The European Journal of Public Health Advance Access originally published online on October 17, 2005
The European Journal of Public Health 2006 16(5):520-525; doi:10.1093/eurpub/cki196
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Overweight and obesity |
Short-term predictors of abdominal obesity in children
Glykeria Psarra1, George P. Nassis1,2 and Labros S. Sidossis1
1 Laboratory of Nutrition and Clinical Dietetics, Department of Nutrition and Dietetics, Harokopio University, Greece
2 Department of Sport Medicine and Biology of Physical Activity, Faculty of Physical Education and Sport Science, University of Athens, Greece
Correspondence: Labros S Sidossis, PhD, Associate Professor and Director, Laboratory of Nutrition and Clinical Dietetics, Department of Nutrition and Dietetics, Harokopio University, El. Venizelou 70, Athens 17671, Greece, tel: +30 210 9549154, fax: +30 210 9549141, e-mail: lsidossis{at}hua.gr
Received November 19, 2004, accepted August 30, 2005
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Abstract |
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Background: The aim of this study was to examine the short-term tracking of abdominal adiposity in children. Methods: A total of 918 children (477 boys) aged 6–12 years at baseline were followed-up for 2 years. Central obesity was assessed by waist circumference (WaistC), whereas body fat distribution by waist-to-hip ratio. Maturity was assessed by the Khamis–Roche method. Parental fatness and children's cardiorespiratory fitness (CRF) were also evaluated. Multiple and logistic regressions were employed to identify the predictors of BMI and WaistC. Results: Tracking of body fatness and body fat distribution was high (r = 0.69–0.86, P < 0.01). More boys remained obese than girls (P < 0.05), whereas a greater percentage of boys moved to a higher quartile of WaistC after the 2-year follow-up (22.0 vs 14.1%, P < 0.01). Sex, child's maturity and WaistC at baseline, CRF, and maternal BMI explained 76% of the variability in BMI and WaistC at the follow-up (n = 290). Children with high WaistC at baseline and low CRF presented 1.9- and 4.3-fold increased risk of remaining in the upper quartile of WaistC at the follow-up (P < 0.01; n = 552). Conclusion: Youth with increased WaistC at baseline and low CRF presented an increased chance of maintaining central obesity at the follow-up. More boys than girls moved into a higher quartile of abdominal obesity during the 2-year follow-up period and this should be taken into account in designing programmes for childhood obesity.
Keywords: central adiposity, children, fatness, fitness, tracking, waist circumference
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Introduction |
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Prevalence of childhood and adolescent obesity is increasing worldwide.1 Paediatric obesity is associated with major health complications in childhood as well as in later life, such as hyperinsulinaemia, insulin resistance, hyperlipidemia and hypertension.2–4 Thus, there is a growing interest in understanding better the factors that affect weight gain in childhood.
Maternal nutrition at pregnancy, child's birth weight, breast feeding, child's dietary habits and physical activity, parental obesity and family socioeconomic status have been identified as predictors of adolescence and adult obesity.5 With regard to physical activity, longitudinal studies have shown that an increased level of physical activity is associated with lower body fat.6,7 This view is contradicted by other studies.8,9 The conflicting views could be due to methodological limitations in studying physical activity accurately.10,11 Accordingly, a number of studies have proposed the use of cardiorespiratory fitness (CRF) instead as a valid index of physical activity.12,13
The study of tracking may help in identifying the obesity indices with the greater stability and thus it may assist in the construction of more effective intervention programs. Several studies have examined tracking of body composition in children,14–16 but relatively little is known about the stability of central or abdominal adiposity and body fat distribution in both boys and girls.15,17 This is despite the facts that (i) central obesity presents a stronger association with several risk factors for cardiovascular diseases than total body obesity4,18 and (ii) central obesity has increased at a higher rate than total body obesity in children over the last 10–20 years, at least in a European sample.19 These findings together with the absence of adequate knowledge on the predictors of central adiposity in childhood and adulthood make the study of central adiposity of high importance to public health.
The purpose of the present study was to examine the tracking of abdominal and total body obesity in a group of Greek boys and girls and, furthermore, to identify certain factors that might affect the stability of body composition and in particular of the central obesity in this group of Caucasian children with a high prevalence of overweight and obesity. Indeed, overweight and obese Greek children aged 6–11 years constitute 31% of the population and this is one of the highest rates in the Mediterranean countries.20
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Methods |
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Sample
A total of 918 healthy children aged 6–12 years (477 boys and 441 girls), all living in Athens, Greece, participated in this study (table 1). Children were from a large school complex in the south part of Athens. Subjects' overweight and obesity rates were very similar to the national data.20 Signed informed consent was obtained from their parents and the study was approved by the Harokopio University ethics committee.
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Study design
Baseline anthropometric data were collected between the fall of 2000 and the spring of 2001. Follow-up measures were taken after 12 and 24 months, with a deviation of 10–15 days from the baseline measurement. All measurements were taken during the morning hours in a school setting by the same investigator. Repeatability of anthropometric measures was examined in a subgroup of 40 children and the correlation coefficient was >0.94 (P < 0.05) for all the variables.
Anthropometric measures
Sature was measured, without shoes, to the nearest 0.1 cm using a stadiometer. Body mass was determined with the subject in light clothing to the nearest 0.1 kg using a digital scale. Waist circumference (WaistC) was taken at the level of the ‘natural’ waist, between ribs and iliac crest, at the end of a normal expiration, whereas hip circumference was taken at the maximum posterior extension of the buttocks.21 Abdominal or central obesity was assessed by WaistC, whereas body fat distribution was assessed by the waist-to-hip ratio (WHR). The percentage of body fat was estimated using an electronic body composition analyser (Tanita, TBF-300, USA). Briefly, the measurement was taken during morning hours after an overnight fast, while children had emptied their bladder. Subjects were instructed not to drink water in the morning of the measurement day. They were also advised to avoid intense exercise and alcohol the day before.21
Maturity was assessed by the percentage of predicted adult stature achieved at the baseline. Predicted adult stature was calculated by the Khamis–Roche method.22 According to this method, adult stature is predicted from the child's height and weight and parents' stature. In the present study, children's anthropometric data at the baseline were used for the adult height prediction. Self-reported height and weight of parents of 529 children were taken once. Parents were classified as obese (BMI 
30 kg/m2) and underweight (BMI < 18.5 kg/m2).
Cardiorespiratory fitness assessment
CRF was assessed using the endurance 20 m shuttle run test23 in 323 children at the follow-up. Subjects started running at 8.0 km/h and the speed was gradually increased by 0.5 km/h every minute. Children were always made to run between two lines, which are 20 m apart. The pace was recorded by an audiotape. The actual score of the test was the last stage completed before the subjects quit. The test is suggested as a valid tool to evaluate maximum oxygen uptake.24,25 Mean difference for repeated shuttle-runs by 55 participants of the present study was 0.13 stages, while 95% confidence intervals were –0.13 and 0.39 stages, respectively.26 Based on the test performance, children were classified as unfit (1st quartile) or fit (4th quartile), according to sex-specific percentiles. CRF, which is the body's maximal ability to transport and utilize oxygen, was used as an index of physical activity level, as previously suggested.13 CRF reflects the effects of genetic factors as well as the influence of accumulated physical activity.27
Calculations
Relative body mass index
Body mass index (BMI) was calculated by dividing body mass (kg) by height (m2). Overweight and obese children were identified using the age- and sex-specific BMI cut-offs28 and underweight using the age- and sex-specific 5th percentile of BMI.29 Relative BMI (relBMI) was calculated using the following equation:
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Statistical analysis
Normality of distribution
Data for each year were tested separately for normality of distribution using the Kolmogorov–Smirnov test. All the variables examined were non-normally distributed and the non-parametric Spearman's correlation coefficient was used.
Tracking analysis
Tracking was defined as (i) the correlation between measurements at years 2000 and 2002 and (ii) the maintenance of a relative position in the population over time.30 Poor tracking was defined by a correlation coefficient <0.3, moderate tracking by a coefficient between 0.3 and 0.6 and high tracking by a coefficient >0.6.30 All variables examined during the 2-year follow-up were converted into percentiles and subsequently divided into quartiles. This was carried out separately in males and females. Each percentile represented the relative position of the subjects in the sample for each year of measurement. Tracking was defined as the proportion of children who remained in the same quartiles after the 2-year follow-up.30 The k-value was used to check the movement of children from one quartile to another after 2 years. The value of k ranges from 0.0 to 1.0. For k-values <0.40 tracking is poor, for k-values between 0.40 and 0.75 tracking is moderate and for k-values >0.75 tracking is high.30 Tracking was examined separately for boys and girls as well as for different age groups (6–9 years and 9–12 years). To evaluate gender and age-group tracking patterns (i.e. number of boys vs number of girls who moved from one quartile to another), the z-criterion was calculated:
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is the standard error of the difference between P1 and P2.
Regression analyses
Multiple regression analysis was performed to identify the factors that predict changes in BMI and WaistC over time. Logistic regression analysis was employed on the group of children who remained in the higher quartile of WaistC and relBMI (fattest group) to study the predictors of fatness tracking. For the analyses, sex, percentage of predicted adult stature, WaistC or BMI at baseline, parental obesity and overweight, and children's CRF were used as independent variables. All statistical analyses were carried out using the SPSS version 10.0 software for Windows package and the level of significance was set as P 
0.05.
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Results |
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Subjects' characteristics
About half of the children (48.8%) were girls and 64.4% of them were aged 6–9 years at baseline. Of the children 17% were overweight, 5.0% obese, and 6.3% were underweight (table 1).
Tracking of body composition and body fat distribution
Tracking of body composition was high at the follow-up (n = 918). The correlation coefficient between years 2000 and 2002 was 0.84 for relBMI (r = 0.85 for boys; r = 0.83 for girls; P < 0.01) and 0.82 for percentage of body fat. At follow-up, about half of the children (54.7%) remained in the same quartile of relBMI, whereas only 10.0% moved into a higher one. The k-value for the movement between quartiles was 0.39 (P < 0.01) and this suggests a moderate tracking.30 Tracking patterns of relBMI were similar in both sexes (56.0% of boys and 53.3% of girls remained at the same quartile). No difference in tracking patterns of relBMI was found between the age groups. About eighty five per cent (84.5%) of the children at the lowest quartile (lower BMI) and sixty five per cent (65.9%) of the children at the highest quartile (higher BMI) remained at the same quartile after the 2-year follow-up.
After 2 years 24% of the children who were underweight at baseline remained underweight, whereas 80% of the obese children remained obese at the follow-up (table 2). Obesity was tracked better in boys than in girls (95.0% of the boys and only 69.2% of the girls remained obese; z = 2.19). Gender difference was presented only in 10- to 12-year-old children (table 2). Abdominal obesity and body fat distribution presented high tracking (r = 0.86 for WaistC; r = 0.69 for WHR; P < 0.01). A greater percentage of boys moved into a higher quartile of WaistC compared with girls (z = 3.06; table 3).
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Regression analyses
Sex, child's BMI and WaistC at baseline, percentage of predicted adult stature, CRF and maternal BMI explained 76% of the variability in BMI and WaistC at the final measure (n = 290; table 4). Multiple regression analyses performed separately for boys and girls as well as for the two age groups (6–9 years and 9–12 years) did not differ in their results (results not shown). The equation for BMI at the follow-up was BMI = –3.43 – 0.30 (sex) + 0.33 (percentage of predicted adult stature) + 0.71 (BMI at baseline) + 0.11 (maternal BMI) – 0.17 (CRF), R2 = 0.76, P < 0.01 (all predictors were significant at P < 0.01 except for the intercept). BMI at the baseline, parental obesity and low level of fitness were the main predictors of the 2-year tracking of body fatness (n = 562; table 5). Finally, WaistC at baseline and CRF level were the only significant predictors of high WaistC after 2 years (n = 552; table 5).
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Discussion |
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The main finding of the present study was that sex, child's WaistC and maturity at baseline, CRF, and maternal BMI explained 76% of the variability in WaistC after the 2-year follow-up. In addition, the risk of maintaining abdominal obesity was increased in those children with high WaistC at baseline and low CRF. Sex was identified as an independent short-term predictor of central obesity. Actually, boys were more likely to preserve obesity and to move into a higher quartile of abdominal obesity than girls in this study.
The moderate to high tracking of body composition found in the present study is in agreement with previous investigations, which have studied body fat stability for extended periods.14–16 Besides the shorter follow-up period in the present study, these findings might suggest an influence of socioeconomic and environmental factors on the tracking of obesity in diverse populations. In Greece, the socioeconomic status and the nutritional habits have been in a transitional state over the past 15–30 years.31 This could partially explain the increasing prevalence of obesity in Greece,32 although longitudinal data on physical activity are lacking.
It has been recently shown that WaistC increased at a higher rate than the total body obesity over the past 10–20 years in children.19 This suggests an increasing risk of metabolic diseases in paediatric population. Despite this, very limited information exists on the stability and the predictors of central adiposity.15 WaistC presented high tracking in the present study (table 3), and this is in agreement with the previous data.15
Given the lack of data on WaistC stability, other indices such as waist-to-hip circumference ratio and trunk-to-extremity skinfolds ratio are used to evaluate central obesity, although these ratios describe the relative fat distribution in the body.33,34 In one such study Casey et al.33 showed that the WHR at the peak height velocity explained 58% of the variance in males and 51% of the variance in females of the WHR at the age of 30 years. The positive influence of high CRF on WaistC was also reported in a recent study that showed lower WaistC in overweight and obese children with high CRF compared with overweight and obese children with low CRF.35 Increased physical activity and elevated CRF after an exercise training programme in children resulted in significant improvements in health indices, such as insulin resistance, in another study.2 These results together with those of the present study suggest that more emphasis should be given to the improvement of CRF for the avoidance and/or treatment of central obesity in children.
The findings of the present study are also in agreement with previous investigations, showing high baseline child's fatness, low physical activity, and parental obesity to be the predictors of youth's obesity.6,7,36–38 Mother's, and not the father's, obesity was associated with children's BMI and WaistC in the present study (table 4). This association suggests both genetic susceptibility and environmental influences on children's body composition. As a matter of fact children's food environment is usually shaped more by mothers than fathers,39 and this could explain the association between the child's BMI and maternal obesity in the present study.
In this group the 2-year tracking of abdominal adiposity was stronger in boys than in girls (table 3). Rolland-Cachera et al.17 and Malina et al.,40 have shown that trunk-to-extremity skinfold ratio is similar in boys and girls until the age of 14–15 years. Thus, the tendency for increased central obesity in boys than in girls in the present study was not due to different maturity level between sexes. An excess accumulation of fat in the abdomen or in the trunk is associated with hyperinsulinaemia, insulin resistance, and elevated blood lipid concentrations in childhood.3 The results of the present study suggest an earlier predisposition to the development of a favourable background for diseases in males than females.
One interesting finding of the present study was that boys had a greater chance to preserve obesity than girls (table 2). This sex difference was more evident in the age group of 10–12 years. This is in agreement with Wang et al.,16 who reported that overweight tracked better in boys than in girls over 6 years. In a recent study, more boys aged 11.5 years at baseline remained overweight and obese than girls after a mean follow-up of 1.6 years.41 One possible explanation for this gender difference in tracking is that females are more likely to develop an early concern for their body shape and image than males. Therefore, girls might have intervened in their body weight though dieting and other methods16 and this could also have affected tracking. Sex-differences at the onset of puberty could also have had an influence on body composition tracking.42
The majority of previous studies did not account for the influence of maturity on body composition stability. This is despite the well-known effect of maturation on child's growth and development.41 Multiple regression identified child's baseline percentage of predicted adult stature as a predictor of BMI and WaistC after 2 years (table 4). This association was a positive one indicating that advanced maturity is related to higher BMI and WaistC in these children. In the Fels Longitudinal Study, rapidly maturing males had larger values for total body fat, percentage of body fat, and fat free mass than slowly maturing males, independent of the age.42 At similar age, rapidly maturing females had only higher fat free mass compared with slowly maturing females in the same study.
The sample employed, the short time of follow-up, and the fact that only a subgroup, for which the complete data were available, was used in the multiple and logistic regressions are limitations of this study. Thus, present findings need to be confirmed in a large-scale study, with a representative sample and with a longer follow-up period. Parental BMI was self-reported and this could have affected the results. Maturity was not directly assessed but it was estimated from the predicted adult stature. The 90% error of adult's height prediction was 0.83 cm for males and 0.66 cm for females aged 4–17.5 years.22 This error was slightly higher than that with the method that uses skeletal age for adult stature prediction (90% errors: 0.70 for males and 0.59 cm for females22).
In conclusion, these results showed that central and total body obesity presented high tracking after a 2-year follow-up period in 6- to 12-year-old children. More boys moved into a higher quartile of central obesity than girls, suggesting a gender difference in abdominal obesity stability. Finally, obesity was tracked better in children with one or more obese parents and in those with low CRF. It seems that health policy-makers should target the avoidance and/or treatment of central adiposity. Improvements in children's fitness through elevated physical activity and management of parental body weight should be of priority for successful interventions. More emphasis should also be given in implementing programmes for boys, who presented a greater chance of maintaining obesity, at least under the present circumstances.
Key points
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Acknowledgments |
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No funding was available for this study.
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References |
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