The Effect of Short Term Aerobic Training on Serum Insulin and Insulin Resistance in Adult Obese Females

he increasing prevalence of obesity and insulin resistance among Iranian women is remarkable (1). In the last decade 48% of Iranian women were obese or overweight (2). Increased levels of obesity and body fat percentage along with age increase in children navigating through maturity and adulthood, increased insulin resistance, type 2 diabetes, and cardiovascular diseases, have always been recognized over recent years (2,3). Scientific studies addressing adults have indicated that chronic inflammation plays a T 1. Assistant Professor, Department of Physical Education and Sport Sciences, Amirkabir University of Technology, Tehran, Iran. 2. Assistant Professor, Department of Exercise Physiology, Faculty of Humanities Sciences, Islamic Azad University, Saveh Branch, Saveh, Iran.


Introduction
he increasing prevalence of obesity and insulin resistance among Iranian women is remarkable (1). In the last decade 48% of Iranian women were obese or overweight (2). Increased levels of obesity and body fat percentage along with age increase in children navigating through maturity and adulthood, increased insulin resistance, type 2 diabetes, and cardiovascular diseases, have always been recognized over recent years (2,3). Scientific studies addressing adults have indicated that chronic inflammation plays a T significant role in the pathogenesis of diseases such as atherosclerosis (2,3), type 1 and type 2 diabetes (4,5), cancer (6), some neurological diseases (7), and immune system diseases (8). The inflammatory markers are associated with obesity and metabolic syndrome (9). The cytokines such as interleukin 6 (IL-6), tumor necrosis factor-α (TNF-α), and interleukin-1 beta (IL-1B) are often categorized as inflammatory cytokines associated with obesity and metabolic syndrome (9). It has been indicated that resistin plays a crucial role in obesity and insulin resistance in diabetic patients due to pro-inflammatory pathways (10). The resistin, as an adipocytesecreted hormone, belongs to either a family of proteins with a cysteine-rich carboxylic terminus called resistin-like molecule or proteins in the inflammatory regions. The increased systemic levels of resistin are associated with impaired insulin function and imbalance of blood glucose levels (11). However, the molecular mechanisms through which the resistin receptors mediate their effects on the hypothalamus in controlling homeostasis and key insulin-sensitive tissues are still unidentified (12). Although the role of genetics and heredity in the development of obesity and its associated disorders such as hormonal disorders is irrefutable, more recent scientific evidence states that the role of environmental factors such as nutrition and physical activity or inactivity also play a major role in this regard. However, in terms of their responses to exercise training, various controversial findings are present about their response to a variety of exercise trainings. For example, in a recent study, serum resistin level was significantly decreased in response to 12 months of training in obese subjects (13); however, another study indicated no significant change after 14 months of training in this regard (14). Some studies have also suggested that longer training programs with a weight loss of at least 5% in obese individuals lead to a significant reduction in inflammatory cytokines (15), while others did not report any significant change at cytokine levels despite weight loss following long-term training programs (16). Also, a significant decrease in the level of cytokines such as resistin and IL-6 without any weight loss was reported (17). Moreover, despite extensive studies addressing the effect of various exercise training on the levels of inflammatory or anti-inflammatory cytokines among healthy or patient populations, the response of serum resistin to aerobic training on obese women is less pronounced. Hence, the present study aim was measurement of six weeks aerobic training effect on serum resistin levels as well as insulin resistance in adult obese women.

Materials and Methods
In this quasi-experimental study, thirty inactive obese women aged 35-45 years old were recruited and divided into exercise (n= 15) and control (n= 15) group. Sample size was decided according two mean comparison equation. All participants were asked to provide informed consent before carrying out the study. Inclusion criteria for study groups were determined obesity (30 ≤ BMI ≤ 36). All subjects were non-athletes and no smoker, not pregnant and non-alcoholics. They were included in the study if they had not done any physical activity in the previous 6 months; they did not use medicines or therapies for obesity and had no history of injury or disease that would prevent daily exercise. Individuals with history of chronic or acute respiratory infections, cardiopulmonary disease, and neuromuscular disease were excluded from the study. Individuals having overt diabetes were also excluded from the study. In the physiology laboratory, anthropometric measurements were obtained (in the morning) from all subjects who had light clothing with no shoes. Two measurements were made every 1 minute and the average of two measurements was used for analysis. Obesity was measured by body mass index (BMI) as division of body weight (kg) by the square of height (in meters) (18). Weight was measured to the nearest 100 IRANIAN JOURNAL OF DIABETES AND OBESITY, VOLUME 12, NUMBER 2, SUMMER 2020 g using digital scales (OMRON) (19). After a normal expiration, waist circumference was determined under the midline of the participant's armpit, at the midpoint between the top of the hip and the lower part of the last rib. Hip girth was obtained at the level of the greatest protrusion of the gluteal muscles with underwear. Body fat percentage was measured using bioelectrical impedance method (Omron Body Fat Analyzer, Finland).

Training program
The exercise group took part in an aerobic exercise training intervention for 6 weeks (3 days a week). Each training session was monitored by an exercise physiologist or a physician. The participants fulfilled a 5-10 min warm-up each session, accompanied by aerobic exercise training at 60-75% (heart rate maximum) HRmax and a 5-min cool down (20). The exercise intensity in each session was controlled and recorded based on the percentage of the maximum heart rate by the Polar pacemaker. The main program of each session was in the form of running in defined intensities, so that the participants exercised the first two weeks at %60-65HRmax, weeks 3 and 4 at %65-70 Hrmax, and the last 2 weeks at %70-75 HRmax (20) (table 1). The control group were trained to maintain their routine activities. The subjects were trained to have their habitual diet during the study. In fact, this study was conducted solely to indicate the impact of aerobic training without calorie restriction on dependent variables.

Laboratory Analyses
For measuring biochemical variables, fasting blood samples were collected prior to and 48 h after last exercise session. The participants were asked not to do any heavy physical activity before blood sampling for 48 h. Serums were separated and stored at -80° immediately after each blood sampling until the assays were performed. ELISA method (Enzyme-linked Immunosorbent Assay for quantitative detection of human resistin) was applied to determine serum resistin, using a Biovendor-Laboratorial kit (Biovendor Company, Czech). The intra-assay coefficients of the method sensitivity and variation were, respectively, 0.033 and 2.8% ng/mL. Oxidase method (Pars Azmoon kit, Tehran) determined glucose.
ELISA method (Demeditec, Germany) was used to determine insulin. The inter-assay and intra-assay coefficients of the method variation were, respectively, 2.88 and 2.6%. Insulin resistance was indicated based on the HOMA-IR as the fasting plasma glucose (mM) product, and insulin (µU/ml) was divided by the constant 22.5 (21).

Data analysis
Data were analyzed by computer using the Statistical Package for Social Sciences (SPSS) for Windows, version 15.0. The Kolmogorov-Smirnov normality test was used to analyze normal distribution of data. Variables between two groups were compared at baseline using independent student's T-test. The mean differences between pre and post-training values on all metabolic and anthropometric variables were determined by paired T-test. The significance level was indicated at a P-value≤ 0.05.

Ethical considerations
The study was approved by the ethics committee of Amirkabir University of Technology (IR.SSU.REC.1397.426).

Results
Baseline (pre-training) and post-training data are presented in Table 1. Data are expressed as mean ± standard deviation (SD). The anthropometrical indexes were not significantly different between groups at baseline (P-value> 0.05, Table 2). Aerobic training significantly decreased body weight, BMI, body fat percentage, abdominal obesity and no change in visceral fat. All these variables remained no change in control group (P-value> 0.05, Table 2). Based on independent T-test, no baseline differences were found between groups for serum resistin, glucose, insulin and insulin resistance (P-value> 0.05,

Discussion
Despite the fact that clinical trials have supported higher levels of resistin in the presence of obesity and metabolic syndrome (22,23), its serum levels after six weeks of aerobic training in adult obese women did not change significantly in the present study. Moreover, although the mentioned training program led to a significant decrease in fasting glucose level in this population that previously had inactive lifestyle, it did not result in a significant change in the insulin resistance index.
Recent studies acknowledge the role of exercise such as long-term aerobic and  resistance trainings in reducing the chronic inflammation, especially in individuals who have high levels of inflammatory mediators (24). In this regard, most studies have supported the effects of a variety of short-or long-term training programs on the levels of inflammatory or anti-inflammatory mediators in healthy or sick and obese or normal populations (25)(26)(27). Studies that have accentuated the impact of long-term training programs on the levels of these cytokines in obese populations have attributed improvements in the inflammatory profile to weight loss or reduction of body fat percentage. For example, in one study, weight loss was associated with decreased level of leptin and increased level of IL-10 in obese mice as a result of long-term resistance trainings in the form of running on treadmill (28). Some other studies have also reported a decrease in the serum resistin levels in obese or diabetic patients in response to two and three months of aerobic exercise, respectively (25,26). The lack of changes in serum resistin level in the present study or some other studies may be explained by considering the fact that some studies have specified at least 10% of weight loss as a pre-requisite to improve the metabolic or inflammation profile in obese populations (29). Although reduction of weight and body fat percentage was significant in the present study, their level was far less than 5% as compared to the levels prior to the training program. Some findings have indicated that three months of moderateintensity aerobic exercise can lead to an improvement in oxidative stress indices such as superoxide dismutase as well as an improvement in insulin resistance and inflammatory profile independent of weight loss (30). In a relatively recent study, although a 3month cycling program (twice per week for up to 45 min) was associated with a decrease in serum resistin levels in a group of type 2 diabetic men, it did not result in a significant change in serum levels of adiponectin and leptin (25). In another study, 12 weeks of resistance training with no changes in body composition resulted in a significant reduction in CRP, leptin, and TNF-α and no significant change in IL-10 in adult obese women (31). The incompatibility of the response of inflammatory cytokines with cardiovascular risk factors to long-term aerobic trainings has also been reported by some other studies (32). In line with no changes in the serum resistin levels in the present study, the 6-week exercise training did not affect insulin resistance in middle-aged obese women although the fasting glucose levels decreased. Some studies have pointed to the effective role of resistin in insulin resistance and development of type 2 diabetes (33). Researchers believe that increasing the expression of resistin in muscle myocytes by inhibiting GLUT4 mRNA reduces the absorption of insulin-dependent glucose by 28-31% (34). The mentioned findings specify the role of resistin in the pathogenesis of insulin resistance in skeletal muscles. Moreover, it has been revealed that resistin leads to an increased glucose production in hepatic gluconeogenesis by over-expression of genes that encode enzymes of hepatic gluconeogenesis pathways, glucose-6-phosphatase (G6Pase), and AMPKdependent phosphoenolpyruvate (35). Despite the presented piece of evidence, some studies have reported the incompatibility of resistin response and insulin resistance with aerobic trainings. For example, in the study conducted by Giannopoulou et al., it has been indicated that 14 weeks of aerobic training led to a significant decrease in insulin resistance without any changes in the resistin level (36), while Jones et al reported a significant reduction in serum resistin levels without any changes in insulin resistance following eight weeks of aerobic training (37). On the other hand, some studies have mentioned that resistin had no effect on glucose oxidation and glycogen synthesis (34). Based on the mentioned evidence, reduction of fasting glucose levels in response to the training program in the present study may be attributed to other environmental factors or other hormonal changes, independent of the serum resistin levels, following the training program. Moreover, lack of diet management during the training programs is one of the main limitation and constraints affecting the ultimate outcome, so that lack of improvements in some cytokines may be attributed to the lack of diet management. Some studies have always emphasized the potential impact of diet regime along with exercise trainings on inflammatory profile. In another study, a 3-month aerobic training without diet management did not lead to changes in serum resistin levels in obese men (38). In this regard, based on the obtained findings, some researchers have suggested that the combination of exercise training with a diet regime, rather than any of them alone, always has more effects on the levels of cytokines and insulin resistance levels (39)(40)(41). In addition, one of the limitations of this study is the small number of patients. Also, lack measuring other inflammatory or antiinflammatory cytokines that affect blood glucose is the weak points or limitation of our study. However, improving blood glucose in the absence of changes in insulin resistance and serum resistin as an inflammatory mediator is one of the strengths of the study.

Conclusions
Six weeks of aerobic training cannot affect serum resistin and insulin resistance in adult obese women. This lack of change can be attributed to the short training period, slight reduction in body weight and body fat percentage as well as small number of samples studied. Despite the lack of changes in resistin and insulin resistance, the reason for significant reductions in glucose level was probably the change in other hormonal mediators in response to exercise training.