This study was conducted to investigate the effect of different dietary ratios of 13C to 12C or 15N to 14N on their relative incorporation into tissues. Eighty male rats were used in two 21-day feeding trials in which they were fed diets with either high δ13C levels (δ13C = 13.89% and δ15N = 2.37% in experiment 1 and δ13C = 19.34% and δ15N = 4.73% in experiment 2) or low δ13C levels (δ13C = 17.90% and δ15N = 3.08% in experiment 1 and δ13C = 21.76% and δ15N = 0.53% in experiment 2), meanwhile, the dietary δ15N levels were designed to two ranks. Blood, liver, adipose and muscle tissues were collected on day 0, 3, 7, 14, and 21 for determination of 13C, 12C, 15N and 14N isotopes. Rat growth rate, antioxidant capacity and metabolic parameters were also assessed. The results indicate that adipose tissue tend to deplete 13C before the stable isotopic ratios achieved final equilibrium. Therefore, feeds with different isotopic signatures had different incorporation rates into tissues. Low dietary 13C levels decreased tissue δ13C values whereas high dietary 13C levels did not alter tissue δ13C values during the 21-d experiment. Blood δ15 N values were a reliable parameter in assessing the relative contribution of dietary nitrogen to tissues. This study revealed a relationship between dietary isotopic signatures and their incorporation rates into rat tissues. However, more studies are needed to illustrate the mechanism through which dietary isotopic ratios influence the extent of isotopic incorporation into the tissues.
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