Aerobic Exercise Training Effects on Omentin-1, Insulin Resistance, and Lipid Profile Among Male Smokers

ABSTRACT Background: Omentin-1 is a circulating adipokine that can serve as a biomarker for assessment of metabolic risk factors. We investigated the effect of eight weeks of aerobic exercise training on serum omentin-1, insulin resistance and lipid profile in nonsmokers and smokers. Methods: Nineteen male nonsmokers (aged 27.88 ± 2.47 years, and with BMI of 22.69 ± 1.77 kg.m−2) and twenty male smokers (aged 30.11 ± 1.96 years, and with BMI of 23.12 ± 1.91 kg.m−2) were randomly assigned into nonsmokers control group (C), nonsmokers exercise group (E), control smoker group (CS), and exercise smoker group (ES). Exercise groups participated in an eight-week aerobic exercise training program (three times a week, 20–35 min per session at 55%-70% of maximum heart rate). Serum omentin-1 and insulin values were determined by ELISA. The Homeostatic Model Assessment for Insulin Resistance (HOMA-IR), glucose level and lipid profile were measured before and after the intervention. Pearson correlation test, Eta test, paired samples t-test, one and two-way analysis of variance (ANOVA) and post-hoc Tukey test were applied for data analysis (p < .05). Results: Aerobic exercise improved both serum omentin-1 and high lipoprotein cholesterol (HDL-C) in the exercise groups (P < .05). Also, exercise training reduced insulin, glucose, HOMA-IR, triglyceride (TG), total cholesterol (TC) and low-density lipoprotein cholesterol (LDL-C) levels (p < .05). Omentin-1 was significantly correlated with glucose, insulin, HOMA-IR, TG, TC, LDL-C and HDL-C in both nonsmokers and smokers. Conclusions: The findings suggest that aerobic exercise-induced changes in omentin-1 in the exercise-trained groups may be associated with the beneficial effects of exercise on reduced insulin resistance and lipid profile.

Smoking is linked to higher risk of developing many respiratory infection diseases and cardiovascular diseases in the smoker population (Malenica et al., 2017). The results of a study showed that current smokers (aged under 65 years) with a normal weight, had an increased risk of death compared to the very obese and nonsmokers (Freedman et al., 2006). The evidence confirmed that smoking cigarettes was associated with altered normal status of the lipid profile. So, there was a significant increase in the levels of total cholesterol (TC), triglyceride (TG), low density lipoprotein (LDL-C), very low density lipoprotein (VLDL), and reduced level of high lipoprotein cholesterol (HDL-C) among smokers (Hallit et al., 2017). Due to the global obesity epidemic and increase in smoking rates, the incidence of obesity and insulin resistance among smokers is a big concern (Chiolero et al., 2008). The association between smoking and obesity is not fully understood, and some studies have reported conflicting results. Collectively, the evidence for any protective effect of smoking against obesity is weak (Dare et al., 2015).
The adipose tissue, as an active endocrine organ, produces bioactive peptides and proteins called "adipokines." Some of these adipokines lead to the prevalence of insulin resistance and cardiovascular complications linked to obesity. Omentin-1, a novel adipokine, is the main isoform of mentions in circulating plasma levels of human, which is mainly secreted from the visceral adipose tissue (Saremi et al., 2010;Yang et al., 2006). Recent evidence points to the possible role of omentin-1 in the pathophysiology of obesity and insulin resistance (Hossein-Nezhad et al., 2012). The omentin-1 circulating levels are inversely related to insulin resistance and decrease in obesity as well as type 2 diabetes (T2D) (Verheggen et al., 2016). Khadem Ansari et al. (2018) recently reported a decrease in omentin-1 serum level among smokers as a metabolic risk factor and as a major prognostic agent of lung cancer in these individuals. Low omentin-1 levels in response to cigarette smoking can cause an immunomodulatory effect on the immune system that appears to raise the risk of exposure to infections in smokers (Jaikanth et al., 2013). However, the exact mechanism underlying the reduced omentin-1 level observed in smokers is little understood.
Exercise training is an important non-pharmacological strategy to improve insulin sensitivity via several metabolic/ physiological changes (Tavassoli & Heidarianpour, 2021;Tavassoli et al., 2019;Verheggen et al., 2016). Since regular exercise training diminishes the risk factors for metabolic diseases such as obesity, T2D, and cardiovascular diseases, (Lazarevic et al., 2008) it may modify circulating omentin-1 level. Saremi et al. (2010) reported an incremental change in omentin-1 concentration following 12 weeks of aerobic training in obese participants, but (Faramarzi et al., 2016) observed no statistically significant change in its level after 12 weeks of exercise program in overweight women. However, until now, the effect of exercise training interventions on omentin-1 level has not been explored in smokers. Therefore, the changes of omentin-1 and the possible association between its serum level and metabolic parameters following exercise training can provide new insight into the mechanisms underlying the benefits of exercise training in smokers. To the authors' knowledge, this is the first study to determine the effect of an exercise intervention program on circulating omentin-1 level and its associations with some metabolic parameters in apparently healthy smokers with normal weight.

Participants
This was a quasi-experimental study with pre-and posttraining and control groups design. For this purpose, nineteen nonsmokers and twenty young smokers (all male) were randomly assigned into nonsmokers control; C group (n = 9; aged 27.67 ± 2.52 years, and with BMI of 22.61 ± 1.95 kg.m −2 ), nonsmokers exercise; E group (n = 10; aged 28 ± 2.74 years, and with BMI of 22.77 ± 1.77 kg.m −2 ), control smoker; CS group (n = 9; aged 29.33 ± 2.52 years, and with BMI of 23.59 ± 2.23 kg.m −2 ) and exercise smoker; ES group (n = 11; aged 30.50 ± 1.76 years, and with BMI of 22.72 ± 1.69 kg.m −2 ). The smokers consumed approximately 11-16 cigarettes per day at least for five years. The subjects were non-athletes, meaning that they had not participated in any regular exercise during the last six months. We excluded subjects who had metabolic or chronic inflammatory disorders, cardiovascular diseases or any other major illness that could affect the results. The exercise groups participated in an eight-week aerobic training program, while the control groups maintained their normal lifestyle. The physical training intensity of the participants was determined during one of the experimental sessions. All experimental procedures were approved by the ethics committee of Bu-Ali Sina University (Hamedan, Iran). All the participants filled in written informed consent forms after being briefed on all the necessary research-related details.

Aerobic exercise training program
The aerobic training program was performed 20-35 min a day, 3 days a week for eight consecutive weeks. All exercise trainings were supervised by an expert exercise physiologist. Each session was executed in three continuous stages as follows: warm-up by exercises including marching, walking briskly, and jogging for 10 min; main activity (aerobic training program); and cooldown by walking, slow jogging and typical post-running stretches for 10 min.
The exercise sessions lasted 20 minutes and the initial intensity of training was set at 55-65% of an individual's maximal heart rate (HRmax) for the first four weeks and was progressively increased to 70% of HRmax for 35 min in the 8th week of the protocol (Vatansev and Çakmakçi (2010). We used an age-based prediction equation (220-age) for calculating the predicted HRmax (Fox et al., 1971). Heart rate was checked continuously during all exercise sessions using a Beurer's PM 100 beltless heart rate monitor (made in Germany) after calibration according to the manufacturer guidelines.

Experimental measurements
Venous blood samples were collected in the morning (8 a.m.) after a 10-12 hour overnight fast. All baseline measurements were performed prior to the beginning of the protocol, and post testing evaluations were made 48 h after the last session of the training program. Omentin-1 serum level was analyzed by a commercial ELISA kit (Eastbiopharm, Hangzhou, China). The intra-assay and inter-assay coefficients of variation were less than 12%, and sensitivity of the assay was 1.03 ng/mL. The serum values of HDL-C, LDL-C, TG, and TC concentrations were measured by an enzymatic colorimetric method using biochemical Auto-analyzer Prestige 24i (Made in Japan). The glucose level was determined using the glucose oxidase method kit (Pars Azmoon, Tehran, Iran). Insulin concentration was assayed using the chemiluminescence method (LIAISON®, Germany). The Homeostasis model assessment for insulin resistance (HOMA-IR) was used to calculate insulin resistance, and it was estimated from fasting glucose and insulin, according the following equation: [Fasting glucose (mg/dl) × fasting insulin (U/l)/405] (Matthews et al., 1985)

Statistical analysis
The results are expressed as the mean ± standard deviation. All the statistical analyses were conducted on the SPSS (version 22) software at the level of significance of 0.05. Assumption of normal distribution of the data and homogeneity of variances were verified by the Shapiro-Wilk test and Levene's test. Paired sample t-test was performed to examine within group differences before and after the eight-week exercise intervention in each group. One way analysis of variance (ANOVA) followed by Tukey's post hoc test were applied to find the differences among the groups and two-way ANOVA was conducted to assess the main effect of each variable. The correlations between the variables were estimated by Pearson's correlation and Eta tests.

Results
Anthropometrical and metabolic characteristics (mean ± SD) of the participants at baseline and after the eight-week exercise intervention are presented in Table 1. After eight weeks of aerobic training, the ES group experienced weight loss, which was accompanied by significant reductions in body mass index (BMI) (P = .04), glucose (P = .006), insulin (P = .01), insulin resistance index (P = .006), TG (P = .025), TC (P = .024), and LDL-C (P = .038) levels. In addition, omentin-1 (P = .025 for E group, and P = .014 for ES group) and HDL-C (P = .032 for E group, and P = .009 for ES group) serum concentrations increased significantly in the exercise groups (Table 1 and Figure 1).
The results showed a significant difference in circulating level of omentin-1 between nonsmokers and smokers (56.70 ± 4.83 ng/l and 49.68 ± 8.70 ng/l, respectively (P = .023). The result of two-way ANOVA revealed that smoking was an effective factor in omentin-1 level (P = 003, partial eta squared = 0.356). An analysis using ETA proved that omentin-1 was highly associated with smoking (Eta test = 0.462; Table 2).
After the 8 weeks of aerobic exercise training, significantly inverse correlations were identified between omentin-1 and glucose (r = −0.41 for E group, and r = −0.30 for ES group), insulin r = −0.38 for E group, and r = −0.29 for ES group), HOMA-IR (r = −0.47 for E group, and r = −0.39 for ES group), TG (r = −0.37 for E group, and r = −0.37 for ES group), TC (r = −0.22 for E group, and r = −0.53 for ES group), and LDL-C (r = −0.25 for E group, and r = −0.48 for ES group). There was no significant correlation between BMI and omentin-1 in both nonsmoker and smoker groups (r = −0.21 for E group, and r = −0.33 for ES group). Also, a significantly positive correlation was found between omentin-1 and HDL-C (r = 0.42 for E group, and r = 0.34 for ES group; Table 3). Values are presented as mean± standard deviation.*P < 0.05, significant difference between pre-and post-8 weeks. # P < 0.05, significant difference between control and exercise groups after 8 week Note: HOMA-IR = homeostasis model assessment for insulin resistance; TG = triglyceride; TC = total cholesterol; LDL-C = low-density lipoprotein cholesterol; HDL-C = high-density lipoprotein cholesterol.

Figure 1.
Serum omentin-1 concentration in all participants before and after eight weeks of aerobic training. *p < .05; significant difference between pre-and postexercise training, and # p < .05; significant difference between exercise group and control group.

Discussion
In the present study, we investigated the effect of aerobic exercise training on circulating omentin-1 level and examined the omentin-1 response in relation to insulin resistance and lipid profile in the smokers and nonsmokers. Body variables (i.e. body weight and BMI), insulin resistance, and blood lipid profile decreased in the training groups after the eight-week exercise intervention. Our results are in line with those of previous observations (Lazarevic et al., 2008;Tavassoli & Heidarianpour, 2021;Tavassoli et al., 2019;Verheggen et al., 2016) and reinforce the beneficial effect of exercise training on these important metabolic factors in confronting many diseases such as metabolic syndrome, T2D, and cardiovascular diseases (Chang et al., 2009;Davidson et al., 2009;Saremi et al., 2010;Tavassoli & Heidarianpour, 2021;Tavassoli et al., 2019) The result of two-way ANOVA and ETA tests revealed that smoking was an effective factor in omentin-1 level and also, omentin-1 was highly associated with smoking. The circulating omentin-1 level was higher in nonsmokers compared to smokers. It was proposed that lower levels of omentin-1 in response to smoking may contribute to increased susceptibility to infections in smokers (Jaikanth et al., 2013) The findings showed that the aerobic exercise training led to a marked increase in omentin-1 concentration in the trained groups. Some studies have been carried out on the effect of exercise training on omentin-1 (Lesná et al., 2015). Unfortunately, to the best of our knowledge, no study to date has examined the effect of exercise on circulating omentin-1 level in smokers. Our result was consistent with that of (Wilms et al., 2015;Ouerghi et al., 2017) and (Saremi et al., 2010), who found an increase in omentin level following exercise, and is inconsistent with the results of (Faramarzi et al., 2016) and (Urbanová et al., 2014) Such inconsistency in the results could be attributed to the differences in participants with overweight in the study of Faramarzi et al., (2016) Also, the hyperlipidic diet feeding may have influenced the results of (Urbanová et al., 2014) The results of the studies indicated that omentin induces anti-inflammatory, anti-atherogenic and anti-diabetic effects (De Souza Batista et al., 2007); Watanabe et al., 2016). Recently, two studies have found a relationship between omentin-1 and increased risk of T2D and cardiovascular diseases ; Wittenbecher et al., 2016). It is reported that omentin-1 was inversely correlated with insulin resistance and this adipokine could improve glucose metabolism and insulin sensitivity (Ouerghi et al., 2020) In line with our findings, previous studies have reported that omintin-1 has a negative correlation with fasting insulin, and HOMA-IR (Cai et al. 2009;Tan et al., 2008). Therefore, the mechanism for increased omentin is directly related to weight loss and a decrease in BMI after exercise training. In this regard, (Moreno-Navarrete et al., 2010) reported that the concentration of circulating omentin-1 rises after weight reduction, which is consistent with our observation. Despite the fact that we observed a significant reduction in BMI in the training groups, there was no significant correlation between BMI and omentin-1 in both nonsmoker and smoker groups. This may be related to the subjects that were within the normal BMI range before participating in exercise training. Also, smoking causes an increase in metabolic rate and induces a reduction of food intake in smokers. Although the evidence for this is conflicting, smoking and body weight are inversely related, and smokers frequently gain weight after quitting smoking (Dare et al., 2015) Not much research has been done on the mechanism of omentin-1 and its association with glucose and insulin levels, but the few studies in this regard have confirmed the role of omentin-1 in transmitting insulin signaling via kinase B protein/Akt activation and increasing the insulinstimulated glucose uptake into the adipose tissue (Cai et al., 2009). Omentin-1 can improve glucose metabolism and insulin sensitivity by increasing glucose transport into the muscles following exercise training (Castro et al., 2019). According to the results of (Castro et al., 2019) there is an association between skeletal muscle and adipose tissue in relation to omentin-1. In fact, exercise training causes an increase in omentin gene expression in adipocytes tissue and improves insulin sensitivity (Alizadeh et al., 2017). Nonetheless, there is an inconsistency in terms of the association between omentin level and insulin resistance between the results of the present study and those of (Hossein-Nezhad et al., 2012) which might be due to various subject populations or other undefined components that may influence omentin-1 level.
In the current study, we found inverse correlations between serum omentin-1 level and some lipid profile parameters in nonsmokers and smokers subjects ( Table 3). As opposed to our results, (Hossein-Nezhad et al., 2012) and (Elsaid et al., 2018) did not observe any significant correlation between serum omentin-1 and lipid levels. Moreno-Navarrete et al., (2010) reported that omentin level was correlated with some lipid metabolic parameters such as TC, LDL-C, and TG. Abd-Elbaky et al., (2015) also found an inverse correlation between omentin-1 and TC levels in adults, which is concurrent with our findings. It is proposed that the differences in body fat distribution in the studied subjects may have impacted on the production or secretion of adipokines and the results of investigations Wajchenberg (2000). Omentin seems to play an important role in lipid metabolism regulation and also against diabetic dyslipidemia as a compensatory mechanism (Moreno-Navarrete et al., 2010) since it has been shown that omentin-1 promotes 5-AMP-activated protein kinase phosphorylation, which acts as an inhibitor of endogenous cholesterol synthesis (Lesná et al., 2015) Moreover, in agreement with earlier findings, (De Souza Batista et al., 2007;Yang et al., 2006) the serum level of omentin-1 was positively associated with HDL-C level in the both nonsmokers and smokers. It was suggested that omentin has an anti-atherogenic behavior; thus, it can affect HDL-C level through modulating insulin action (Watanabe et al., 2016;Yan et al., 2011). The potential mechanism involved in the increased HDL-C level following aerobic exercise training may be linked to modifications in the activities of some enzymes such as lipoprotein lipase and lecithin-cholesterol acyltransferase and hepatic triglyceride lipase (Ferguson et al., 1998)

Limitations
Lack of diet control and the small number of participants (sample size) can be considered as the major limitations of the present study. Further investigation based on exercise intervention and implementation of de-training along with diet control in a larger population is suggested to clarify the underlying mechanisms of omentin-1 in smokers.

Conclusion
The results of this study suggest that increased omentin-1 levels following aerobic exercise training may be associated with the beneficial effects of exercise on reduced insulin resistance and lipid profile. Thus, it can be a beneficial intervention in order to prevent or delay the progression of the metabolic disorders, especially in smoker individuals. However, implementing a de-training period (cessation of the training after the training) may clarify the physiological and regulatory role of omentin-1. Further experimental studies need to be conducted on smokers with different BMI.