Abstract
Background:
Serum (ApoB/ApoA-1) ratio is considered a stronger predictor of systemic inflammation and atherosclerosis than serum total cholesterol/HDL (TC/HDL) ratio among adults. We evaluated the relationships between ApoB/ApoA-1 and TC/HDL ratios with surrogate markers of inflammation and insulin resistance (IR) among obese adolescents.
Methods:
Body mass index z-score (BMI-z), body composition, fasting glucose, insulin, lipids, high-sensitive c-reactive protein (hs-CRP), hemoglobin A1c (HbA1c), and homeostatic model assessment for insulin resistance (HOMA-IR) were evaluated in 143 obese adolescents.
Results:
Male subjects had higher BMI-SDS, fat-free mass (FFM), and glucose than female subjects (P < 0.01). Furthermore, 54.5% met diagnostic criteria for metabolic syndrome (MS) and displayed higher SBP, BMI-SDS, fat mass (FM), HOMA-IR, hs-CRP, TG, TC/HDL, TG/HDL, ApoB/ApoA-1, and HbA1c, but lower HDL and ApoA-1 than the non-MS group (P < 0.05) with similar gender distribution. In the entire cohort, TC/HDL and ApoB/ApoA-1 ratios were strongly correlated (r = 0.81, P < 0.0001). Receiver operating characteristic analysis indicated that the area under the curve in MS subjects for ApoB/ApoA-1 and TC/HDL-C ratios was not statistically different. ApoB/ApoA-1 and TC/HDL-C ratios were positively correlated with SBP (r = 0.29; P = 0.0004) and (r = 0.43; P < 0.0001), respectively. Finally, ApoB/ApoA-1 and TC/HDL-C ratios were correlated with hs-CRP (r = 0.21; P = 0.014) and (r = 0.26; P = 0.0016), respectively. However, the relationships between ApoB/ApoA-1 and TC/HDL ratios with HOMA-IR were not statistically significant.
Conclusions:
Unlike in the adult population, serum ApoA-1, ApoB, and ApoB/ApoA-1 ratio may not have significant advantage over conventional lipoproteins in evaluating the presence of systemic inflammation, MS, and risk of atherosclerosis in obese adolescents.
Introduction
M
Dyslipidemia is an independent major risk factor for progression of cardiovascular disease (CVD) 7 and serum levels of low-density lipoprotein cholesterol (LDL-C) HDL-C and total cholesterol to HDL-C (TC/HDL-C) ratio have been utilized for determination of risk of CVD in adults. 8,9 Indeed, Castelli suggested that the total cholesterol to HDL-C ratio or Castelli index (CI) was a strong predictor of coronary risk based on preliminary analysis of the Framingham study. 8 Furthermore, elevated TC/HDL-C ratio has been associated with a proinflammatory state in adults and adolescents. 10,11
On the contrary, it has been suggested that apolipoproteins (Apo), ApoB and ApoB/ApoA-1 ratio, may be better markers of CVD than conventional LDL-C, HDL-C, and TC/HDL-C ratio. 12 –14 Indeed, Juonala et al. suggested that serum levels of ApoB and ApoA-1 measured in children and adolescents reflect a lipoprotein profile predisposing to the development of subclinical atherosclerosis in adulthood. 15 In addition, several studies have reported a strong association of systemic inflammation with the serum ApoA-1 and ApoB/ApoA-1 ratio as in adults and children. 16 –18 However, the apolipoproteins superiority to conventional lipoproteins in assessing CVD risk factor and inflammatory state has not been confirmed in other studies. 19 –21 To date, there has been no comparison of standard lipoprotein levels with apolipoproteins in relationship with low-grade inflammation in adolescents with and without MS. Therefore, we compared the relationship between TC/HDL-C ratio and ApoB/ApoA-1 ratios and some of the components of MS and high-sensitivity c-reactive protein, a biomarker of low-grade inflammation in a group of obese adolescents with and without MS.
Subjects and Methods
Subjects and design
One hundred forty-three adolescents (age: 12.1–17.4 years) who met the criteria for obesity [body mass index (BMI) >95th percentile for age] 22 were included in the study. Participants were evaluated at the Children's Hospital of Wisconsin (CHW) Endocrine Clinic for evaluation of MS between May 2008 and April 2011. Race/ethnicity was self-assigned: Caucasian (C, n = 61; 42.7%), Mexican American [Hispanic (H), n = 44; 30.8%], and African American (AA, n = 38; 26.6%). Children were excluded if they had hepatic or renal disease, metabolic rickets, malabsorptive disorders (Crohn's disease, cystic fibrosis, and celiac disease) or cancer, or were taking multivitamin supplements, anticonvulsants, or systemic glucocorticoids. The CHW Institutional Review Board (IRB) approved the retrospective review of patients' clinical charts; therefore, informed consent was not required.
As a part of routine care, participants and/or their guardians completed a questionnaire detailing their medical history and medications. Two well-trained clinicians determined pubertal maturation (Tanner stage). Data were collected on patients, including age, gender, and self-declared ethnicity, height, weight, blood pressure, and body composition analysis by bioelectrical impedance (TANITA-TBF-410; TANITA Corporation of America, Inc., Arlington Heights, IL) for evaluation of fat mass (FM), fat-free mass (FFM), and total body water (TBW). 23,24 Fasting serum samples were obtained for glucose, insulin, hemoglobin A1c (HbA1c), lipid profiles, and high-sensitivity c-reactive protein (hs-CRP).
Laboratory studies and calculations
All blood samples were obtained between 0800 and 1100 h after an overnight fast. Serum glucose was measured by an autoanalyzer (Orthodiagnostics Fusion 5.1; Ortho-Diagnostics, Rochester, NY). The hs-CRP assays were carried out at Quest Diagnostics (San Jose, CA) using a polystyrene particle-enhanced immunonephelometric method (Dade Behring BNII). The detection limit of this assay was 0.20 mg/L, with measuring range of 0.18–1150 mg/L, and with intra-assay and interassay coefficients of variance of 2.65% and 3.6%, respectively. The hs-CRP values >10 mg/L were excluded to avoid influence of acute infection. 25 HbA1c was determined by the Bayer DCA (Bayer Diagnostics, Inc., Tarrytown, NY) 2000 instrument (nondiabetic range of 4.5%–5.7%).
Fasting serum insulin was measured by Nichols radio-immunoassay (RIA) (Nichols Institute, San Clemente, CA) with intra-assay and interassay coefficients of variation (CV) of 2.4%–6.3% and 5.2%–13.0%, respectively. The homeostatic model assessment estimates for insulin resistance (HOMA-IR) [(blood glucose mM × insulin μU/mL)/22.5) was calculated. 26
Total cholesterol (TC), HDL-C, and triglycerides (TG) were determined by colorimetric methods (Beckman spectrophotometer, Fullerton, CA). LDL-C was calculated using Friedewald's equation. 27 The levels of apolipoproteins were measured by kinetic immunonephelometry. The coefficients of variation for interassay and intra-assay corresponding to the methods used to measure were the followings: TC (1.06% and 0.68%), HDL-C (3.51% and 0.96%), ApoB (4.3% and 2.93%), and ApoA-1 (4.5% and 2.5%).
Blood pressure (BP) measurements were taken twice with the patient in sitting position. Elevated systolic or diastolic pressure (SBP and DBP, respectively) was defined as a value above the 95th percentile for age and gender. 28
Modified National Cholesterol Education Program (NCEP) criteria 2 for diagnosis of MS were defined as the presence of three or more of the following: age-adjusted BMI >95th percentile, systolic or diastolic BP >90th percentile, TG >90th percentile, HDL cholesterol <5th percentile, and impaired fasting glucose >5.6 mM.
Statistical analyses
Statistical analyses were carried out using SPSS (version 24.0). Data are expressed as mean ± SD. Body mass index (BMI) values were converted into standard deviation scores (SDS), which were normalized for age and gender based on 2000 Center for Disease Control (CDC) growth charts. The natural logarithmic transformation of the variables was used in the correlation and regression analyses when they were found to be skewed. Differences between those with MS and non-MS groups were estimated using unpaired Student's t-tests. Chi-square analyses were used to compare prevalence of MS. Spearman's correlations were performed to examine the associations between ApoB/ApoA-1 and TC/HDL-C ratios as well as among SBP, hs-CRP, and HOMA-IR. Fisher Z transformation was used when comparing correlations. P < 0.05 was considered significant.
Results
Findings stratified by gender
Table 1 summarizes the clinical and biochemical characteristics of the entire participant cohort stratified according to gender. There were no differences in FM, fasting insulin, HbA1c, hs-CRP, HOMA-IR, TG, TG:HDL-C, cholesterol, LDL-C, HDL-C, TC: HDL-C, ApoA-1, ApoB, and ApoB:ApoA-1 ratio between male and female subjects. However, male subjects had higher, BMI-SD, FFM, FFM:FM, TBW, and fasting glucose than female subjects (P < 0.01).
FM, fat mass; FFM, fat-free mass; TBW, total body water; HOMA-IR, homeostatic model assessment estimates for insulin resistance; MS, metabolic syndrome; NA, not applicable; NS, not significant.
The AA subgroup displayed significantly higher BMI-SDS, FM, and HbA1c than H and C subgroups (data not shown, P < 0.01). There were no ethnic differences in fasting insulin concentrations, HOMA-IR values, hs-CRP levels, and lipoprotein profiles in our cohort (data not shown).
Findings stratified by presence and absence of MS
In our cohort, 54.5% of adolescents met diagnostic criteria for MS displaying higher SBP, BMI SDS, FM, glucose, HbA1c, insulin, HOMA-IR, hs-CRP, triglycerides, triglycerides:HDL-C ratio, cholesterol:HDL-C ratio, and ApoB/ApoA-1 ratio than non-MS group (Table 2). There were no differences in total cholesterol and ApoB levels in MS compared with non-MS group. However, subjects in MS group had lower serum HDL-C and ApoA-1 levels than non-MS group.
NA, not applicable; NS, not significant.
Findings in the entire cohort
Using receiver-operating characteristic analysis indicated that the area under the curve in MS subjects for ApoB/ApoA-1 and TC/HDL-C ratios were not statistically different (0.76 ± 0.47 vs. 0.85 ± 0.36; P = 0.18). Total cholesterol/HDL-C and ApoB/ApoA-1 ratios were strongly correlated (r = 0.81, P < 0.0001) (Table 3). In addition, LDL-C/HDL-C and ApoB/ApoA-1 ratios were also highly correlated (r = 0.79; P < 0.0001) (data not shown). Although the relationships between ApoB/ApoA-1 and TC/HDL-C ratios with FM were not statistically significant, ApoB/ApoA-1 and TC/HDL-C ratios were positively correlated with SBP (r = 0.29; P = 0.0004) and (r = 0.43; P < 0.0001), respectively. Also, ApoB/ApoA-1 and TC/HDL-C ratios were correlated with hs-CRP (r = 0.21; P = 0.014) and (r = 0.26; P = 0.0016), respectively. Furthermore, there was a strong positive relationship between serum TG and hs-CRP levels (r = 0.0001). There were positive correlations between HOM-IR and FM (r = 0.48; P < 0.0001), TG (r = 0.26; P = 0.0014), HbA1c (r = 0.30; P = 0.0002), and hs-CRP (r = 0.29; P = 0.0004), respectively. However, the relationships between ApoB/ApoA-1 and TC/HDL-C ratios with HOMA-IR were not statistically significant.
P < 0.01.
P < 0.05.
Discussion
In the present study, TC/HDL-C and ApoB/ApoA-1 ratios were strongly correlated, and TC/HDL-C and ApoB/ApoA-1 ratios showed similar association with MS. Also, ApoB/ApoA-1 and TC/HDL-C ratios were positively correlated with SBP. Finally, ApoB/ApoA-1 and TC/HDL-C ratios displayed similar correlations with hs-CRP in the entire cohort. However, the relationships between ApoB/ApoA-1 and TC/HDL ratios with HOMA-IR were not significant.
While the alterations in cholesterol, triglycerides, LDL-C, and HDL-c metabolism have already been identified as risk factors for CVD, other lipoproteins have been implicated to establish more dependable biochemical factors which can predict CVD. Juonala et al. reported that ApoB/ApoA-1 ratio, measured in a cohort of nonobese and obese adolescents, was a stronger predictor of adulthood carotid intima media (CIMT) and brachial flow-mediated dilation (FMD), as markers of subclinical atherosclerosis, than LDL-C/HDL-C ratios. 15 However, ApoB/ApoA-1 and LDL-C/HDL-C ratios in adolescents were similarly correlated with adulthood SBP. Also, it has been suggested that there are advantages in measuring ApoB and ApoA-1 levels, since their concentrations influence the number of particles of their corresponding lipoprotein classes with resultant opposite risk level. 29 Therefore, a high ApoB/ApoA-1 ratio corresponds to the number of atherogenic lipoprotein particles, which are expected to be deposited in the arterial wall. However, Raitakari et al. have previously reported that conventional risk factors such as LDL-C, SBP, and obesity during adolescence are associated with increased adulthood CIMT. 30 While we did not assess any vascular markers of atherosclerosis, both ApoB/ApoA-1 and TC/HDL-C ratios were correlated with SBP in our obese adolescent cohort. Indeed, TC/HDL-C ratio appeared to have a stronger correlation with SBP than ApoB/ApoA-1 ratio.
In addition, ApoB/ApoA-1 ratio has been reported to be strongly associated with the presence of MS and insulin resistance in obese adults and children. 16,31 In our cohort, ApoB/ApoA-1 and TC/HDL-C ratios were significantly higher in obese adolescents with MS than those without MS. Indeed, there was no difference between ApoB/ApoA-1 and TC/HDL-C ratios and its relationship to MS. On the contrary, we did not observe any correlation between ApoB/ApoA-1 and TC/HL-C ratios and index of insulin resistance (HOMA-IR).
Some studies have reported a strong association of systemic inflammation with the serum ApoA-1 and ApoB/ApoA-1 ratio in adults and children. 16 –18 However, the advantage of ApoB/ApoA-1 ratio versus TC/HDL-C ratio in evaluating CVD risk factor and inflammatory state has not been confirmed in other studies. 19 –21 Recently, Agirbasli et al. reported that TC/HDL-C ratio correlates with hsCRP levels in a group of children and adolescents. 11 Furthermore, we observed that ApoB/ApoA-1 and TC/HDL-c had similar correlation with hs-CRP, a biomarker of low-grade inflammation.
In this cohort, male subjects had higher BMI SDS and higher fasting glucose levels than female subjects especially among AA and H subgroups. However, higher BMI SDS in AA and H male subjects was not associated with higher rate of dyslipidemia. It is likely that higher BMI SDS among males was due to their higher FFM and FFM:FM ratio than female subjects. 32 Therefore, male subjects were less likely to have dyslipidemia due to lower relative body fat. While male subjects had higher fasting glucose levels than female subjects overall, their HbA1c levels were not significantly different from HbA1c levels among females. Finally, we observed a tendency for higher prevalence of the MS among females despite having a lower BMI SDS than males. Since 58.1% of our cohort were females with lower FFM:FM ratios compared with males, it is likely that this lower FFM:FM ratios is what may have influenced the development of insulin resistance in the females. 33
Limitations to this study include the retrospective design of the study and lack of oral glucose tolerance data to assess glucose homeostasis and β-cell function in relationship to the ApoB/ApoA-1 and TC/HDL-C ratios, particularly since HbA1c values were higher in AA group compared with C and H groups. The possible presence of impaired glucose tolerance among some of the participant cohort may have influenced our results. 34 Another limitation to the study is that there were no age- and sex-matched normal weight controls for each racial/ethnic group. Also, the accuracy of bioelectrical impedance (BIA) for assessment of body composition has been questioned because of larger errors in individual estimates of body fat compared with DXA method. 24 However, BIA has been deemed accurate for assessing body composition in large groups of normal weight or obese pediatric subjects compared with DXA. 35
In conclusion, ApoB and ApoB/ApoA-1 ratio does not have significant advantage over conventional lipoprotein ratios in evaluating the presence of systemic inflammation, MS, and risk of atherosclerosis in obese adolescents. Since apolipoprotein and lipoprotein ratios share similar dietary and lifestyle factors and are highly correlated, either of these ratios may be an adequate index of dyslipidemia. Further studies are needed, however, to evaluate importance of ApoB/ApoA-1 ratio in predicting atherosclerosis in adolescents with MS.
Footnotes
Acknowledgment
This study was funded by the Diabetes Research Fund, Department of Pediatrics, Medical College of Wisconsin, Milwaukee, WI
Authors' Contributions
Ramin Alemzadeh collected and analyzed the data as well as wrote and edited the article; Jessica Kichler analyzed the data as well as wrote and edited the article.
Author Disclosure Statement
No competing financial interests exist.
