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.
Highlights
Irisin and FGF21 are novel hormones implicated in the modulation of energy homeostasis [1], and more recently with bone metabolism
Percent body fat was lower in both groups of athletes versus non-athletes and in amenorrheic athletes (AA) compared to eumenorrheic athletes (EA)
We found that irisin levels were lower in AA compared to EA and non- athletes, and these differences persisted after controlling for fat and lean mass
Summary
Irisin and FGF21 are novel hormones implicated in the modulation of energy homeostasis [1], and more recently with bone metabolism. A recently discovered myokine and adipokine, irisin has been proposed to be an important mediator of the beneficial metabolic effects of exercise [2]. It is released systemically from skeletal muscle and induces the ‘‘browning’’ of subcutaneous white adipocytes, uncoupling protein 1 (UCP1)mediated thermogenesis, and increased energy expenditure [2]. Irisin secretion increases in men who exercise [2,3]. FGF21 is secreted into the circulation from the adipocytes and liver, and is expressed in fat, skeletal muscle and the pancreas. FGF21 promotes conversion of white to beige adipose tissue, activation of UCP1-
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