Relationship of serum irisin levels and various demographic and biochemical factors in non-diabetic hemodialysis patients

under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Relationship of serum irisin levels and various demographic and biochemical factors in non-diabetic hemodialysis patients J Prev Epidemiol. 2020;5(2):e31 Original Article


Introduction
Chronic kidney disease (CKD) is a public health priority that affected 10-12% of the population (1). The advanced stage of CKD is the end-stage renal disease (ESRD) which leads to an irreversible reduction in renal function. ESRD may result in death if it is not treated by dialysis or kidney transplantation (2). The prevalence of metabolic imbalance and abnormal energy hemostasis in CKD patients is estimated to be 20%-80% (3)(4)(5). In addition, changing energy balance in these patients could lead to overweight, protein-energy waste, obesity, and increased mortality (6). It has been indicated that an active and dynamic lifestyle can play an important role in preventing CDK among different individuals through exercise and physical activity (7). In fact, muscle tissue produces various cytokines and myokines such as irisin and also regulates various physiological and metabolic pathways. Irisin is a newly discovered myokine that released into the circulation through cleavage of fibronectin type III domain containing protein 5 (FNDC5) (8,9). Irisin regulates energy and metabolic homeostasis and improves body weight, glucose tolerance, and insulin resistance (10). Several studies have investigated irisin levels in various diseases such as type 2 diabetes mellitus, nonalcoholic fatty liver disease, cardiovascular diseases, metabolic syndrome, and CKD (11)(12)(13)(14). Irisin is not excreted by the kidneys and its level is reduced in the advanced stage of CKD. Therefore, the level of irisin can be considered a substantial indicator of renal function (8). Furthermore, some studies have revealed that irisin level was lower in CKD patients with hemodialysis than normal individuals (15,16). Considering the importance of irisin level in chronic renal failure and the socio-

Key point
In a study on 61 non-diabetic stable hemodialysis patients, we found positive correlation of serum irisin with hemodialysis doses and a negative correlation of serum irisin with BMI.
10.34172/jpe.2020.31 doi economic and psychological burden, that hemodialysis imposes on patients, and the need to improve the quality of life in these patients, it seems necessary to conduct further studies in this field.

Objectives
The purpose of the present study is to investigate the relationship between serum irisin level and various demographic and biochemical factors in non-diabetic hemodialysis patients.

Patients
In this analytical cross-sectional, 61 subjects (35 men and 26 women) with ESRD under hemodialysis were referred to Noor hospital, Isfahan in 2015. Demographic information including hypertension, starting date of hemodialysis, number of dialysis sessions per week, dialysis duration, medication consumption, and body mass index (BMI) was recorded for all subjects. Inclusion criteria included patients without diabetes, chronic or acute infection, cancer, and muscular diseases, obvious ischemia detected by electrocardiography, and presence of heart failure as well as consumption of corticosteroids and immunosuppressive medications. The hemodialysis dose was calculated by multiplying the time of hemodialysis duration and the number of dialysis sessions per patient until the test was taken. Dialysis adequacy was also calculated by Kt/V.

Laboratory assessments
Five ml of venous blood samples were taken from all patients after an 8-hour fast and pre-hemodialysis for three consecutive days, and then the samples were immediately transferred to the laboratory. Complete blood count was assessed and blood serum was separated. Serum irisin was measured using an ELISA kit (EASTBIOPHARM, Hangzhou Eastbiopharm Co. Ltd. China) according to manufacturers' instructions. Levels of alkaline phosphatase, triglyceride, total cholesterol, C-reactive protein, albumin, calcium, phosphorus, and vitamin D were evaluated by standard methods.

Ethical issues
The research followed the tenets of the Declaration of Helsinki. The institutional ethical committee at Isfahan University of Medical Sciences approved all study protocols. Accordingly, written informed consent was taken from all participants before any intervention. This study was extracted from M.D., thesis of Fereshteh Rafi'i at this university (Thesis #394752).

Statistical analysis
SPSS software, version 20 (SPSS Inc., Chicago, IL) was applied to analyze all data. Data were expressed as mean ± SD for continuous variables. To determine the relationship between variables, independent t test, chi-square, analysis of variance (ANOVA) and Pearson's correlation coefficient tests were used. P < 0.05 was considered statistically significant. Table 1 represents the results of correlations between various demographic data and irisin level using Pearson's correlation coefficient and independent t-test. According to Table 1, a significant correlation was observed between irisin level and gender (P = 0.01). Therefore, irisin level was significantly higher in men than in women (Figure 1). Furthermore, an inverse and significant difference between irisin level and BMI (P = 0.03, r=-0.26, Table 1; Figure 2). Linear regression was used to determine the relationship between the irisin level, as a dependent variable, and the other variables, as independent variables. We found a positive and significant relationship only with the dialysis dose (P = 0.01, r=0.32; Figure 3). No meaningful difference was revealed between biochemical parameters and irisin levels (P > 0.05; Table 2).

Discussion
This study aimed to investigate the relationship between serum irisin levels and various demographic and biochemical parameters in non-diabetic hemodialysis  patients in 2015. The findings indicated that serum irisin level has a significant relationship with gender that was higher in men.
In the study, conducted by Ebert et al on 532 CKD patients from 1-5 stages, serum irisin levels were significantly greater in women than in men. In their study, only 169 out of 532 CKD patients were in the 5 th stage and under hemodialysis (17). This stands as a possible reason for dissimilar findings with our findings.
Moreover, the present study showed an inverse relationship between serum irisin levels and BMI. A study conducted on 69 non-diabetic individuals with the mean BMI of 27.61 ± 3.8 kg/m 2 reports that serum irisin level negatively correlates with BMI and fat mass (15). Choi et al found that serum levels of irisin have a negative correlation with BMI (18), which is consistent with the results of our study.
In a study involving three groups (group 1; patients with anorexia nervosa, group 2; those with normal weight, and group 3; patients with morbid obesity), serum irisin level in patients with obesity was higher than the other two groups, suggesting that irisin level positively correlated with BMI (13,14). In contrast, the study conducted by Wen et al on CKD patients indicated no correlation between serum irisin and BMI (16). This is not similar to the findings of the current study which can be possibly attributed to the higher BMI mean score (23.06 km/m 2 ) and the larger number of men than women (10 women, 28 men) which showed in the study by Wen et al (16).
In this study, a positive correlation between serum levels of irisin and dialysis dose was seen. The literature, however, does not seem to show a similar study conducted in this regard.
In our study, none of the biochemical variables including plasma hemoglobin, hematocrit, and serum calcium, phosphorus, alkaline phosphates, triglyceride, cholesterol, vitamin D and c-reactive protein showed a correlation with serum irisin levels. In the study by Cavalier et al serum irisin levels were measured after a dose of 100 000 units of vitamin D in healthy volunteers, however, no relationship was found between vitamin D and irisin levels after the treatment (19).
In the study Ebert et al, 38 patients with chronic renal failure on stage five showed a positive correlation between serum irisin levels and HDL-c (17), while our study found no association between serum irisin levels and lipid profile. It should be noted, however, that in our study the total cholesterol level was measured as opposed to separate HDL-c and LDL-c values. This can probably be the cause for this inconsistency.
In the current study, the average cholesterol was calculated as 163.4 mg/dL; this amount is 195.2 mg/ dL in the study by Ebert et al (17). Furthermore, in the study by Ebert et al, a positive correlation was reported between serum levels of irisin and plasma hemoglobin levels in patients (17), whereas in our study no correlation between these two variables and serin irisin was detected. The study conducted by Ebert et al involved 38 end-stage renal failure patients with 52.6% and 47.4% women and men respectively (17). However, our study included 42.6% of women. The duration of hemodialysis for patients in   the study by Ebert et al was not reported. Thus, it may be due to differences in the duration of hemodialysis in these two studies which resulted in differences in plasma hemoglobin levels between the two studies.

Conclusion
In this study, we found that serum irisin level in hemodialysis patients correlates significantly with their gender such that it is more in men than women. It correlates negatively with BMI too. It correlated positively with dialysis dose too. Irisin is probably a weight-associated parameter whose shortage can lead to obesity. However, further studies are required as this is a novel issue. An important point to be taken into account in future research -possibly conducted with a larger sample size -is whether irisin is itself an agent or whether it plays the role of an epiphenomenon.

Limitations of the study
This study was conducted on 61 non-diabetic hemodialysis patients; wider research is required on all patients in other medical centers.