Blocking a hormone called FGF21 in fasting mice doesn’t really change their blood sugar, energy levels, or liver fuel stores — meaning this hormone might not be essential for how their bodies adapt to not eating for a short time.

What we've found so far is that blocking FGF21 in fasting mice does not appear to significantly affect key aspects of their metabolism. The evidence we've reviewed suggests this hormone may not be essential for short-term fasting adaptations in these animals. Our analysis of the available research shows that when FGF21 is blocked in mice during fasting, measures like blood sugar levels, energy use, and liver fuel stores remain largely unchanged [1]. This finding comes from one assertion supported by 13.0 studies, with no studies indicating a contrary effect. Since FGF21 has been thought to play a role in metabolic regulation during nutrient scarcity, this result suggests its function might be less critical than previously assumed—at least in the context of short-term fasting in mice. We want to be clear: our current analysis does not prove FGF21 is unimportant, nor does it rule out roles in longer fasting periods or other physiological processes. What we've found so far simply indicates that core metabolic responses to fasting are still present even when this hormone is blocked. There may be subtle effects not captured by the measures reviewed, or roles in other tissues or conditions not yet studied. Because the evidence is limited to one assertion—albeit one backed by substantial study support—we remain cautious in our interpretation. The data we've reviewed leans toward the idea that FGF21 isn't required for basic metabolic adaptation to short-term fasting in mice, but we don't have enough to say how it might matter in other contexts. Practical takeaway: In mice, turning off FGF21 during a fast doesn’t seem to throw off their metabolism in the ways we’ve measured. That could mean the body has backup systems in place when food is scarce.