CYP1A1 and obesity due to melanocortin 4 receptor deficiency: These results suggest that the effect on liver metabolism by Western diet via AHR signaling is not predominately through the Cyp1 xenobiotic pathway (Lahvis et al. 2000) and that some component(s) in Western diet affects the AHR to directly and/or indirectly induce nonxenobiotic signaling, which in turn, causes large changes in liver metabolism that can affect the propensity for obesity.