The effect of enzymatic hydrolysis of a dietary protein on the excretion of urinary nitrogen metabolites : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in Nutritional Science at Massey University, Palmerston North, New Zealand
Hydrolysed milk proteins are used for many purposes in human nutrition. Although it is assumed that the nutritive value of a protein hydrolysate is the same, or even superior to the corresponding intact protein, there is limited research available to support this assumption. The aim of this study was to compare amino acid utilisation and the pattern of excretion in the urine of the nitrogenous metabolites (urea, ammonia and creatinine) as an immediate response to the ingestion of a meal containing an intact protein or its enzymatic hydrolysate. This involved a novel technique, 'acute urine collection' (AUC), in which urine was drained from the bladder at short time periods (30 min to 2 hr) through a catheter. The performance and nitrogen balance results indicated that the two sources of amino acid were equally effective in supporting nitrogen retention and growth of the pigs. Nevertheless, the pattern of excretion of the metabolites of nitrogen digestion suggested important differences in the metabolism of the pigs on the two diets. Both groups of pig excreted creatinine nitrogen, at constant and comparable rates over the sampling period indicating similar rates of catabolism in the muscle. The total excretion of nitrogen by AUC by the two groups was similar but the pattern of excretion over the day differed which indicated a difference in the metabolism of the amino acids in the diets. This may have been in part due to a more rapid absorption of amino acids from the hydrolysed diet and in part due to a higher rate of glutamine and asparagine breakdown in the gut of pigs fed the hydrolysate. Excretion of nitrogen as urea and ammonia was similar for the two groups but there were differences between the groups in the pattern of excretion of these metabolites. In addition, the excretion of ammonia was significantly lower (P <0.0001) in the pigs fed the hydrolysate. This was due to a higher content of fixed cations in the diet containing the hydrolysate that led to a compensatory reduction in ammonia excretion. There was a proportional increase in the excretion of urea in the pigs on the hydrolysed diet as a result of the reduction in ammonia excretion but the differences were small relative to the total urea excretion and not significant. AUC not only gives comparable information to the nitrogen balance if it is carried out over a 24 hr period but it also provides detailed information about the protein utilisation during the immediate postprandial period. In particular, AUC can indicate differences and/or similarities in protein absorption by allowing the observation of the pattern of production of urea directly related to the catabolism of dietary amino acids. In addition, it may be possible to use this technique to estimate the optimum time between meals.