FGF21 (Fibroblast Growth Factor 21)

FGF21 is a hormone released by the liver, and to a lesser extent muscle and fat, in response to fasting and mitochondrial stress. In rodents it improves insulin sensitivity and extends lifespan, which led to the assumption that higher is better. In humans the direction of the association is the opposite: FGF21 rises with age and rises further in obesity, type 2 diabetes and mitochondrial disease, a pattern usually explained as tissue resistance to its signal.

Lynae Hanks, Orlando Gutierrez and Krista Casazza measured FGF21 across 184 healthy people aged 5 to 80 and found a stepwise rise: median 156.0 pg/mL in children aged 5 to 12, 211.0 in young adults aged 20 to 29, 267.0 in adults aged 30 to 50, 292.0 in older adults aged 55 to 64, and 358.5 pg/mL (IQR 238.5-481.0) in elder adults aged 65 to 80 [1]. The rise held independently of body composition indices [1].

Maria Conte, Claudio Franceschi and Stefano Salvioli profiled 693 subjects aged 21 to 113, split into 79 young adults aged 21 to 59, 336 elderly aged 61 to 79, 155 oldest-old aged 82 to 99 and 123 centenarians aged 100 to 113 [2]. Cluster analysis split the elderly into a low-mitokine group of 273 with median FGF21 284.1 pg/mL and a high-mitokine group of 63 with median 431.5 pg/mL, p = .000. The high group also had higher glycemia (103.2 vs 98.2 mg/dL, p = .012), higher insulin (10.0 vs 8.6 microIU/mL, p = .007), higher HOMA-IR (2.6 vs 2.0, p = .002) and higher CRP (1.3 vs 1.0 mg/L, p = .012) [2]. In nonagenarians the low-mitokine group had stronger handgrip (p = .007) and longer estimated survival (p = .000) [2].

How it is measured: ELISA on plasma or serum. Values are not standardised across kits, and fasting status matters because FGF21 is a fasting-response hormone. Leila Motlagh Scholle and Stephan Zierz reviewed its use as a diagnostic marker for mitochondrial myopathy, where it is compared head to head with GDF-15 [3].

How to move it: nothing has a clean randomised answer. Because elevated FGF21 in humans marks metabolic stress rather than a deficiency state, the target is the upstream driver. Berberine cut HbA1c by 0.99 percentage points (95% CI -1.16 to -0.83, P < 0.001) over 12 weeks in the 409-person PREMOTE trial, which addresses the metabolic state FGF21 reports on, though that trial did not measure FGF21 [4].

Read FGF21 as a stress gauge, not a longevity dial.