Abstract
Cats apparently have a high requirement for dietary protein compared to omnivores and herbivores. This was attributed to metabolic inflexibility, due to the inability of the hepatic ureagenic, catabolic and gluconeogenic enzymes to adapt to protein intake. Other animals adapt to dietary protein by a variety of mechanisms including changes in ureagenesis, amino acid oxidation (hereafter referred to as protein oxidation) and protein turnover. Two of these mechanisms were examined in vivo in the cat; protein turnover and ureagenesis, in cats fed either moderate protein (MP; 20% dietary energy as protein) or higher protein (HP; 70% dietary energy as protein) diets. The single dose end product method was used to measure whole body protein turnover. Protein synthesis was higher (p<0.05) on the HP diet (75±10 mmolN/kgBW/d, 6.6±0.9g protein/kgBW/d) than the MP diet (38±5 mmolN/kgBW/d, 3.3±0.4g protein/kgBW/d). Protein breakdown was higher (p<0.05) on the HP diet (72.2±8 mmolN/kgBW/d, 6.3±0.7g protein/kgBW/d) than the MP diet (44.3±3.1 mmolN/kgBW/d, 3.9±0.3g protein/kgBW/d). These results show that feline protein turnover adapts to dietary protein as has been shown in other species and does not support a theory of metabolic inflexibility. For the study of ureagenesis, a single dose of [15N15N] urea was administered, and a model applied to calculate ureagenesis. There was a significant relationship between nitrogen intake (mmolN/day) and urea production (mmolN/day) (simple linear regression, intercept=-25.76, slope=0.92, R2=94.92, p<0.0001). These results show that feline ureagenesis adapts to dietary protein as has been shown in other species and does not support a theory of metabolic inflexibility. Cats were shown to be metabolically flexible in the current thesis, adapting to dietary protein as do omnivores and herbivores. It is therefore unknown why cats require so much dietary protein, and alternative theories were suggested.