It has thus been recommended that 0.75–0.8 g/kg/day would be a minimum recommended daily intake for adults, but would be higher in babies and children undergoing critical growth phases (Tessari 2006). Notwithstanding these variations, consensus evaluations of nitrogen turnover have been determined, whereby 0.6 g/kg/day of protein would be sufficient to achieve a zero nitrogen balance. The rates of utilisation depend on factors such as genetics as well as diet, physical activity, pathogenic challenges, and climate. Assessment of the nitrogen balance is difficult, as it needs to encompass protein turnover in tissues, organs and muscles as well as requirements for energy metabolism and the utilisation of amino acids in numerous biochemical pathways and regulatory systems. 1994 Fielding and Parkington 2002 Millward 2004 Millward et al. The study of nitrogen balance has been extensively researched, revealing complex relationships between various pools of metabolites that act to maintain metabolic support for homeostasis and exercise activities (el-Khoury et al. This would especially occur under conditions of fitness training, maintaining intensive exercise regimes, facilitating responses to pathogenic challenge, or recovery from injury. It was concluded that these amino acids represent points of limitation to anabolic metabolism by restriction of their supply at critical times of demand. 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. Conversely, leucine, isoleucine, and valine were conserved during nitrogen flux and resulted in positive balances. 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. The model calculated the nitrogen balances for a range of amino acids to determine the amino acid requirements to support daily exertion. The amino acid excretion profiles for urine and sweat were constructed for males and females from published data. 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. 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 nitrogen balance is regulated by factors such as diet, physical activity, age, pathogenic challenges, and climatic conditions.
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