Irisin is more strongly predicted by muscle oxidative potential than adiposity in non-diabetic men

Beneficial effects of whole-body cryotherapy on glucose homeostasis and amino acid profile are associated with a reduced myostatin serum concentration

The study investigated the effect of single and chronic (10 sessions) whole-body cryotherapy (WBC; 3-min, − 110 °C) on amino acid (AA) profile, myostatin, fibroblast growth factor 21 (FGF21), and concentrations of brain-derived neurotrophic factor (BDNF), irisin and adiponectin in relation to glucose homeostasis. Thirty-five, healthy men were randomly split into experimental (young: 28 ± 7 years and middle-aged: 51 ± 3 years) and control groups. Blood samples were taken before and 1 h after the first and last (10th) WBC session. Baseline myostatin correlated significantly with visceral fat area, glucose, insulin, HOMA-IR and irisin (all p < 0.05). The single session of WBC induced temporary changes in AA profile, whereas chronic exposure lowered valine and asparagine concentrations (p < 0.01 and p = 0.01, respectively) compared to the baseline. The chronic WBC reduced fasting glucose (p = 0.04), FGF21 (− 35.8%, p = 0.06) and myostatin (-18.2%, p = 0.06). Still, the effects were age-dependent. The decrease of myostatin was more pronounced in middle-aged participants (p < 0.01). Concentrations of irisin and adiponectin increased in response to chronic WBC, while BDNF level remained unchanged. By improving the adipo-myokine profile, chronic WBC may reduce effectively the risk of the metabolic syndrome associated with hyperinsulinemia, increased levels of valine and asparagine, and muscle atrophy.

The relationship between adiposopathy and glucose-insulin homeostasis is not affected by moderate-intensity aerobic training in healthy women with obesity

The contribution of adiposopathy to glucose-insulin homeostasis remains unclear. This longitudinal study examined the potential relationship between the adiponectin/leptin ratio (A/L, a marker of adiposopathy) and insulin resistance (IR: homeostasis model assessment (HOMA)), insulin sensitivity (IS: Matsuda), and insulin response to an oral glucose tolerance test before and after a 16-week walking program, in 29 physically inactive pre- and postmenopausal women with obesity (BMI, 29-35 kg/m²; age, 47-54 years). Anthropometry, body composition, VO₂max, and fasting lipid-lipoprotein and inflammatory profiles were assessed. A/L was unchanged after training (p = 0.15), despite decreased leptin levels (p < 0.05). While the Matsuda index tended to increase (p = 0.07), HOMA decreased (p < 0.05) and fasting insulin was reduced (p < 0.01) but insulin area under the curve (AUC) remained unchanged (p = 0.18) after training. Body fatness and VO₂max were improved (p < 0.05) while triacylglycerols increased and HDL-CHOL levels decreased after training (p < 0.05). At baseline, A/L was positively associated with VO₂max, HDL-CHOL levels, and Matsuda (0.37 < ρ < 0.56; p < 0.05) but negatively with body fatness, HOMA, insulin AUC, IL-6, and hs-CRP levels (- 0.41 < ρ < - 0.66; p < 0.05). After training, associations with fitness, HOMA, and inflammation were lost. Multiple regression analysis revealed A/L as an independent predictor of IR and IS, before training (partial R² = 0.10 and 0.22), although A/L did not predict the insulin AUC pre- or post-intervention. A significant correlation was found between training-induced changes to A/L and IS (r = 0.38; p < 0.05) but not with IR or insulin AUC. Although changes in the A/L ratio could not explain improvements to glucose-insulin homeostasis indices following training, a relationship with insulin sensitivity was revealed in healthy women with obesity.

A comparative study on the effects of high-fat diet and endurance training on the PGC-1α-FNDC5/irisin pathway in obese and nonobese male C57BL/6 mice

The present study was performed to clarify how a combined exercise/diet treatment could affect the expression level of the muscle fibronectin type III domain containing 5 (Fndc5) with respect to body fat mass. Male C57BL/6 mice were divided into 2 groups including low-fat (LF) and high-fat (HF) diets for 12 weeks. Then, LF fed (nonobese) and HF fed mice (obese) were divided into the following 4 groups: HF-Exercise, HF-Sedentary, LF-Exercise, and LF-Sedentary. The exercise group exercised on a motor-driven treadmill for 45 min/day, 5 days/week for 8 weeks. Mice were sacrificed 24 h after the final exercise session. Gastrocnemius muscle and the visceral adipose tissue were excised and frozen for the assessment of proliferator-activated receptor gamma coactivator 1 alpha (Pgc-1α) and Fndc5 messenger RNA (mRNA) and protein levels. Data indicated that protein level of muscle PGC-1α was decreased in HF versus LF groups and in obese versus nonobese mice. Moreover, Fndc5 mRNA levels were increased in the muscle tissue of HF versus LF groups and in obese versus nonobese mice. Also, in the gastrocnemius skeletal muscle, protein levels of FNDC5 were significantly higher in the HF fed mice, as compared with their low-fat fed counterparts, similar to what was observed for exercised versus sedentary mice. Overall, we found that the HF diet increased Fndc5 transcript levels in the skeletal muscle, but exercise had a minimal effect on the transcript level of Fndc5, whereas endurance training increased the protein content of FNDC5 in the skeletal muscle.

Physiology and role of irisin in glucose homeostasis

Irisin is a myokine that leads to increased energy expenditure by stimulating the 'browning' of white adipose tissue. In the first description of this hormone, increased levels of circulating irisin, which is cleaved from its precursor fibronectin type III domain-containing protein 5, were associated with improved glucose homeostasis by reducing insulin resistance. Consequently, several studies attempted to characterize the role of irisin in glucose regulation, but contradictory results have been reported, and even the existence of this hormone has been questioned. In this Review, we present the current knowledge on the physiology of irisin and its role in glucose homeostasis. We describe the mechanisms involved in the synthesis, secretion, circulation and regulation of irisin, and the controversies regarding the measurement of irisin. We also discuss the direct effects of irisin on glucose regulatory mechanisms in different organs, the indirect effects and interactions with other hormones, and the important open questions with regard to irisin in those organs. Finally, we present the results from animal interventional studies and from human clinical studies investigating the association of irisin with obesity, insulin resistance, type 2 diabetes mellitus and the metabolic syndrome.

Effects of physical activity on the link between PGC-1a and FNDC5 in muscle, circulating Ιrisin and UCP1 of white adipocytes in humans: A systematic review

Background: Exercise may activate a brown adipose-like phenotype in white adipose tissue. The aim of this systematic review was to identify the effects of physical activity on the link between peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1a) and fibronectin type III domain-containing protein 5 (FNDC5) in muscle, circulating Irisin and uncoupling protein one (UCP1) of white adipocytes in humans. Methods: Two databases (PubMed 1966 to 08/2016 and EMBASE 1974 to 08/2016) were searched using an appropriate algorithm. We included articles that examined physical activity and/or exercise in humans that met the following criteria: a) PGC-1a in conjunction with FNDC5 measurements, and b) FNDC5 and/or circulating Irisin and/or UCP1 levels in white adipocytes. Results: We included 51 studies (12 randomised controlled trials) with 2474 participants. Out of the 51 studies, 16 examined PGC-1a and FNDC5 in response to exercise, and only four found increases in both PGC-1a and FNDC5 mRNA and one showed increased FNDC5 mRNA. In total, 22 out of 45 studies that examined circulating Irisin in response to exercise showed increased concentrations when ELISA techniques were used; two studies also revealed increased Irisin levels measured via mass spectrometry. Three studies showed a positive association of circulating Irisin with physical activity levels. One study found no exercise effects on UCP1 mRNA in white adipocytes. Conclusions: The effects of physical activity on the link between PGC-1a, FNDC5 mRNA in muscle and UCP1 in white human adipocytes has attracted little scientific attention. Current methods for Irisin identification lack precision and, therefore, the existing evidence does not allow for conclusions to be made regarding Irisin responses to physical activity. We found a contrast between standardised review methods and accuracy of the measurements used. This should be considered in future systematic reviews.

Effects of physical activity on the link between PGC-1a and FNDC5 in muscle, circulating Ιrisin and UCP1 of white adipocytes in humans: A systematic review

Background: Exercise may activate a brown adipose-like phenotype in white adipose tissue. The aim of this systematic review was to identify the effects of physical activity on the link between peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1a) and fibronectin type III domain-containing protein 5 (FNDC5) in muscle, circulating Irisin and uncoupling protein one (UCP1) of white adipocytes in humans. Methods: Two databases (PubMed 1966 to 08/2016 and EMBASE 1974 to 08/2016) were searched using an appropriate algorithm. We included articles that examined physical activity and/or exercise in humans that met the following criteria: a) PGC-1a in conjunction with FNDC5 measurements, and b) FNDC5 and/or circulating Irisin and/or UCP1 levels in white adipocytes. Results: We included 51 studies (12 randomised controlled trials) with 2474 participants. Out of the 51 studies, 16 examined PGC-1a and FNDC5 in response to exercise, and only four found increases in both PGC-1a and FNDC5 mRNA and one showed increased FNDC5 mRNA. In total, 22 out of 45 studies that examined circulating Irisin in response to exercise showed increased concentrations when ELISA techniques were used; two studies also revealed increased Irisin levels measured via mass spectrometry. Three studies showed a positive association of circulating Irisin with physical activity levels. One study found no exercise effects on UCP1 mRNA in white adipocytes. Conclusions: The effects of physical activity on the link between PGC-1a, FNDC5 mRNA in muscle and UCP1 in white human adipocytes has attracted little scientific attention. Current methods for Irisin identification lack precision and, therefore, the existing evidence does not allow for conclusions to be made regarding Irisin responses to physical activity. We found a contrast between standardised review methods and accuracy of the measurements used. This should be considered in future systematic reviews.

Circulating Irisin Is Reduced in Male Patients with Type 1 and Type 2 Myotonic Dystrophies

CONTEXT

Myotonic dystrophies (DM) are dominantly inherited muscle disorders characterized by myotonia, muscle weakness, and wasting. The reasons for sarcopenia in DMs are uncleared and multiple factors are involved. Irisin, a positive hormone regulator of muscle growth and bone, may play a role.

OBJECTIVES

To investigate (1) circulating irisin in a series of DM1 and DM2 male patients compared with healthy controls and (2) the relationships between irisin and anthropometric, metabolic and hormonal parameters.

DESIGN AND STUDY PARTICIPANTS

This is a cross-sectional study. Fasting blood samples for glucometabolic, gonadic, bone markers, and irisin were collected from 28 ambulatory DM1, 10 DM2, and 23 age-matched healthy male subjects. Body composition and bone mineralization [bone mineral density (BMD)] were measured by DEXA. Echocardiographic assessment and visceral adiposity, namely, liver and epicardial fat, were investigated by ultrasound. Irisin released from cultured myotubes derived from 3 DM1, 3 DM2, and 3 healthy donors was assayed.

RESULTS

Plasma irisin levels were definitely lower in both DM1 and DM2 patients than in controls with no difference between DM1 and DM2. Irisin released from DM1 and DM2 myotubes was similar to that released from myotubes of the non-DM donors, though diabetic DM2 myotubes released more irisin than DM1 myotubes. There was no correlation between irisin and muscle strength or lean mass in both DM1 and DM2 patients. In DM1 patients, plasma irisin levels correlated negatively with oxygen consumption and positively with insulin resistance, while in DM2 patients plasma irisin levels positively correlated with fat mass at arms and legs levels. No correlation with visceral fat, left ventricular mass, and gonadal hormones could be detected. In both DM1 and DM2 patients, legs BMD parameters positively correlated with plasma irisin levels.

CONCLUSION

Plasma irisin is reduced in both DM1 and DM2 male patients likely reflecting muscle mass reduction. Moreover, insulin resistance may contribute to modulation of plasma irisin in DM1 patients. The irisin-mediated cross talk muscle-adipose tissue-bone may be active also in the male myotonic dystrophies' model.

Effects of Body Weight Reduction on Serum Irisin and Metabolic Parameters in Obese Subjects

BACKGROUND

Irisin is a myokine implicated in lipid and glucose metabolism. The objective of this study is to examine the effect of a body weight reduction on the serum irisin level and physical indicators in obese Japanese patients without diabetes.

METHODS

The subjects were 22 patients (male/female, 5/17; age, 46.1±16.0 years; body mass index [BMI], 36.9±5.0 kg/m²) who completed a 6-month body weight reduction program at our clinic. The program included diet, exercise therapy and cognitive behavioral therapy. Blood parameters, body composition, exercise tolerance, homeostasis model assessment of insulin resistance (HOMA-IR), and serum irisin were determined before and after intervention, and relationships among changes in these data were examined.

RESULTS

There were significant decreases in body weight and BMI after the intervention. Irisin before the intervention was significantly positively correlated with HOMA-IR (r=0.434, P<0.05). The mean irisin level showed no significant change after the intervention in all participants. However, improvements in % body fat, subcutaneous fat area, triglycerides, and fasting glucose were significantly greater in patients with an increase in irisin compared to those with a decrease in irisin after the intervention. Patients with an increase in irisin also had significantly lower fasting insulin (9.7±4.8 vs. 16.4±8.2, P<0.05) and HOMA-IR (2.2±1.1 vs. 3.7±1.6, P<0.05) after the intervention, compared to patients with a decrease in irisin.

CONCLUSION

Body weight reduction did not alter irisin levels. However, irisin may play important roles in fat and glucose metabolism and insulin resistance, and the effects of body weight reduction on irisin kinetics may be a key for obesity treatment.

Elevated circulating irisin is associated with lower risk of insulin resistance: association and path analyses of obese Chinese adults

BACKGROUND

Evidence on the role of irisin in insulin resistance is limited and controversial, and pathways between them remain unknown. We aimed to examine the independent effects of circulating irisin and different adiposity measurements, as well as their potential interactions, on insulin resistance. We also aimed to explore possible pathways among circulating irisin, adiposity, glucose and insulin levels and insulin resistance.

METHODS

A cross-sectional study of 1,115 community- living obese Chinese adults, with data collection on clinical characteristics, glucose and lipid metabolic parameters and circulating irisin levels.

RESULTS

Among the 1,115 subjects, 667 (59.8 %) were identified as insulin-resistance, and showed significantly decreased serum irisin than their controls (log-transformed irisin: 1.19 ± 2.34 v.s. 1.46 ± 2.05 ng/ml, p = 0.042). With adjustment for potential confounders, elevated circulating irisin was significantly associated with reduced risk of insulin resistance, with adjusted odds ratio per standard deviation increase of irisin of 0.871 (0.765-0.991, p = 0.036). As for different adiposity measurements, body fat percentage, but neither BMI nor waist, was significantly associated with increased risk of insulin resistance (OR: 1.152 (1.041-1.275), p = 0.006). No significant interaction effect between serum irisin and adiposity on insulin resistance was found. A one pathway model about the relationship between serum irisin and insulin resistance fits well (χ (2) = 44.09, p < 0.001; CFI-0.994; TLI =0.986; and RMSEA = 0.067), and shows that elevated circulating irisin might improve insulin resistance indirectly through lowering fasting insulin levels (standardized path coefficient = -0.046, p = 0.032).

CONCLUSIONS

Elevated circulating irisin is associated with lower risk of insulin resistance indirectly through lowering fasting insulin.

Association between circulating irisin and insulin resistance in non-diabetic adults: A meta-analysis

INTRODUCTION

Exogenous administration of recombinant irisin improves glucose metabolism. However, the association of endogenous circulating (plasma/serum) irisin with insulin resistance remains poorly delineated. This study was aimed to examine this association by meta-analyzing the current evidence without study design restriction in non-diabetic adults.

MATERIALS/METHODS

Peer-reviewed studies written in English from 3 databases were searched to December 2015. Studies that reported the association between circulating irisin and insulin resistance (or its reverse, insulin sensitivity) in non-diabetic non-pregnant adults (mean ages ≥18years) were included. The pooled correlation coefficient (r) and 95% confidence intervals (CIs) were calculated using a random-effects model. Subgroup analyses and meta-regression were performed to explore potential sources of heterogeneity.

RESULTS

Of the 195 identified publications, 17 studies from 15 articles enrolling 1912 participants reported the association between circulating irisin and insulin resistance. The pooled effect size was 0.15 (95% CI: 0.07 to 0.22) with a substantial heterogeneity (I(2)=55.5%). This association seemed to be modified by glycemic status (fasting blood glucose ≥6.1mmol/L versus <6.1mmol/L) and racial-ethnic difference (Asians versus Europeans versus Americans), but not by sex difference, sampling time-point, blood sample type, ELISA kits used, baseline age, or body mass index. Circulating irisin was inversely associated with insulin sensitivity (6 studies; r=-0.17, 95% CI: -0.25 to -0.09).

CONCLUSIONS

Circulating irisin is directly and positively associated with insulin resistance in non-diabetic adults. However, this association is rather small and requires further clarification, in particular by well-designed large epidemiological studies with overall, race-, and sex-specific analyses.