In mice, a hormone called FGF21 helps turn on liver genes that burn fat and make sugar when food is scarce — without it, those genes don’t turn on as much.

What we've found so far does not support the idea that FGF21 regulates liver genes for fat burning and sugar production during fasting in mice. Our current analysis shows the evidence leans against this claim. We reviewed 13 assertions about the role of FGF21 in regulating liver genes during fasting in mice. All 13 refute the idea that FGF21 is necessary for turning on genes involved in fat burning and sugar production in the liver under fasting conditions [1]. While one assertion suggested FGF21 helps activate these genes, our analysis of the evidence behind it found no supporting data — only refuting evidence [1]. This means that in the studies we examined, mice without FGF21 still showed normal or near-normal activation of these liver genes during fasting. Our analysis does not rule out any role for FGF21 in metabolism, but based on what we've reviewed so far, it does not appear to be a key regulator of these specific processes in the mouse liver during fasting. There may be other hormones or signals at work, but we cannot determine that from this evidence alone. We recognize this is a narrow question and our understanding could change as more data becomes available. Right now, the balance of evidence we’ve analyzed points away from a major role for FGF21 in this specific function. Practical takeaway: In mice, the hormone FGF21 may not be as important as once thought for turning on liver genes that burn fat or make sugar when food is missing.