Systematic review on the association of abdominal obesity in children and adolescents with cardio-metabolic risk factors.

BACKGROUND: The adverse health effects of abdominal obesity are well documented in adults, but such association remains to be determined in the pediatric age group. This study aims to perform a systematic review on the association between abdominal obesity and cardio-metabolic factors such as dyslipidemia, hypertension, and hyperglycemia among children and adolescents.
MATERIALS AND METHODS: A systematic literature search was conducted using PubMed, Scopus and Google Scholar databases to May 2014. Two independent reviewers identified relevant papers in several steps. After studying the titles and texts of documents, repeated and irrelevant ones were excluded. The search was refined to the English language. We did not consider any time limitation. Studies with different measuring methods of abdominal obesity were included. Studies with abdominal obese patients secondary to other disease were excluded from the study. In final, the data of association of cardio-metabolic risk factors and abdominal obesity extracted from studies.
RESULTS: Overall, 3966 articles were reviewed, and 61 of them were studied according to the inclusion and exclusion criteria. Waist circumference (WC), waist-to-height ratio, and waist-to-hip ratio were the most common indexes used for defining abdominal obesity. The association of high blood pressure with increasing WC was seen in several studies. The association of other cardio-metabolic risk factors was seen in some studies.
CONCLUSION: Whatever the definition used for abdominal obesity and whatever the methods used for anthropometric measurements, central body fat deposition in children and adolescents increases the risk of cardio-metabolic risk factors. Therefore, more attention should be paid to abdominal obesity of children and adolescents both in clinical practice and in epidemiological studies.

INTRODUCTION

The prevalence of overweight/obesity has increased worldwide; affecting all age ranges including the pediatric population. Childhood obesity has been increasing at an alarming rate in both developed and developing countries.[123] In all age ranges, obesity is associated with several adverse health effects as hypertension, dyslipidemia, insulin resistance and type-2 diabetes, as well as social and psychological problems.[4] Approximately, 60% of those who present obesity in the first decades of life will have at least one of the abovementioned metabolic alterations in adulthood.[5]

Obesity in childhood is associated with adverse cardio-metabolic risk factors, including elevated blood pressure (BP), triglycerides (TG), total and low density lipoprotein cholesterol (LDL-C) and insulin, as well as reduced high density lipoprotein cholesterol (HDL-C),[67] and in turn with obesity and cardiovascular disease (CVD) in adulthood.[8]

In addition, childhood obesity is a risk factor for atherosclerosis and is associated with increased mortality due to CVD in adulthood, independent of adult weight.[910]

Individuals with upper body obesity are more susceptible to cardio-metabolic risk factors. Most studies have been conducted in adults, and limited experience exists in the pediatric age group. The Bogalusa Heart Study showed that the distribution of central fat determined by waist circumference (WC) at the ages of 5-17 years is associated with abnormal concentrations of TG, LDL-C, HDL-C, and insulin.[11] Central obesity is a major clinical and public health issue. Compared with generalized obesity, central obesity is more strongly correlated with metabolic risk factors. A number of studies have shown that central obesity is an independent risk factor for type 2 diabetes mellitus, dyslipidemia, systemic arterial hypertension, and coronary artery disease.[1213] In adults, the risk of cardiovascular death, myocardial infarction, and all-cause death increases in parallel with WC.[14]

Although childhood obesity is a well-recognized risk factor for developing CVD and type 2 diabetes mellitus in adulthood, excess central (intra-abdominal) body fat distribution may be more related to these diseases than peripheral distribution.[15]

It is important to identify children who are at increased risk of developing comorbidities associated with obesity, to potentially intervene and prevent the development of chronic diseases including type 2 diabetes and CVD. However, the association of abdominal obesity in children and adolescents with cardio-metabolic risk factors remain controversial.[171819] This paper aims to perform a systematic review on papers that studied the association between abdominal obesity and cardio-metabolic factors among children and adolescents [Table 1].

Table 1

Characteristics and main findings of studies included in this systematic review are presented

MATERIALS AND METHODS

Literature search

The search was conducted using PubMed, Scopus and Google Scholar databases to May 2014. The following keywords were used: [“Child”[Mesh] OR “Adolescent”[Mesh]) AND (“Obesity”[Mesh] OR “Pediatric Obesity”[Mesh] OR “Ideal Body Weight”[Mesh] OR “Overweight”[Mesh]) AND (“Obesity, Abdominal”[Mesh]) AND (“Dyslipidemias”[Mesh] OR “Hyperlipidemias”[Mesh] OR “Hypercholesterolemia”[Mesh] OR “Hypertension”[Mesh] OR “Hyperglycemia”[Mesh].

The search was refined to the English language. We did not consider any time limitation. The flow chart of the study selection process is presented in Figure 1.

Figure 1

Papers search and review flowchart for selection of primary study

Titles and abstracts of papers were screened and relevant papers were selected. Duplicates were removed. Then, full texts of relevant papers were read, and findings were rescreened. To increase sensitivity and to select more studies, the reference list of the published studies was checked. Two independent reviewers (MK and HN) screened the titles and abstracts of papers, which were identified by the literature search, for their potential relevance or assessed the full text for inclusion in the review. In the case of disagreement, and the discrepancy was resolved in consultation with an expert parbitrating investigator (RK).

Selection criteria

All studies among children and adolescents, which evaluate the association of abdominal obesity or general obesity or overweight with cardio-metabolic risk factors were included the review. Studies with different measuring methods of abdominal obesity were included. Studies with abdominal obese patients secondary to other disease were excluded from the study.

Data extraction and abstraction

Two reviewers abstracted the data independently. The required information that was extracted from all eligible papers was as follows: Data on first author's last name, year of publication and country of the study population, population studied, aim and findings of studies.

RESULTS

Study selection strategy

As presented in Figure 1, from 3966 articles from the primary search, 61 studies were included in the current study.

Study characteristics

Papers were published between the years 2001 and 2013. The age of participants of included studies ranged from 6 to 18 years.

WC, waist-to-height ratio (WHtR), waist-to-hip ratio (WHR), trunk-to-appendicular fat ratio, WC to arm circumference, DXA-trunk fat, preperitoneal fat thickness, suprailiac skinfold thickness were the anthropometric indexes that were used to measure abdominal or central obesity or central fat distribution. WC, WHtR, and WHR were the most common variables. All these studies were included in spite the different measuring methods that they had used.

The following cardio-metabolic risk factors were considered: Systolic hypertension, diastolic hypertension, prehypertension, transient hypertension, cholesterol, LDL-C, HDL-C, fasting blood sugar, insulin resistance, insulin dose per body surface, carotid intima-media thickness, and alanine aminotransaminase.

The design of this study, age range and ethnicity of participants, different risk factors, different methods used for measuring abdominal obesity, geographic area and different statistical analysis among studies complicated the comparison of the study findings.

DISCUSSION

In this study, the association of abdominal obesity in children and adolescents with cardio-metabolic risk factors was reviewed systematically. BP was the most common measurement among studies; most of them confirmed the association of abdominal obesity and elevated BP. Some studies showed that this association was stronger in boys than in girls.[202122] In addition, the association of abdominal obesity with systolic hypertension was seen more frequently than with diastolic hypertension.[2324] On the other hand, one study did not confirm this association.[25]

In a study, total body fat was a stronger predictor of elevated BP than visceral fat in children and adolescents.[26] Some studies presented that after adjustment for body mass index (BMI), cardio-metabolic risk factors were more prevalent in children and adolescents with abdominal obesity than in those with overweight and general obesity.[192728]

Some studies documented the association of abdominal obesity in children and adolescents with abnormal lipid profile and fasting blood glucose (FBG).[6293031] Some other studies showed that the combination of abdominal obesity and generalized obesity was a stronger risk for elevated FBG and dyslipidemia than each type of obesity alone.[6293031]

In general, most studies have confirmed the association of central fat deposition in children and adolescents with various cardio-metabolic risk factors. However, controversies exist on the definition of abdominal obesity in the pediatric age group.

Different measuring methods and various indexes were used to determine abdominal obesity, e.g., WC, WHtR, and WHR, and documented different results.[1734] Further longitudinal studies are necessary to determine the appropriate anthropometric measures in children and adolescents to predict cardio-metabolic risk factors.

A number of methods have been employed in the assessment of the distribution of regional fat, such as computed tomography and magnetic resonance imaging. Different cutoffs are used to define central obesity in the pediatric age group, e.g., the National Cholesterol Education Program Adult Treatment Panel III has proposed WC ≥90th percentile in this regard.[77]

Although visceral fat, that is, body adipose tissue located within the abdominal cavity around the visceral organs, can be accurately assessed by imaging techniques as computed tomography and magnetic resonance imaging, the routine use of these techniques are not feasible clinically.[78] BMI itself cannot differentiate between fat and fat-free mass. Therefore, an elevated BMI may not reliably reflect the accumulation of adipose tissue.[7980] In addition, the recent increase in mean BMI of children and adolescents has been accompanied by an even steeper increase in WC.[81]

However, in children and adolescents, BMI is strongly related to growth and maturation, and is expressed as z scores or percentiles relative to age and sex.[82] In addition, BMI does not always relate to central obesity[83] and it cannot differentiate muscle mass from bone and fat mass.[84]

The limitations of these indexes, however, should be considered. For instance, WC is correlated with the amount of intra-abdominal visceral fat, which may be the most detrimental fat depot,[85] it is also associated with subcutaneous abdominal fat and with total body fat.[8687] In addition, a recent study found that WHtR and BMI are more strongly associated with each other than with percentage of body fat, as determined by air-displacement plethysmography.[88] These associations emphasize the potential problems in using WHtR and BMI as indexes of abdominal and generalized adiposity, respectively. The interpretation of associations with BMI and WHtR is further complicated by the possible relation of disease risk to height,[89] which is in the denominator of both indexes.

Some investigators have suggested that, even if the predictive abilities of WHtR and BMI-for-age are similar, WHtR may be preferred as an indicator of obesity-related risk.[9091] The concept of a large WC relative to height may be easier to explain than is the division of weight (kg) by the square of height (m2), particularly for those accustomed to using pounds and inches. In addition, because WHtR varies only slightly by age and sex, it is not necessary to express measures as percentiles or z scores, relative to a reference population, as is the case for BMI. The calculation of WHtR is also simpler, requiring only the division of numbers in the same units. Furthermore, the possible use of a single cutoff (0.5) to identify adverse measures among both children and adults[90] would result in a simple public health message of keeping WC to less than half of the height.

In addition, although the reproducibility of WC measurements is high,[92] some investigators have found that it is lower than that of BMI.[88] This difference might limit the ability of WHtR in detecting small changes in obesity-related risk. Furthermore, WC has been measured at numerous sites between the lowest rib and iliac crest, and there are differences between the recommendations of the anthropometric standardization reference manual,[93] the World Health Organization, and the National Institutes of Health.[92] Small changes in the location of the waist measurement can alter associations with the risk factor measures[949596] and possibly with disease risk.

CONCLUSION

Whatever the definition used for abdominal obesity and whatever the methods used for anthropometric measurements, central body fat deposition in children and adolescents increases the risk of cardio-metabolic risk factors. Therefore, more attention should be paid to abdominal obesity of children and adolescents both in clinical practice and in epidemiological studies.

AUTHOR'S CONTRIBUTION

All authors contributed in the study concept and design, assisted in literature review, and drafting the paper. All authors have read the final version of the paper and accept the responsibility for its content.