Together these observations suggest that CR regulates IGF-1 expression downstream of GH. The mechanism of CR-induced regulation of IGF-1, however, remain unknown. Recently, fibroblast growth factor 21, a novel endocrine-like member of the FGF superfamily highly expressed in the liver, has been implicated as a negative regulator of IGF-1 expression and has also been reported to extend lifespan in mice when over-expressed. Indeed, FGF21 transgenic mice are smaller, exhibit reduced hepatic GH sensitivity downstream of JAK2, have reduced circulating IGF-1 levels and exhibit a 36% increase in median lifespan relative to WT controls. In addition, treatment of WT mice with recombinant human FGF21 reduces circulating IGF-1 levels, while treatment of chondrocytes with FGF21 attenuates GH-induced IGF-1 mRNA expression. Furthermore, long-term CR in mice and severe CR in young developing mice increases FGF21 expression relative to AL controls. Importantly, in studies using whole-body FGF21-knockout mice, FGF21 is required to permit the full undernutrition-related reduction in both hepatic mRNA and circulating IGF-1 levels. Interestingly, earlier work demonstrated that circulating FGF21 and hepatic FGF21 mRNA levels are rapidly increased in response to fasting in a peroxisome proliferator-activated receptor alpha -dependent manner. However, the effect of moderate CR in adult mice on FGF21 expression and the role of FGF21 in mediating the IGF-1 and cell proliferation responses to this CR regimen have not been previously explored. Accordingly, the goal of the present work was to determine the effect of moderate CR on GH secretory dynamics, hepatic GH signaling and FGF21 expression in adult C57BL/6 male mice. In addition, we sought to determine whether FGF21 is necessary for the reductions in circulating IGF-1 levels and cell proliferation rates in response to this CR regimen. Future studies should focus on other factors that may regulate changes in IGF-1 expression in response to moderate short-term CR in adult WT mice, including protein intake, insulin and thyroid hormone. Interestingly in humans, long-term CR does not reduce circulating IGF-1 levels and a very low calorie diet in obese diabetics KRX-0401 actually reduces circulating FGF21 levels, with the caveat that baseline circulating FGF21 levels are elevated in this population. More studies are needed to confirm and clarify the effect of varying degrees of CR on circulating IGF-1 and FGF21 levels and the potential interplay between these two hormones in healthy humans. Despite the lack of an effect of FGF21 on the IGF-1 and cell proliferation responses to moderate CR in adult mice, FGF21 may have an important role in other responses to this CR regimen. FGF21 is highly expressed in the pancreas and has well-established glucose-lowering and insulin-sensitizing effects in vivo.