A random sample of 2500 citizens of Palanga aged 35–74 was drawn from the National Population Register in 2002. The citizens of Palanga were chosen as an object of investigation, because there was close community with minor migration reflecting the population of West part of Lithuania. The optimal size of the sample, ensuring representativeness of the population of Palanga aged 35–74 years, was calculated (1630 ± 33 individuals). From the sample of 2500 citizens, 160 selected persons were not invited to participate in the study, because they were not found at the given addresses, 1602 persons (600 males and 1002 females) participated in the survey in 2003. The response rate for the first survey was calculated in the following: (1602/2340) × 100 = 68.5%. In the period from 2003 until 2013, 158 persons (9.9%) of those, who had participated in the first survey in 2003, died, 47 (2.9%) had changed their living address, 20 (1.2%) declined to participate, 11 (0.7%) could not participate as a result of serious health problems, and 435 (27.2%) people did not respond to the multiple invitations sent to them by post. During the second survey, data from 931 people, 322 males and 609 females, aged 45–84 years, were collected in 2013. The first and the second surveys were approved by the Ethics Committee for Biomedical Research at Lithuanian University of Health Sciences, Kaunas, Lithuania (protocol code BE-2-25 and 14 June 2012 of approval). Informed consent was obtained from all participants during both surveys. However, blood samples were collected from only 850 subjects and fifteen (1.8%) subjects, whose blood tests showed severe thyroid dysfunction, were excluded from the analysis. The final study cohort consisted of 835 subjects: 300 (35.9%) men and 535 (64.1%) women. The mean age of the study subjects was 63.5 ± 10.3 years. The original source of method descriptions is described elsewhere12.
All study participants were evaluated according to for socio-demographic characteristics (i.e., age, sex, height, weight, education and marital status), behavioral factors, HRQoL and self-perceived health using questionnaires. Fasting blood samples were draw from all participants and biochemical tests were performed for the glucose and insulin. The IR was calculated according to the formula HOMA-IR (HOMA-IR = (fasting plasma insulin [µIU/ml]) × (fasting plasma glucose [mmol/l])/22.5); normal rate of HOMA = ≤ 2.7.
WHO-5 wellbeing test
The WHO-5 Well-being Index13 questionnaire consisted of 5 questions reflecting the well-being of a person during the last 2 weeks: I feel cheerful and in good spirits; I feel calm and relaxed; I feel active and vigorous; I wake up feeling fresh and rested; my daily life is filled with things that interest me. The raw score is calculated by totaling the figures of the five answers. The raw score ranges from 0 to 25; 0 representing the worst possible and 25 representing the best possible quality of life, Cronbach α = 0.876. The respondents who scored 50 and more did not have depressive mood. For the respondents who scored less than 50, depressive mood was identified, and they were ascribed to the group of an increased risk of depression.
The questionnaire on general data, behavioral factors and self-perceived health
The questionnaire on general data14 was used to collect the information about the marital status, education, employment and income of respondents. The questionnaire on behavioral factors14 consisted of questions about smoking, alcohol consumption, and physical activity during the last year. The self-perceived health questionnaire14 consisted of questions about complaints and diagnosed diseases, medications used during the last year, frequency of stress events, and visits to any doctor.
Arterial blood pressure (mmHg) was measured twice with a quicksilver sphygmomanometer on the right hand while a person was sitting, with the precision of 2 mm according to the methodological guidelines15. The average of two measurements was used for the analysis. If the participants' systolic blood pressure was greater than 140 mmHg and/or their diastolic arterial blood pressure was greater than 90 mmHg in the previous two weeks, they were graded as hypertensive.
Body height was measured in stocking feet (without shoes) with a medical height rod. Body weight was measured without shoes using a medical scale. Body mass index (BMI) was calculated according to the following formula: BMI = body mass (kg)/height (m) 2 using the data of height and weight measurement. Overweight was diagnosed when BMI was 25.0–29.9 kg/m2, obesity when BMI was 30.0 kg/m2 and more.
36-Item Short Form Medical Outcome Questionnaire (SF-36)
The 36-item Short Form Medical Outcome Questionnaire (SF-36) consists of 8 multi-item subscales that assess HRQoL on 8 domains: physical functioning, social functioning, role limitations due to physical problems, role limitations due to emotional problems, mental health, energy/vitality, pain, and general health perception. Each of the 8 SF-36 domains is scored on scales from 0 to 100, with higher scores indicating better HRQoL16. Internal reliability (ɑ coefficients) of 8 subscales has been found to range between 0.71 and 0.85.
The clinical and the sociodemographic characteristics were reported by frequencies and percentages for the categorical variables, and with means and standard deviations for the continuous variables. Variable distribution of similarity to normal was assessed visually and using the Kolmogorov-Smirnov and Shapiro-Wilk'o tests. The data characteristics were compared between groups without IR and with IR were using Fisher’s χ2 test, for the parametric two-tailed Student's t-test test or nonparametric Mann-Whitney U test. A logistic regression analysis using an enter method was used to investigate if 10-year follow-up period (time), sex, age, IR were related to different areas of quality of life (Model 1) and additionally adjusted by family status; education; employment; self-perceived health; frequent stressful events; depression mood; alcohol used; smoking; illness during past 12 month; and obesity (Model 2). Statistical analyses were performed with the Statistical Package for the Science Software v.22 (SPSS, Chicago, IL). The level of significance was set at p < 0.05.