Exploring Circulating Irisin as a Biomarker: An Analysis in Relationship with Glucose and Bone Status Evaluation in Adults with Vitamin D Deficient Versus Sufficient Status

Background: Irisin, a muscle-derived hormone, enhances the energy metabolism by activating the brown adipose tissue and acts as a bone-forming agent across the entire life span. No consistent clinical data in humans have been published so far to highlight if blood irisin as glucose/bone biomarker should be refined based on the vitamin D status (deficient or sufficient). Therefore, we aimed to objectively assess the level of irisin in female adults with abnormal and normal vitamin D status, as reflected by the level of 25-hydroxyvitamin (25OHD) in relationship with glucose and bone metabolic parameters. Methods: This pilot, prospective, exploratory study included eighty-nine menopausal women aged over 50. We excluded subjects with malignancies, bone and metabolic disorders, insulin treatment, and active endocrine disorders. Fasting profile included glycaemia, insulin, and glycated haemoglobin A1c (HbA1c). Then, 75 g oral glucose tolerance test (OGTT) included glycaemia and insulin assay after 60 and 120 min. Bone status involved bone turnover markers and central dual-energy X-ray absorptiometry providing bone mineral density (BMD) and trabecular bone score. Results: Eighty-nine subjects were included in the following two groups depending on 25OHD: vitamin D-deficient (VDD) group (N = 48; 25OHD < 30 ng/mL) and vitamin D-sufficient (VDS) group (N = 41; 25OHD ≥ 30 ng/mL). The two groups had similar age and menopausal period (62.29 ± 10.19 vs. 63.56 ± 8.16 years, respectively; 15.82 ± 9.55 vs. 16.11 ± 9.00 years, p > 0.5 for each). A statistically significant higher body mass index (BMI) was found in VDD vs. VDS group (32.25 ± 5.9 vs. 28.93 ± 4.97 kg/m2, p = 0.006). Circulating irisin was similar between the groups as follows: median (IQR) of 91.85 (44.76–121.76) vs. 71.17 (38.76–97.43) ng/mL, p = 0.506. Fasting profile and OGTT assays showed no between-group difference. Median HOMA-IR in VDD group pointed out insulin resistance of 2.67 (1.31–3.29). Lowest mean/median T-scores at DXA for both groups were consistent with osteopenia category, but they were confirmed at different central sites as follows: femoral neck in both groups [VDD versus VDS group: −1.1 (−1.20–−0.90) vs. −1.1 (−1.49–−0.91), p = 0.526, respectively], only at lumbar spine for VDS group (T-score of −1.18 ± 1.13). The correlations between irisin and the mentioned parameters displayed a different profile when the analysis was performed in the groups with different 25OHD levels. In VDD group, irisin levels statistically significantly correlated with serum phosphorus (r = −0.32, p = 0.022), osteocalcin (r = −0.293, p = 0.038), P1NP (r = −0.297, p = 0.04), HbA1c (r = 0.342, p = 0.014), and BMI (r = 0.408, p = 0.003). Conclusions: This pilot study brings awareness in the analysis of irisin in relationship with glucose and bone-related biomarkers correlates, showing a distinct type of association depending on 25OHD level, which might represent an important crossroad in the multitude of irisin-activated signal transduction pathways.