Irisin

Irisin is a 112-amino acid peptide hormone cleaved from the extracellular domain of the transmembrane protein FNDC5 (fibronectin type III domain-containing 5). It was discovered in 2012 by Bruce Spiegelman's laboratory at Harvard University and named after the Greek messenger goddess Iris. Irisin was identified as a myokine — a hormone released from skeletal muscle during exercise — that mediates some of the beneficial metabolic effects of physical activity.

The primary discovered function of irisin is the induction of "browning" of white adipose tissue. When irisin acts on white fat cells, it upregulates the expression of uncoupling protein-1 (UCP1) and other thermogenic genes, converting them into "beige" or "brite" (brown-in-white) adipocytes that dissipate energy as heat rather than storing it. This thermogenic effect increases total body energy expenditure and was proposed as a mechanism by which exercise confers metabolic benefits independent of caloric expenditure during the activity itself.

Since its discovery, research on irisin has expanded dramatically. Studies have linked irisin to improvements in glucose homeostasis, enhanced cognitive function, bone metabolism, and cardioprotective effects. In bone, irisin has been shown to stimulate osteoblast differentiation and promote cortical bone formation in mouse models. In the brain, irisin may enhance brain-derived neurotrophic factor (BDNF) expression in the hippocampus, potentially contributing to the cognitive benefits of exercise.

However, irisin has also been the subject of scientific controversy. Questions have been raised about the accuracy of some irisin detection assays, the true circulating concentrations in humans, and whether the original findings have been fully replicated. Mass spectrometry-based studies have confirmed irisin's existence in human plasma, though at lower concentrations than initially reported by immunoassay-based methods.