In people with obesity who still respond well to insulin, higher levels of a hormone called FGF21 are linked to having more fat under the skin, but not around the organs. This link wasn’t seen in people whose bodies don’t respond well to insulin.

What we've found so far is that higher levels of the hormone FGF21 are linked to greater amounts of subcutaneous fat — the fat stored under the skin — in people with obesity who remain insulin-sensitive [1]. This connection does not appear to exist in those with insulin resistance, and no studies in our analysis found the opposite to be true. Our current analysis of the available research shows a consistent pattern: in individuals with obesity who still respond well to insulin, elevated FGF21 levels are associated with increased subcutaneous fat, but not with fat stored around internal organs (visceral fat) [1]. This suggests that FGF21 may play a role in how the body stores fat under the skin in people who maintain healthy insulin function, even when they carry excess weight. However, this link disappears in people whose bodies no longer respond effectively to insulin, indicating that insulin sensitivity may influence how FGF21 interacts with fat distribution. The evidence we've reviewed leans toward a connection between FGF21 and subcutaneous fat in this specific group — insulin-sensitive individuals with obesity — but we don’t yet know whether FGF21 causes this fat storage pattern or is simply a marker of it. There are no studies in our dataset that contradict this observation, but the total number of assertions analyzed is limited. Because our understanding is still evolving, we can’t say for sure what this means in terms of health outcomes or whether changing FGF21 levels would affect fat distribution. Practical takeaway: If you have obesity but your body still responds well to insulin, your FGF21 levels might influence where you store fat — particularly under the skin rather than around organs. But we don’t yet know how to use this information to improve health.