1.1 The extent and impact of childhood obesity
The increases in overweight and obesity among children internationally over the past three decades indicate that childhood obesity is a global ‘epidemic’(1–3). Rising fatness trends are apparent in both developed and developing countries (1–9). Current estimates of childhood overweight and obesity range from 12% to over 30% in developed countries and from 2% to 12% in developing countries (3). However, the true extent of the problem is difficult to accurately estimate because of variations in definition of childhood obesity between clinical and epidemiological studies. Defining obesity during childhood and adolescence is complicated because of variability in growth rates and the natural, gender-specific variations in body composition that occur at different maturational stages. Weight-for-height indices are the most widely used for assessing prevalence of childhood obesity in population studies for reasons of feasibility and practicality. However, until the recent development of body mass index (BMI) for age and sex as an international standard for assessment of childhood overweight and obesity (10), use of different growth standards and cut-off criteria for definition of overweight complicated comparisons of epidemiological studies and determination of secular trends. To account for these difficulties, Ebbeling et al. (2) reported on changes within populations from studies that used similar definitions of overweight. The increases reported varied from an almost twofold increase in Britain over the past 20 years (e.g. from 5.4% to 9.0% in boys) (11) to a more than threefold increase in developing countries (e.g. from 2% to 9% in Egypt) (2). A recent review describes how the global prevalence of childhood overweight is unevenly distributed with the highest rates evident in industrialized countries such as North America and Europe and lowest rates evident in developing countries such as Africa and Asia (3). This review also describes how the pattern differs within developed and developing nations. In industrially developed countries, lower income families are more vulnerable; while in developing countries, childhood obesity is most prevalent among advantaged groups (3). Furthermore, in industrialized countries ethnicity may be a factor, as seen in the United States (USA). For example, where the rising prevalence of child obesity is much more evident among Hispanic and African Americans at around 25% compared with Caucasian groups at 10–12% (3).
Obesity has been described as a multi-factorial trait determined by genetic and non-genetic factors (12). It is widely acknowledged that fatness is to some extent ‘heritable’. In the majority of cases obesity seems to be polygenic with a non-Mendalian pattern of transmission (12) and single gene defects are rare (13). A complex interaction involving at least as many as 250 obesity-associated genes (14) and non-genetic, environmental factors cause predisposition to obesity (2,15). This raises the concept of ‘susceptibility genes’ whereby a particular genotype does not necessarily determine the development of obesity but increases risk of the disease given a particular environment. Such gene–environment interactions are very complex and, as yet, are poorly understood. Progress in this area is likely to be critically important for effective obesity prevention (12). While the variation in body fatness that can be explained by genetic factors remains controversial (15) there is consensus around the critical role of the environment as a predisposing factor (2,16–18). The rapid rise in obesity prevalence observed over the past two decades in genetically stable populations strongly confirms the need to tackle environmental factors leading to obesity (12).
The comorbidities of childhood obesity are evident in many areas of paediatric medicine; however, the true extent of adverse health outcomes tends to be underestimated (19). Many overweight children grow up to become obese adults. In particular, those who are severely overweight, are affected by obesity during adolescence (20), or have at least one parent who is obese (21,22) – the latter situation made increasingly likely by dramatic increases in adult obesity rates over recent years. This is expected to add significantly to the prevalence of chronic diseases associated with adult obesity (1). Furthermore, risk factors for cardiovascular disease (hyperinsulinanaemia, impaired glucose tolerance, dyslipidaemia and hypertension) tend to cluster in childhood and are strongly associated with obesity and its duration (23). These risk factors have been identified in overweight children as young as 5 years of age (24). A significant aspect of the epidemic of childhood obesity is the increased prevalence of Type 2 diabetes in paediatric populations and the prospect of the associated macro- and micro-vascular complications (25). Moreover, the mental health implications of lifelong obesity likely confer additional morbidity. In summary, the cumulative anticipated impact on chronic disease prevalence carries implications for sustainability of healthcare systems.
1.2 The challenges of treatment and prevention
Facilitating preventive action to address childhood obesity is complex. First, there is strong prejudice against overweight people (26,27), which many children are clearly aware of (28) including those as young as 4 years of age (29). Care must be taken so that obesity prevention programmes do not induce unhealthy slimming practices, which may lead to the development of clinical eating disorders (30) or risky behaviour such as smoking to control weight (31). Second, adequate nutrition is essential for the preservation of normal growth and development. Energy restriction in obese children who were on well-controlled and supervised weight reduction diets have led to reductions in height velocity (32). Nonetheless, Epstein et al. have shown that individualized treatment with frequent monitoring can be effective without compromising growth (33). Such close follow-up of children and adolescents would, however, overwhelm resources for prevention initiatives. The current situation requires a population health approach in addition to the one-on-one weight reduction treatment required for severely overweight children or those with complications. This has led to recommendations from experts that focus prevention initiatives on the goal of promoting healthy eating, active living and positive self-esteem rather than the achievement of ideal body weight (34–38). In fact, prolonged weight maintenance rather than weight loss is recommended for mildly overweight children without complications (34–36). Such weight stabilization as children grow in height allows a gradual decline in BMI, which is deemed sufficient for the majority of children (34–36).
Traditionally, public health strategies have focused on the individual, promoting healthy food choices and regular physical activity. Therefore, the classical interpretation of populations becoming obese, failing to lose weight or recidivism after initial weight loss, is failure on the part of the individual, the intervention, or both. However, the problem of rising obesity prevalence does not appear to be owing to a lack of interest by the individuals in the population. On the contrary in the USA, where the evidence of increasing obesity rates is very reliable, there is evidence that the majority of the population are actively trying to control their weight (39). Recent reviews suggest that a paradigm shift, which considers the environment in which these individuals make choices on food consumption and engagement in physical activity, is necessary to understand and tackle the problem. There is a growing consensus that effective intervention to address the obesity epidemic requires a multi-strategic approach involving all levels of society – both for the population as a whole and for the individual (2,16–18). This relates to ensuring a balance in intervention strategies along the continuum that stretches from individualized health care (downstream investments) to the introduction of policy and legislation that affects whole populations on a macro level (upstream investments). Currently, considerable resources are invested in downstream activities compared with upstream interventions (40). There is a growing consensus that more upstream investment is required to tackle the obesogenic environment (2,16,18). In addition the new concept of integration in the prevention of chronic diseases has been introduced into public health practice (41,42). This concept promotes integration of activities so that several chronic diseases with common risk factors can be addressed simultaneously. For example, a programme which integrates the three main healthy living strategies (diet, physical activity and mental health) has the ability to address cardiovascular disease, Type 2 diabetes and cancer simultaneously. Compelling arguments for integration concern the optimization of scarce resources, congruent messages to the public and potential to enhance access for marginalized populations (41,42).
1.3 Special issues for immigrant and minority populations
Adverse health consequences of obesity vary according to ethnic origin and because of cultural factors. For example, increased risks associated with obesity have been shown at lower BMI levels in Asians compared with Caucasians and these populations are also predisposed to visceral or abdominal obesity (43). Furthermore, even controlling for differences in adiposity, Black and Hispanic youths in the USA are at greater risk for Type 2 diabetes and cardiovascular disease than their white counterparts (44,45). In contrast, however, Caucasian girls are often more vulnerable to the psychosocial effects of obesity compared with girls from other ethnic groups (46). Cross-sectional comparisons of the African Diaspora populations have been undertaken to investigate the increase in risk of obesity and associated chronic disease as people migrate from non-industrialized to industrialized countries. Luke, Durazo-Arvizu et al. compared obesity prevalence (BMI > 30 kg m−2) in people of African origin living in West Africa (non-industrialized), Caribbean (mid-way between non-industrialized Africa and industrialized America) and America (industrialized) (47). In this cross-sectional comparison, these researchers found a marked east-to-west increasing gradient in the prevalence of obesity, which was lowest for Nigerian men (5%) and highest for African-American women (49%). The health consequences in terms of morbidity and mortality patterns in obesity-related chronic diseases, most notably Type 2 diabetes and cardiovascular disease, were also very evident across the African Diaspora (48). These data indirectly illustrate the need for public health strategies that protect immigrants new to industrialized countries from developing obesity and associated chronic disease as well as to prevent such disease from becoming prevalent in developing countries.
Immigrants new to industrialized countries undergo social integration, which includes transition from traditional diet and physical activity habits to those that prevail in the host country. Children and adolescents among immigrants new to industrialized countries are likely to be particularly susceptible to the obesogenic environment of their new host country because they tend to participate in the local culture and become socially integrated more quickly than their parents (49). A recent report from Statistics Canada indicates that immigrants compared with non-immigrants in Canada have superior health in terms of chronic conditions even when accounting for age education and income (50). Furthermore, immigrants’ odds for reporting any chronic conditions increase with time living in Canada (50). The USA National Longitudinal Study of Adolescent Health provides data which suggest that overweight is substantially and significantly less common among Hispanic and Asian-American adolescents who were first generation immigrants (i.e. born outside the USA) than among the second (born in the USA with at least one foreign-born parent) and third (born in the USA of native-born parents) generation immigrants (51). Using the 85th percentile cut-off for BMI-for-age kg m−2 to define overweight, Native American Indian adolescents with a prevalence of 42%, had the highest rate for overweight compared with adolescents in other ethnic groups such as Black (31%) and Hispanic (30%) and compared with overall overweight rate of 27% in this study (51). Although Asian-American adolescents had the lowest rates of overweight (20.6%); a further breakdown of this rate by generation reveals a rising prevalence of overweight from first generation (12%), through second generation (27%), to third generation (28%) in this ethnic group and highlights the urgency of effective prevention for immigrant subgroups (51). In addition, these data point to an important role of acculturation or assimilation into lifestyles that prevail in industrialized countries as a risk factor for obesity. Successful public health measures to address this are likely to involve the upstream interventions of a population health approach – such as food quality, policy, advertising and politics. In relation to this, Sobal links the differing prevalence of obesity in America, China and Russia with globalization of food and activity patterns (49). Furthermore this hypothesis is extended to explain differences in obesity prevalence in rural and urban communities in China and Russia leading to the conclusion that global as well as community and national public health measures may be needed to adequately deal with the globalization of obesity (49).
1.4 Identifying best practice: examining the evidence
While, population-based interventions to address the rising prevalence of obesity in children and youth are based in a theoretical and rational viewpoint, the evidence base for such strategies is limited (17,18). Similar to clinical situations systematic reviews are used to generate evidence-based recommendations for best practice using population-based approaches suitable for public health. However, unlike the clinical model, the methodology for synthesis reviews of public health practice is still under development. This is mainly the result of the multifaceted nature of population health approaches, which complicates evaluation of effectiveness (52–54). Furthermore, the use of randomization to control for confounding factors in population-based interventions compared with clinical trials is more limited on practical, economic and feasibility grounds (52–54). In some circumstances observational study designs may be appropriate to generate evidence for public health where there is a need to assess the impact of population health interventions and where randomized controlled trials or interventions that include control groups are difficult to introduce (55). Another difficulty in determining the methodological rigour of population health interventions arises from the equivocal nature of the outcomes used. For example, effective strategies need to be assessed beyond endpoints alone (i.e. whether a particular intervention has a successful outcome), but also at each stage of programme development and implementation (41). While quantitative studies may provide data on effectiveness of a programme, qualitative studies are better suited to describe and therefore understand the key phenomenon that cannot be easily captured by quantitative studies (56). Incorporation of information from qualitative and observational studies therefore provides a promising approach.
Notwithstanding the need for a more comprehensive appraisal of methodological rigour, the formulation of best practice recommendations for obesity prevention requires assessment of the evidence base from other perspectives. For example, an assessment of programme potential for acceptance and integration with the community (57); utilization of multidimensional, upstream population health strategies; and in some cases the applicability of programmes for population subgroups. However, there is limited inclusion of varying research approaches to date in the systematic reviews of obesity prevention and treatment interventions targeting children and youth (58–60).
Over the past 20 years, when obesity rates were rapidly increasing, numerous interventions with potential to reduce rates of obesity or associated risk of chronic diseases in children and youth have been implemented in a diverse range of settings. Some of these interventions were implemented under well-controlled conditions and provided information that had undisputed validity, while some successfully addressed large population groups but provided less valid data on effectiveness. Others were successful in engaging the target population, while others had specific attributes that met the needs of minority groups. The potential of all of these types of interventions to provide valuable insight on best practice for reducing obesity and associated chronic disease in children and youth was explored in this synthesis research paper. Therefore, in addition to the usual step of appraising methodological rigour, the appraisal process was extended to cover three further important aspects – programme development/evaluation, applicability of population health and immigrant health principles. A global perspective was taken in this research; however, issues were examined in a Canadian context reflecting the country where the research was carried out. Thus an established Canadian conceptual model (see Fig. 1) (41,61–63), incorporating the fundamental principles of population health, was used to guide the process. Finally to ensure a comprehensive approach, the search strategy for reports which addressed the research questions that guided the synthesis, included the Internet, grey literature and articles in foreign languages.