Giving a specific protein (FGF21) to mice that can't make it themselves helps them process sugar better and regain healthy fat levels, especially when they're on a bad diet.

What we've found so far is that restoring FGF21 in mice that can't produce it on their own may improve how their bodies handle sugar and help normalize fat levels, especially when they're eating a poor diet [1]. Our current analysis shows the evidence leans toward this effect. We analyzed the available research and found 43.0 supporting assertions and no studies that refute the idea that giving FGF21 to these mice—known as FGF21 knockout mice—can reverse insulin resistance and restore fat mass [1]. These mice are genetically altered so they don’t make FGF21, a protein involved in metabolism. When researchers give them this protein, the evidence shows improvements in insulin sensitivity and fat tissue levels, particularly when their diet is unhealthy [1]. Our analysis of the data suggests that replacing FGF21 may play a role in restoring metabolic balance in these animals. The effects appear more pronounced under conditions of dietary stress, such as when the mice consume a high-fat or otherwise poor-quality diet [1]. However, all the current evidence we’ve reviewed comes from animal studies, and we don’t have data on whether this would apply to humans. We want to be clear: this is what we’ve found so far, based on limited evidence focused entirely on mice. We can’t say how this might translate to people, or whether FGF21 treatments would have similar effects in humans with insulin resistance or weight concerns. Practical takeaway: In mice that can’t make FGF21, adding it back seems to help their bodies manage sugar and fat better—especially when their diet is unhealthy. But we don’t yet know what this means for human health.