Obesity prevalence estimates in a Canadian regional population of preschool children using variant growth references

Globally, obesity is a significant public health problem [1, 2] and a number of studies report an increasing prevalence of overweight and obese children in Canada [3‐6]. The health risks associated with excess body weight are well documented [7, 8]. The age and sex specific body mass index (kg/m²) or BMI is the most common method for assessing weight status and health risk in children [9]. There are three sets of growth references commonly used to assess a child's weight status and health risk; BMI cut-points published by the US Centre for Disease Control and Prevention (CDC), the International Obesity Task Force (IOTF) and those published by the World Health Organization (WHO) [10‐12]. Inconsistent prevalence estimates of childhood overweight and obesity based on variant growth references pose a challenge in defining the burden of childhood obesity at a population level. Recommendations are inconsistent on which method to use [13, 14].

The purpose of this paper is to compare prevalence estimates of overweight and obesity among a regional preschool population living in the province of Newfoundland and Labrador, Canada using the CDC, IOTF and WHO BMI cut-points. A secondary objective is to assess the level of agreement between the growth references.

The Memorial University Human Investigations Committee and the Health and Community Services Boards ethics committees approved this study.

The anthropometric measures and characteristics of the children are presented in Table 1. There were no significant differences in these variables. Figure 1 illustrates that for the overweight group, there were significant differences between CDC and WHO (19.1% and 26.7%) and between IOTF and WHO (18.2% and 26.7%). When children were classified as obese, there were significant differences across the three references (CDC 16.6%, IOTF 8.3%, WHO 11.3%).

In Figure 2, for boys, there were significant differences in overweight between IOTF and WHO (16.4% vs. 28.5%) and between CDC and WHO (18.6% vs. 28.5%). A similar relationship was found for girls. In Figure 3, there were significant differences in the obese category across all three standards for both boys (CDC 17.3%, IOTF 7.8%, WHO 11.7%) and girls (CDC 15.7%, IOTF 8.8%, WHO 10.8%). To determine the level of agreement between growth references, we calculated the kappa statistic (Table 2).

There is a high prevalence of childhood overweight or obesity in this Canadian preschool population, irregardless of growth reference used. Approximately one in three preschool children was overweight or obese. Similar statistics are being reported in other developed counties [18‐27]. The short and long term physical health risks for children associated with excess weight include hypertension, hyperinsulinemia, glucose intolerance, type II diabetes, dyslipidemia, increased risk of early cardiac disease and psychosocial difficulties. Childhood obesity is a significant public health concern and accurate measurement and classification is crucial in determining the burden of the health problem. In the current study, the CDC reported a significantly higher prevalence of childhood obesity compared to the other growth references. The CDC reported more than double the prevalence of obesity compared to the IOTF and approximately 30% more obese than the WHO. Whether this is an accurate reflection or an overestimation of the prevalence of obesity in this population, it is difficult to tell. Conflicting recommendations are provided by various organizations that include the Canadian Pediatric Society [28] and the Clinical Practice Guidelines on the management and prevention of obesity in adults and children [13]. Although there was an overall good level of agreement between the different growth references there was some noticeable inconsistencies in classification which may lead to challenges in assessing the weight status of individual children in a clinical environment.

Several studies are published comparing the prevalence estimates of overweight and obesity in children and youth using various growth references, and the findings are inconsistent [18‐27]. One study recently published on a sample of Canadian children and youth compared the estimates of excess weight using the same three reference cut-points as in the current study and similar findings were reported. The prevalence estimate for the childhood obesity estimate was the same based on the WHO and CDC growth references (13%) but lower based on the IOTF cut-points (8%) [18]. Another study on children between four and six years of age living in the province of Alberta, compared prevalence estimates using CDC and IOTF cut-points. The authors reported that an estimated 13.8% and 11.4% of children were overweight and obese according to the CDC cut-offs, while an estimated 11.5% and 6.8% of children were classified as overweight and obese using the IOTF cut-offs. Similar to the current study, the CDC growth references classified almost twice as many children as obese compared to the IOTF method. The level of agreement between the two methods was .69 (p < .01), significantly lower then that in the current study [22].

In an Italian study on 258 preschool children three to six year of age, the prevalence of overweight and obesity were compared using the CDC and IOTF methods as well as local Italian growth references published by Luciano [23]. All three sets of growth references gave similar estimates of overweight in boys (CDC 16.10%, IOTF 12.90%, Luciano 14.5%) and girls (CDC 15.70%, IOTF 15.7%, Luciano 10%). These findings were not dissimilar to the current study's findings on overweight in boys (CDC 18.6%, IOTF 16.4%) and overweight in girls (CDC 19.6%, IOTF 20.2%). However in the Italian study, the use of the CDC reference led to a prevalence estimate of obesity in boys that was ten times that of the other references (CDC 10.5%, IOTF 0.8%, Luciano 0.8%). In girls the CDC estimate was also significantly higher compared to the other two references (CDC 11.90%, IOTF 6.7%, Luciano 3%), although not as large a difference as in the boys. Based on this study it appears that the reliability of the growth reference used may be affected by the underlying prevalence of childhood obesity in the population, as the Italian rates of childhood obesity tend to be much lower than those found in North America. Ethnic diversity will also have an impact. This provides further challenges when making international comparisons [23]. In contrast to the previous studies, a study recently conducted on 604 Spanish children 6 to 10 years of age, researchers reported that when using the WHO criteria, the combined prevalence of overweight and obesity was 39%, significantly higher than both the CDC estimate of 20% and the IOTF estimate of 17%. In this study, the CDC and IOTF demonstrated a high level of agreement (kappa >.80), however the level of agreement between the CDC and the IOTF and the WHO was poor (kappa < .40). In this study, the authors concluded that it was the WHO criteria that overestimated the prevalence of childhood overweight and obesity, not the CDC as seen in the current study [19].

The significant difference in prevalence estimates of childhood obesity produced by the three growth references make it difficult; to assess the extent of the problem and its associated health burden, to conduct research, to make population comparisons and to inform policy. The CDC and to a more limited extent the WHO references appear to allow for an earlier identification of a larger number of children affected by excess weight compared to the IOTF. If the CDC classifies children accurately than using this reference may prompt health professionals to provide primary prevention to a pediatric population earlier to help reduce the risk of associated health conditions [9]. Although the association between increasing BMI and increased health risk has been substantiated in adults [29] it is important to raise a concern that none of these growth references have been convincingly linked to the future development of adverse health outcomes in children.

An important strength of our study is that we directly measured children's heights and weights and did not rely on self reported survey data which may underestimate the prevalence of overweight and obesity. Our study limitations included a non random sample from one region in Canada. However, the prevalence estimates of overweight and obesity in the current study were very similar to a larger provincial study [5], providing confidence that our study findings are representative of this provincial population. The current research adds to the debate about the relative reliability and validity of the variant growth references used to monitor a child's growth and has implications for future research. It raises several research questions that still need to be answered. Have we determined the ideal definition of overweight and obesity in children so that prevention efforts can be initiated at the earliest opportunity? Are some of these references better suited to particular populations depending on the mix of ethnicity and race? Can we validate these growth references against a gold standard? Should references based on optimal growth be considered the gold standard? Are any of these standards associated with adverse health outcomes in children? It is clear that more research including longitudinal studies is needed in order to answer many of these questions.

Childhood obesity is a public health problem and is associated with increased morbidity and mortality. Obesity tracks through the life cycle [2, 30] suggesting that early identification and primary prevention is key to reversing and preventing the upward trend into adult obesity and its potential future burden of illness [31]. A consensus is urgently needed on the most valid and reliable growth reference to use to measure and monitor a child's growth for both clinical and research purposes.