Effect of dietary stable isotopic ratios of carbon and nitrogen on the extent of their incorporation into tissues of rats

Use of stable isotopes has attracted a great deal of interest in physiological and metabolic research as many researchers are unwilling to use radioactive isotopes [1]. The amount of carbon stable isotope 13？C varies between the C3 and C4 plants due to their use of different photosynthesis pathways while the abundance of stable nitrogen isotope 15？N in plants resembles that of their growing circumstance [2,3]. Since every feedstuff is characterized by its natural stable isotopic signature [4,5], the analysis of stable isotopes in tissues is proposed as a method to evaluate the relative contributions of nutrients from different feed sources to those deposited in tissues [6,7].Many models have been established to estimate the fractional contribution of various isotope profiles in the diet to those deposited in tissues including one compartment, two compartment, multi-compartment and mixed models [8]. However, the effect of different dietary ratios of 13？C to 12？C or 15？N to 14？N on their relative incorporation into tissues remains unknown.Numerous factors such as diet quality, nutritional status, body size, age, dietary ontogeny, tissue and elemental composition have been reported to affect the extent of deposition of 13？C and 15？N into tissues [9-12]. The relationship between discrimination factors and dietary isotopic ratios is consistent and dietary isotopic values have been shown to explain 51 % of the variation in isotopic discrimination [13] . In controlled experiments, dietary isotopic values explained 60 % to 98 % of the variation in isotopic discrimination in different tissues of rats [14]. The stable isotopic analysis, which has been used as an important tool for so many years, is conducted to investigate the turnover and deposition of nutrients from the macroscopic view. It is aimed at not a certain nutrient but the relationship between various nutrients. However, few studies have determined the effect of different dietary ratios of 13？C to 12？C or 15？N to