Irisin Levels Are Lower in Young Amenorrheic Athletes Compared with Eumenorrheic Athletes and Non-Athletes and Are Associated with Bone Density and Strength Estimates

Abstract

Irisin and FGF21 are novel hormones implicated in the “browning” of white fat, thermogenesis, and energy homeostasis. However, there are no data regarding these hormones in amenorrheic athletes (AA) (a chronic energy deficit state) compared with eumenorrheic athletes (EA) and non-athletes. We hypothesized that irisin and FGF21 would be low in AA, an adaptive response to low energy stores. Furthermore, because (i) brown fat has positive effects on bone, and (ii) irisin and FGF21 may directly impact bone, we hypothesized that bone density, structure and strength would be positively associated with these hormones in athletes and non-athletes. To test our hypotheses, we studied 85 females, 14–21 years [38 AA, 24 EA and 23 non-athletes (NA)]. Fasting serum irisin and FGF21 were measured. Body composition and bone density were assessed using dual energy X-ray absorptiometry, bone microarchitecture using high resolution peripheral quantitative CT, strength estimates using finite element analysis, resting energy expenditure (REE) using indirect calorimetry and time spent exercising/week by history. Subjects did not differ for pubertal stage. Fat mass was lowest in AA. AA had lower irisin and FGF21 than EA and NA, even after controlling for fat and lean mass. Across subjects, irisin was positively associated with REE and bone density Z-scores, volumetric bone mineral density (total and trabecular), stiffness and failure load. FGF21 was negatively associated with hours/week of exercise and cortical porosity, and positively with fat mass and cortical volumetric bone density. Associations of irisin (but not FGF21) with bone parameters persisted after controlling for potential confounders. In conclusion, irisin and FGF21 are low in AA, and irisin (but not FGF21) is independently associated with bone density and strength in athletes.

Irisin: A Potential Link between Physical Exercise and Metabolism—An Observational Study in Differently Trained Subjects, from Elite Athletes to Sedentary People

“…In our study, we evaluated the concentrations of irisin in a population of athletes who regularly perform different loads of physical activity and we did not find significant differences in the basal concentrations of this hormone in the different groups studied. These data seem to be similar to results of another study performed on adolescent athletes in a paper of Singhal and colleagues [23]. In this study irisin was compared in two groups (eumenorrheic athletes and nonathletes, resp.)…”

The myokine irisin increases cortical bone mass

“…There is also an inverse correlation between serum irisin levels and vertebral fragility fractures in postmenopausal women (45,46). In contrast, irisin levels in athletes positively correlate with BMD (47). In addition, we provide in vitro evidence for an early up-regulation of Lrp5 and β-catenin, both of which may play key roles in stimulating Wnt signaling.…”

Role of Irisin on the bone–muscle functional unit

“…43 Likewise, post-menopausal women showed inverse correlation between Irisin levels and vertebral fragility fractures. 44,45 Conversely, Irisin measured in athletes was positively associated with bone density and strength, 46 supporting the idea that Irisin could have a protective role on bone health. Unfortunately these studies, as many other human studies in which Irisin levels were measured by ELISA assays, have been debated because of poor antibody specificity that may have given artifacts.…”