42 and 1.58 g/kg BW⁷⁵/day). Whole-body protein synthesis and degradation varied from 4.31 to 7.44 g N/kg BW^0.75/day and from 2.88 to 6.24 g N/kg BW^0.75/day. The respective minimum values were observed at the medium DAA level. It appears that the coupling of anabolic processes of protein metabolism was maximal at medium dietary DAA contents.