Modelling of protein turnover provides insight for metabolic demands on those specific amino acids utilised at disproportionately faster rates than other amino acids

Abstract

The nitrogen balance is regulated by factors such as diet, physical activity, age, pathogenic challenges, and climatic conditions. A paradigm was developed from published recommended rates of protein intake (g/kg/day) with corresponding rates of endogenous protein turnover and excretion, to extrapolate amino acid balances under various conditions. The average proportions of amino acids in the ingested proteins representing a well-balanced diet were used to assess intake and an average human composition profile from five major high-turnover proteins in the body to assess endogenous protein turnover. The amino acid excretion profiles for urine and sweat were constructed for males and females from published data. The model calculated the nitrogen balances for a range of amino acids to determine the amino acid requirements to support daily exertion. Histidine, serine, glycine, and ornithine were in negative balances in males and females and this potential deficit was greater in the higher body-mass ranges. Conversely, leucine, isoleucine, and valine were conserved during nitrogen flux and resulted in positive balances. The model was run under a scenario of high demand for the synthesis of IgG during a response to an infectious challenge which indicated that these were increased requirements for tyrosine, threonine, and valine. It was concluded that these amino acids represent points of limitation to anabolic metabolism by restriction of their supply at critical times of demand. This would especially occur under conditions of fitness training, maintaining intensive exercise regimes, facilitating responses to pathogenic challenge, or recovery from injury.

Modelling of amino acid turnover in the horse during training and racing: A basis for developing a novel supplementation strategy

“…The higher concentrations in the human urine profile were largely catered for by the very high levels of histidine and glycine which were present at levels >1,000μmoles/ L. This would be predicted to have a significant impact on the protein turnover modelling, leading to a different profile of amino acids required for replenishment in horses compared with that proposed for humans [1]. Histidine is essential for both species, but humans can derive larger quantities of this component via the high proportions of carnosine (β-alanyl-Lhistidine) present in meat [1,49,50]. As herbivores, the horses seem to have developed a more efficient system of preventing losses of key amino acids such as histidine and glycine via urine excretion pathways (Table 4).…”

“…Pre-exercise, pre-feeding urinary amino acid concentrations (n = 4) with corresponding percentage relative abundances for Standardbred horses. Amino acids have been listed in order from the highest to lowest urinary concentrations 1 .…”

“…Horse Pre-exercise Urine Concentration (μmole 1 These values have been compared with human equivalent data collected from a previous study from 101 males and 52 females [48]. The human data were recalculated to present the average concentrations found in the combined group of males and females.…”

“…Assessment of the nitrogen balances of individual amino acids to establish whether they may be in deficit or in positive balance is critical to determining whether supplementation of specific amino acids may provide physiological benefits in horses. This approach has been successfully developed in the human context where it was found that histidine, serine, glycine and ornithine were used at disproportionately faster rates than the other amino acids during exercise and were thus potentially limiting under periods of high intensity training [1]. Horses excrete different proportions of amino acids in their sweat and urine compared with humans and thus require a specific approach to developing an equine model to evaluate nitrogen balance for amino acids.…”

“…In addition, some essential amino acids such as histidine may be oxidised or excreted at higher rates than others and must be replenished in the diet. Several amino acids have numerous roles in different metabolic pathways including energy metabolism and generating important complex metabolites, neurotransmitters and hormones [1,19].…”