['Isotopic turnover quantifies the metabolic renewal process of elements in organs and excreta. Knowledge of the isotopic turnover of animal organs and excreta is necessary for diet reconstruction via stable isotope analysis, as used in animal ecology, palaeontology and food authentication. Effects of dietary protein content on the isotopic carbon and nitrogen turnover (i.e. delay, representing the time between ingestion and start of renewal, and half-life) are unknown for most mammalian organs and excreta.', 'To examine the effect of dietary protein content on turnover (delay and turnover rate), we fed 18 rats either a diet at protein maintenance or above protein maintenance, and quantified their isotopic carbon and nitrogen turnover in ten organs and excreta. These included the excreta faeces and urine, the visceral organs blood plasma, liver, kidney, lung and spleen, the cerebral tissue brain, and the muscular tissues heart and muscle. For data analysis, we used piecewise linear/non-linear exponential modelling that allows quantifying delay and turnover rate simultaneously.', 'Delays were ~0.5\u2009days for carbon and nitrogen turnover and were not affected by dietary protein content. Half-lives during the following reaction progress were in the range of 1 to 45\u2009days, increasing from excreta to visceral organs to muscular and cerebral organs. Rats fed the higher protein amount had 30% shorter nitrogen half-lives, and 20% shorter carbon half-lives.'] Does dietary protein content affect isotopic carbon and nitrogen turnover?