Background The objective of this study was to measure associations of circulating interleukin-6 (IL-6) and tumor necrosis factor α (TNF-α) levels with anthropometric and abdominal fat distribution in overweight or obese postmenopausal women.
Methods One hundred eight overweight or obese postmenopausal were evaluated. Demographic and anthropometric measurements were done. Serum IL-6, TNF-α, glucose, and insulin levels were measured. Insulin resistance was calculated by using homeostasis model assessment-insulin resistance (HOMA-IR). The assessment of abdominal fat distribution was performed by ultrasonography. Statistical analysis was made with Pearson and partial correlation analysis.
Results There was a positive correlation between serum IL-6 and TNF-α (r = .19; p = .047). IL-6 was positively correlated with body mass index (BMI) (r = .43; p = .0001), waist circumference (r = .41; p = .0001), and visceral fat layer (r = .33; p = .0001) measurements and HOMA-IR index (r = .31, p = .001). A positive relationship between HOMA-IR and visceral fat layer thickness was observed (r = .320; p = .0001). TNF-α was positively associated with BMI but not with any measures of central obesity. When adjustment for BMI was performed, there were no significant relationships between the studied parameters.
Conclusions There are no significant correlations between abdominal fat distributions measured by ultrasonography and circulating IL-6 and TNF-α levels. BMI may have a stronger association with circulating inflammatory cytokine concentrations than with different measures of central obesity in overweight or obese postmenopausal women.
- tumor necrosis factorα
- abdominal fat layers
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Adipose tissue expresses and secretes multiple cytokine-like molecules.1,2Cytokines have a potential role in the regulation of adipose tissue stores and insulin resistance.3-9Although cytokines have local autocrine and paracrine actions, they are released to the circulation and act as hormones regulating the acute-phase reaction and influencing the major endocrine axis.5,6Interleukin-6 (IL-6) and tumor necrosis factor α (TNF-α) are cytokines with metabolic effects.3,4,10,11Furthermore, TNF-α and IL-6 take part in the inflammation cascade.4They are partially responsible for the disturbances in glucose and lipid metabolism.1,8,9,12Both of these cytokines induce insulin resistance.1,8,9,12,13
The abdominal fat layers comprise subcutaneous, preperitoneal, and visceral fat compartments. The abdominal fat distribution may be more crucial than total body fat in terms of complications of obesity, such as metabolic syndrome. The mechanism underlying metabolic syndrome is not completely understood, but the inflammatory cytokines secreted by abdominal adipose tissue may play a key role in the development of metabolic syndrome.14After menopause, there is a greater production of cytokines, which may contribute to obesity-related inflammatory status.15Therefore, the relationships between serum IL-6 and TNF-α levels and abdominal fat distribution might be more important than we thought in overweight or obese postmenopausal women.
This cross-sectional study was planned to measure associations of circulating inflammatory cytokine levels (IL-6 and TNF-α) with anthropometric and abdominal fat distribution in overweight or obese postmenopausal women. The assessment of abdominal fat distribution was performed by ultrasonography, which was considered a reasonable and easily applicable method. To the best of our knowledge, there are a few studies using ultrasonography to measure abdominal fat layers during the evaluation of relationship between abdominal fat distributions and circulating inflammatory cytokine levels.
One hundred fifty-three overweight or obese postmenopausal women aged 45 to 54 years who were admitted to our obesity research clinic were consecutively enrolled into this study. Menopause status was confirmed by the absence of menstruation for at least 12 months and by high serum levels of follicle-stimulating hormone (> 30 mIU/mL) and low serum levels of estradiol (E2) (< 20 pg/mL). All subjects included were not receiving any type of hormone therapy. An extensive physical examination was performed for each subject. A standardized interview was conducted by trained personnel, and detailed information for each subject was collected on medical history. Exclusion criteria included histories of hyperlipidemia, coronary artery disease, chronic kidney and liver diseases, diabetes mellitus, cerebrovascular and peripheric vascular disease, hypertension, hypothyroidism, chronic and acute inflammatory diseases, asthma, chronic bronchial diseases, alcohol consumption, smoking, and the use of all medications known to alter lipid metabolisms, liver function, and insulin secretion or action. Forty-five women were excluded from the study after the first examination because they did not meet the criteria mentioned above, so the study population consisted of 108 women. The current report represents an audit of an established clinical practice in our unit, for which our institutional ethics committee does not require that written approval form be obtained. However, patients consented to the investigations described as part of their normal medical care, and informed consent was obtained from all participants.
Venous blood samples were drawn from study participants after fasting for 12 hours. Samples were collected in serum separator tubes, allowed to clot for 30 minutes, centrifuged for 15 minutes at 2,000g at room temperature, and aliquoted into polypropylene tubes. For the measurements of IL-6 and TNF-α, one aliquot of serum was stored at −70°C until analyzed and another aliquot was sent to the laboratory for routine blood tests that included blood glucose, insulin, and cytokine measurements. Biochemical measurements were done with commercial kits. Serum glucose measurements were performed by enzymatic methods (Beckman Coulter, Fullerton, CA). Insulin and IL-6 measurements were done by solid-phase chemiluminescence's immunoassay and TNF-α by immunometric assay, “IMMULITE ONE” (DPC Biosystems, CA, Los Angeles, CA). Analytic sensitivity and intra-assay and between-assay coefficients of variation were, respectively, 2 pg/mL, 6.2% and 7.5% for IL-6 and 1.7 pg/mL, 3.6% and 6.5% for TNF-α. Insulin resistance was calculated by using the HOMA-IR score, which employs the formula fasting insulin concentration (mU/L) × glucose (mmol/L)/22.5, as described by Matthews and colleagues.16
All anthropometric measurements were done by the same physician on the day blood specimens were taken. Waist circumference (cm) was taken with a tape measure as the point midway between the costal margin and iliac crest in the midaxillary line with the subject standing and breathing normally, and hip circumference (cm) was measured at the widest point around the greater trochanter. Body mass index (BMI) (body weight (kg)/height m2) and the waist to hip ratio were computed.
Ultrasonography was carried out using a General Electric Logiq 200 Alpha/Pro (Walpole, MA). The linear array probe (7.5 MHz) was used to measure the subcutaneous and preperitoneal abdominal fat layers. The subcutaneous minimum and preperitoneal maximum measurements were taken from the region just below the xyphoid process, whereas the subcutaneous maximum and preperitoneal minimum fat layers were measured from the region just above the umbilicus for accurate assessment of subcutaneous and preperitoneal abdominal fat layers.17The convex-array probe (3.5 MHz) was used for measuring each parameter of visceral abdominal fat layer. Both the distance and the thickness of parameters were measured as follows: (1) the distance between the internal surface of the abdominal muscle and the posterior wall of the aorta on the umbilicus, (2) the distance between the internal surface of the abdominal muscle and the splenic vein, and (3) the thickness of the fat layer of the posterior right renal wall.18The patient was asked to suspend respiration during examination, and special care was taken to keep the probe just touching the skin to prevent compression of fat layers.
Statistical analysis was made with Pearson and partial correlation analysis. The data are expressed as mean ± standard error. Statistical significance was set at p < .05. Data were analyzed with SPSS, version 11.0 (SPSS Inc, Chicago, IL).
The mean values of clinical features including age and duration of amenorrhea, biochemical characteristics, and anthropometric and ultrasonographic measurements of all subjects are given in Table 1.
All parameters that were studied in 108 postmenopausal women were compared with each other. The correlations between the biochemical characteristics and the other parameters (anthropometric and ultrasonographic measurements) are summarized in Table 2. There was a positive correlation between serum IL-6 and TNF-α (r = .19; p = .047). IL-6 was positively correlated with BMI, waist circumference, visceral fat layer measurements, and HOMA-IR index (r = .31; p = .001). A positive relationship between HOMA-IR and visceral fat layer thickness was observed. TNF-α was positively associated with BMI but not with any measures of central obesity.
Partial correlation analysis was also done after adjustment for BMI, and all of the studied parameters were compared with each other again. However, when adjustment for BMI was performed, there were no significant relationships between the studied parameters. The partial correlations between the biochemical characteristics and anthropometric and ultrasonographic measurements are summarized in Table 3.
The inflammatory processes have a clustering role in obesity. The cytokine cascade is responsible for the up-regulation of chronic inflammation in overweight or obese subjects and induces obesity-related complications. After menopause, abdominal obesity is the most frequent feature observed,19and being overweight results in worsening insulin resistance and elevations in adipocytokine levels in postmenopausal women.20Estrogen deprivation after menopause enhances proinflammatory cytokine production,15and the incidence of metabolic syndrome increases from the time of the menopausal transition to postmenopause.19
TNF-α and IL-6 are well-known proinflammatory cytokines3,4,10,11and are expressed by human adipose tissue.7,10,21The positive correlation between circulating IL-6 concentration and the percentage of body fat was already demonstrated.22Thirty percent of circulating IL-6 concentrations in humans is secreted by adipocytes.23It was also shown that serum IL-6 levels are significantly correlated with BMI, waist circumference, and visceral adipose tissue.24,25Our data were consistent with these observations. In this study, serum IL-6 levels were significantly related to visceral fat layer, BMI, and waist circumference; however, serum TNF-α levels were correlated only with BMI. But after correction for BMI, there was no association of visceral fat or any measures of central obesity with circulating IL-6 and TNF-α levels. Thus, our results suggest that there is no independent association between IL-6, TNF-α levels, and visceral adipose tissue accumulation and that this association is largely mediated by the increased BMI levels. It could be hypothesized that visceral adipose tissue may not be a major source of the increased cytokine production after estrogen deprivation in postmenopausal women with normal BMI levels.
TNF-α induces insulin resistance via phosphorylation of insulin receptor and insulin receptor substrate 1.1,12,13Also, IL-6 causes an increase in serum glucose concentration by inducing peripheral insulin resistance because IL-6 stimulates glucagon and other counterregulatory hormones secretions, such as cortisol, growth hormone, and catecholamines.8,9In the current study, insulin resistance as expressed by the HOMA-IR was significantly correlated with serum IL-6 levels and visceral fat layer thickness, whereas there was no relationship between TNF-α and HOMA-IR. Also, IL-6 is a potent cytokine that induces the production of TNF-α and has an autocrine role in the production of other proinflammatory cytokines.2,6We found a positive correlation between serum TNF-α and IL-6 levels. However, these associations were not independent of BMI because we did not observe any significant correlations between these parameters after correction of BMI.
Ultrasonographic examination was recently proposed as an alternative, noninvasive, cheap, and reliable technique to evaluate body fat distribution, including intra-abdominal (visceral and preperitoneal) fat and extra-abdominal (subcutaneous) fat layers.26Computed tomography (CT) has been considered an accurate and reproducible technique of body fat measurement.27However, CT scans are costly and time-consuming and involve exposure to ionizing radiation. It was demonstrated that the ultrasonographic assessment of intra-abdominal fat layer is well correlated with CT-guided measurement of visceral adipose tissue thickness, and it was also proven that ultrasonographic examination is a valuable method in the direct evaluation of intra-abdominal fat distribution.18Comparison of sonographic measurement of subcutaneous and preperitoneal fat thickness with magnetic resonance imaging (MRI) measurement of total abdominal and visceral fat indicated that ultrasonographic examination is a useful technique to evaluate regional distribution of obesity in the assessment of cardiovascular risk.28In a validation study, intra-abdominal adipose tissue was assessed by CT, MRI, anthropometry, and ultrasonography, and it was shown that abdominal ultrasonography is a reliable and reproducible method to assess the amount of intra-abdominal adipose tissue and to diagnose intra-abdominal obesity.29The validity of ultrasonography in evaluation of visceral and subcutaneous fat thickness was also supported by another study.30Interoperator and intraoperator mean variation coefficients of this method were about 7% and 5%, respectively.31
Our study has a limitation: it was a cross-sectional clinical study, and the circulating IL-6 and TNF-α levels were merely evaluated. Therefore, we need transcriptional profiling studies to evaluate the relationship between abdominal fat layers and proinflammatory cytokines in greater detail.
In conclusion, these results suggest that there are no significant correlations between abdominal fat distributions measured by ultrasonography and circulating IL-6 and TNF-α levels independent of BMI in overweight or obese postmenopausal women. BMI may have a stronger association with circulating inflammatory cytokine concentrations than abdominal fat layers or other measures of central obesity in this population.
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