HOMA-IR mean values in healthy individuals: a population-based study in iranian subjects

Homeostasis Model Assessment-estimated Insulin Resistance (HOMA-IR) is an important indicator of insulin resistance. In this population-based investigation, we sought to report the mean value of HOMA-IR in different subgroups of a large population-based database of Iranian healthy subjects. This study recruited adult healthy individuals between the ages of 18 to 70 years old to Massoud Medical Laboratory, Tehran, Iran. Fasting insulin was measured by using the Electro Chemiluminescence method using Roche Cobas 6000 e601/602 instrument. The mean ± SD value of the HOMA-IR index in the studied population was 2.11 ± 0.99 (2.5-97.5% percentiles: 0.66–4.50). In addition, the mean ± SD of HOMA-IR index in male and female groups were 2.35  1.0 (2.5-97.5 percentile: 0.57–4.37) and 2.05 ± 1.0 (2.5-97.5 percentiles: 0.53–4.35), respectively. Interestingly, it was observed a significant increment for the HOMA-IR index in the male group compared with the female group in all age subgroups (P < 0.01). Our findings showed the mean value of 2.11 ± 0.99 HOMA-IR in the Iranian healthy population. Considering the large sample size in our study, more clinical investigations in terms of ethnicity should be done to provide a precise standardized HOMA-IR index in the Iranian population.


Introduction
Insulin resistance (IR) is principally defined as the impaired regulation of insulin production and secretion of hepatic glucose which is recognized as hepatic IR [1]. In the liver, IR refers to impaired suppression of glucose production, whereas peripheral IR is a decreased response to insulin in skeletal muscle or adipose tissue. In the IR state, pancreatic β-cells respond to insulin impairment by the production and secretion of insulin. This compensatory action can maintain glycaemia at the normal values [2]. In clinical settings, several dynamic and static methods such as HOMA-IR, studies investigated metabolic syndrome outcomes in casecontrol studies with small a sample size [10]. In clinical practice, IR assessment through HOMA-IR is considered a vital index for early management and screening of T2DM [11]. In addition, HOMA-IR is useful to distinguish different high-risk groups as screening guidelines are introduced. It has been noted that via HOMA-IR evaluation IR subjects can distinguish from those with healthy status [12][13][14]. To our knowledge, there is no valid reference value for HOMA-IR in the Iranian population. Moreover, few studies have evaluated the relation of IR with a broad group of health related-outcomes in large and population-based human subjects. In this population based investigation, we sought to report the mean value of HOMA-IR in different subgroups of a large population-based database of Iranian healthy subjects.

Subjects
This is a population-based study that recruited adult healthy individuals between the ages of 18 to 80 years old to Massoud Laboratory, Tehran, Iran between January 2015 to August 2020. Inclusion criteria were determined as follows: healthy adult subjects with fasting blood glucose between 70 and 100 mg/dl and fasting insulin value between 2 and 20 (µIU/ ml), glycated hemoglobin (HbA1c) based on the National Glycohemoglobin Standardization Program (NGSP) ≥ 6.5% at baseline [15]. The individuals with chronic kidney disorder, non-alcoholic fatty liver (NAFLD), and any chronic diseases such as inflammatory and autoimmune disorders, and endocrine dysfunction were not included in this study. This study was in accordance with the Helsinki Declaration.

Biochemical assays
Upon 8-12 h of fasting, 5 mL of venous blood was taken from all participants. Then Samples were centrifuged at 4000 RPM for 10 min and serum separated. Serum glucose levels were detected by the Hexokinase method using Roche Cobas 6000 c501/502 auto analyzer. Fasting insulin was measured by using the Electro chemiluminescence method using Roche Cobas 6000 e601/602 instrument. Serum levels of triglycerides, high-density lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL) cholesterol, and total cholesterol (mg/dl) were examined by Roche Cobas 6000 c501/502 instrument. All analyses were performed according to the manufacturer's instructions and after verification of methods. Insulin resistance was assessed by Homeostatic Model Assessment of Insulin Resistance (HOMA-IR) index using the following formula: Fasting Insulin (µIU/mL) × Fasting Serum Glucose(mg/dL) 405

Statistical analysis
Statistical analyses were performed by SPSS version 23.0 (SPSS Inc, Chicago, IL) and Graph Pad Prism software version 8.01 (Graph Pad, San Diego, CA). Data presented as mean ± SD and median and interquartile range (IQR). The interval references of HOMA-IR presented in percentiles ranging from 2.5 to 97.5% based on gender and age groups. The differences in the HOMA-IR values were analyzed using Student's t-test between females and males in all age subgroups. The p-values less than 0.05 were considered statistically significant with a 99% confidence interval (CI).

Baseline and clinical characteristics of the study population
Demographic and biochemical parameters of subjects in the total population and different age subgroups are presented in Table 1. In total, 6637 individuals were 57.7% (3696) women and 44.3% (2940) men. All subjects were divided into 7 age subgroups. The median ± IQR age of the total population was 37.8 ± 12.1 with BMI 26.5 ± 3.8 kg/m 2 . At baseline, the mean values of TG, cholesterol, HDL-c and LDL-c in the studied population were 124.3 ± 7.2, 179.9 ± 36.7, 48.6 ± 11.9, and 108.2 ± 30.0, respectively. The results revealed that the mean serum glucose levels in the studied population were 89.4 ± 5.9 and the age group between 18 and 25 had the lowest (87.5 ± 5.8) level compared with other age subgroups. The mean value of fasting insulin was 9.5 ± 4.3 for the studied population and the age subgroup of 30-35 had the lowest insulin concentration (9.1 ± 4.2). Age subgroups of 18-25 and 40-45 had the highest fasting insulin levels (9.9 ± 5.7 and 9.8 ± 4.4 respectively). Other biochemical parameters such as AST, ALT, urea, Cr, and TSH values are shown as mean ± SD.

HOMA-IR values of the study population
As shown in Table 1, the mean value of HOMA-IR in the studied population was 2.11 ± 0.99. In addition, all participants were sub grouped regarding age and gender (   Up to now, numerous methods have been suggested for the measurement of IR in clinical practice [17]. Some techniques such as a hyperinsulinemic-euglycemic clamp, the gold standard method of IR, are difficult to use in clinical practices and population-based studies [3]. Among them, the HOMA-IR index is proposed as a simple surrogate for wider use in everyday clinical practice [18,19]. HOMA-IR is a well-established index that assesses insulin resistance status based on comparing fasting glucose and circulating insulin levels that higher HOMA-IR indicates more insulin resistance [9]. The reference value of HOMA-IR has been assessed in several human studies [13,14,16,17]. To the best of our knowledge, data regarding reference value and accurate threshold of HOMA-IR are limited in the Iranian healthy population. Therefore, this study sought to report values of the HOMA-IR index regarding age and gender in healthy Iranian individuals. In this study, the mean±SD value of HOMA-IR was 2.11 ± 0.99 in the total population. As noted earlier, mean values of HOMA-IR were reported regarding gender and age differences. The findings of the present study showed higher mean values of HOMA-IR in the men group compared to female subjects. In addition, the HOMA-IR mean values were increased at higher ages in both female and male groups. Previous literatures have evaluated HOMA-IR values in various pathological conditions with different ethnicities and clinical backgrounds [1,4,7,8,18]. One study by Esteghamati et al. investigated 1276 non-diabetic and nonhypertensive subjects in the Iranian population [17]. The authors indicated that the mean value of HOMA-IR was 1.8 ± 1.13 in the whole population and the lower limit of the top quintile of HOMA-IR distribution values in normal subjects was 1.78 (1.69 for men and 1.81 for women). In another study, Yamada et al. reported a mean value of 2.0 for HOMA-IR in non-obese Japanese individuals [12,18]. Nakai et al. reported the mean value of 1.7 (90th percentile) for IR in Japanese subjects [18]. It has been noted that the optimal cut-off point for HOMA-IR was 1.77 in nondiabetic and 3.87 in diabetic Iranian subjects [20]. Gayoso et al. investigated 2459 random Spanish population samples [21]. They proposed the mean value of 2.0 ± 1.1 in the total population and 1.9 ± 1.0 for women and 2.1 ± 1.2 for men. In another multicenter cross-sectional survey, Vilela et al. indicated the mean value of 1.7 (1.1-2.7) for HOMA-IR in 1061 non-diabetic Brazilian individuals [22].
In the present study, the mean values of HOMA-IR were higher than in previous investigations. One reason for the inconsistency could be different laboratory methods of insulin and glucose measurement. It has been argued that various insulin assays reflect different insulin levels and ultimately result in different HOMA-IR values [2]. Another possible explanation for this discrepancy may be population diversity age subgroup had the lowest HOMA-IR range between 0.59 and 4.13 (2.5-97.5 percentile of suggested reference interval) while the HOMA-IR in the > 50 subgroup was greater than other age subgroups (0.62-4.35, 2.5-97.5 percentile of suggested reference interval). In men, the 25-30 age group had the lowest HOMA-IR range (0.73-4.32, 2.5-97.5 percentile of suggested reference interval) and the 35-40 age group had the highest HOMA-IR range between 0.7 and 4.5 (2.5-97.5 percentile of suggested reference interval). As depicted in Fig. 1, there were significant differences in HOMA-IR values between men and females (p < 0.01) but we did not find any significant differences based on age subgroups. Regression analyses for HOMA-IR between female and male subjects are shown in Fig. 2.

Discussion
Insulin resistance is recognized as impaired biological insulin response with normal concentrations that correlate with a cluster of disorders such as cardiovascular disease (CVD), obesity, hypertension, and type-2 diabetes [16].

Declarations
Ethics approval and consent to participate All participants signed an informed consent letter. We obtained ethical approval from the Massoud Laboratory (ML-99-1969).

Consent for publication
All authors support submission to this journal. in terms of ethnicity. It is noteworthy that previous studies applied different inclusion criteria for defining and measurement of insulin resistance. It has been noted that both ethnic differences and gender variations can affect the reference range of HOMA-IR in different populations [5]. In this study, the mean values of HOMA-IR in the women group were slightly lower than the male group. In accordance with these results, Isokuortti et al. reported that after adjustment of age and BMI, the male group had slightly higher HOMA-IRs than female group [5]. Till now, there were rare studies that reported HOMA-IR mean values in healthy individuals according to gender [17,20]. Even though individuals in the present study were non-obese with normal glucose and lipid metabolism, these differences between genders may be related to sexual maturity and adipose tissue accumulation pattern in the adolescent population. It has been argued that in Indian the adolescents HOMA-IR scores increased with sexual maturity and with progression from normal to obese and a HOMA-IR cut-off of 2.5 provided the maximum sensitivity and specificity in diagnosing [23].
In conclusion, the current results showed the mean value ± SD of 2.11 ± 0.99 for HOMA-IR in the Iranian healthy population. In addition, the findings revealed a higher HOMA-IR value in men compared with females.
As noted in the current study, we reported different reference values of HOMA-IR in healthy subjects compared to previous literature. Considering the large sample size in our study, more clinical investigations in terms of ethnicity should be done to provide a precise standardized HOMA-IR index in Iranian population. Reference values for HOMA-IR, even in healthy individuals may be population-specific and every laboratory should establish its own reference value for HOMA-IR, or at least understand how its insulin assay compares with other laboratories.