Childhood Obesity

Indices Based on Weight for Height Measures

A variety of different terms, metrics, and cutoff points have been used to define childhood overweight and obesity based on weight and height, such as 110–120% of ideal weight for height, or weight-for-height greater than 1 or 2 standard deviations above a predefined mean, or gender-specific 85th and 95th percentiles of body mass index-for-age (Ogden et al. 2010). There are international (International Obesity Task Force (IOTF) and World Health Organization (WHO)) and national growth references (Center for Disease Control (CDC), Spain-SRS, Italy-Luciano) to describe and assess overweight and obesity in children. These growth references show weight, height, and BMI for children by age and gender in use, with different cutoff values and give therefore slightly different estimates of overweight and obesity prevalence (Quelly 2014). In 2002, the International Obesity Taskforce (IOTF) developed international standards for classifying overweight and obesity which enabled comparison of prevalence globally (Nader et al. 2006). The IOTF recommend using these international growth charts and age and gender specific cutoff points because, on average, they correspond to the adult thresholds. The IOTF classification has been shown to have high specificity but low sensitivity (Han et al. 2010). However, many countries continued to use their own country-specific charts, including the United States (CDC), where standards are based on a national survey from early 1960s, i.e., before the current epidemic.

In 2006, the World Health Organization released new international Child Growth Standards for children 0–5 years, based on data from international optimally nourished breast-fed infants (WHO Multicentre Growth Reference Study Group 2006). The standards depict normal early childhood growth under optimal environmental conditions and can be used to assess children everywhere, regardless of ethnicity, socioeconomic status, and type of feeding. Naturally, there are individual differences among children, but across large populations, regionally and globally, the average growth is remarkably similar. For example, children from India, Norway, and Brazil all show similar growth patterns when provided healthy growth conditions in early life. The WHO 2006 Child Growth Standard shows that differences in children’s growth to age five are more influenced by nutrition, feeding practices, environment, and healthcare than genetics or ethnicity (WHO Multicentre Growth Reference Study Group 2006). There are also reference charts available for children aged 5–19 years (based on a revision of US data collected in 1977 adapted to match the standards for 0–5-year-olds). Since 2006, there are 125 countries that have adopted the WHO standards, and many countries switched from weight-for-age only to multiple indicators. Weight-for-age was adopted almost universally, followed by length/height-for-age (104 countries) and weight-for-length/height (88 countries). Most countries opted for sex-specific charts and the z-score classification (22).

Consequences of Different Growth Standards

There are important differences in the prevalence of overweight and obesity between the WHO, IOTF, and national reference standards due to the choice of cut-offs and to design and characteristics of used samples (de Onis et al. 2007; Monasta et al. 2011). Overweight and obesity prevalence estimates among children based on IOTF and CDC definitions are substantially lower than estimates based on WHO definitions (Monasta et al. 2011; Wijnhoven et al. 2013). This means that using the WHO standards will result in lower rates of undernutrition (except during the first 6 months of life) and higher rates of overweight and obesity than when CDC or IOTF standards are used (de Onis et al. 2007). This finding was confirmed in a study comparing the WHO 2006 Child Growth Standard and the UK 1990 growth standard. It showed by the WHO 2006 that standard infants were less likely to be classified as underweight or having poor weight gain in the first year but more infants would be classified as overweight in the preschool years (Wright et al. 2008).

Several studies reporting on the prevalence of childhood obesity in high income countries have used the IOTF cutoff points. However, IOTF can only be applied to children 2–18 years old where the WHO reference covers the full spectrum of childhood. When comparing the CDC with the WHO standards, the CDC charts reflect a heavier, shorter sample than the WHO standards (O’Neill et al. 2007). The WHO standards weight-for-age mean Z-score seem to better track healthy breast-fed infants track and would be the better tool for monitoring the rapid and changing rate of growth in early infancy (de Onis et al. 2007).

Skinfolds and Body Circumference

BMI is of limited use in capturing subtle alterations in growth and body composition, such as total and regional body fatness, limb/trunk length, and skeletal muscle mass (SMM), which are needed to establish the early life origins of noncommunicable diseases such as type 2 diabetes (Lobstein et al. 2003; McCarthy 2014). Triceps and subscapular skinfold measurements assess the thickness of subcutaneous tissue and reflect fatness primarily and are a useful addition to the battery of growth standards for assessing childhood obesity (O’Neill et al. 2007). Waist circumference is a validated measure to measure deep subcutaneous as well as intra-abdominal fat accumulation, which represents risk of ill health (Cole et al. 2000). Waist circumference is a measure of abdominal fatness in children and is associated with higher fasting glucose and insulin concentrations and altered lipid profiles (Brambilla et al. 2013). Waist circumference is now endorsed by the International Diabetes Federation and National Institute for Clinical and Health Excellence for diagnostic and monitoring purposes (McCarthy 2014). In addition, chest circumference is associated with obesity in young children and is positively correlated with rapid growth. Therefore, chest circumference may be a useful marker for rapid growth and may help clinicians to identify obese children at 3 years of age (Akaboshi et al. 2012).

Childhood Obesity in LMICs

In contrast to these findings in high income countries, the prevalence of overweight and obesity is still rising in children and adolescents in many LMICs (Popkin et al. 2012). From 1980 to 2013, prevalence rates in these countries have increased from 8% to 13% for both boys and girls (Ng et al. 2014). However, there are now several LMICs countries that describe prevalence rates of childhood obesity of more than 15% in children and adolescents, such as Mexico (42%), Brazil (22%), India (22%), and Argentina (19%) (Gupta et al. 2012).

Many LMICs are undergoing rapid demographic and socioeconomic changes that have caused changes in dietary habits and sedentary lifestyles. This has led to a shift in consumption patterns from a traditional low fat and high fiber diet to more “Western” diets that are energy dense, low fiber diet, the “nutrition transition”(Drewnowski et al. 1997; Popkin 2001). Initially, it was thought that in high-income countries obesity levels would be highest in poor people and rural areas and that the opposite was to be observed in low and middle income countries. However, emerging evidence suggests that the burden of childhood obesity in developing countries is shifting toward the urban poor (Popkin et al. 2012).

Childhood Obesity and Social Economic Class

The causal direction of the relationship between childhood obesity and social economic class is complex (Wang et al. 2012). In general, socioeconomic groups with greater access to energy-dense diets are at increased risk of being obese than their counterparts. Since 2005, the prevalence of childhood obesity is increasing in children in all socioeconomic groups in developed countries, but mostly in children from low socioeconomic groups (Grow et al. 2010). Initially, there was a greater increase among higher-SES children in developed countries, especially after 1997, when income inequality dramatically increased (Singh et al. 2008; Wang et al. 2012). However, it seems that in developed countries the gap has closed in the wrong way and children in more deprived population groups have now overtaken those in least deprived population groups (Brunt et al. 2008). Obesity, overweight and central obesity, and sedentary behavior coexist with undernutrition, such as in urban areas in India (Singh et al. 2007).

Developmental Mismatch

One of the pathways leading to childhood obesity involves developmental mismatch. The theory of predictive adaptive responses (PARs) posits that prenatal environmental conditions such as maternal diet induce phenotypic changes in the developing offspring, which uses these cues about the current environment to predict aspects of its future postnatal environment (Gluckman et al. 2005). PARs have arguably evolved because this opportunity to alter phenotype may provide a Darwinian fitness advantage in the predicted environment (i.e., survival to reproductive age and successful reproduction). PARs may therefore potentially become maladaptive later in life should there be a disparity between the anticipated and actual environment. This may arise from cues being inaccurately transmitted (due perhaps to placental dysfunction), or not reflecting the later environment, due to socioeconomic and nutritional transition between generations. The latter can occur in the case of maternal ill-health such as preeclampsia, because the mother consumes an unbalanced diet not representative of the population or, most importantly in the contemporary context, because diet and lifestyle influences change between one generation and the next. The outcome is a situation of developmental mismatch, where the offspring phenotype is not best suited to its environment, leading to greater risk of disease (Gluckman et al. 2007). Maternal undernutrition is relatively common in developing countries, and this is associated with gender equality issues; in developed countries, despite plentiful food diets are often unbalanced in relation to both macro- and micronutrients (Hanson et al. 2009). Even within the normal range of Western diets in the UK, a less prudent diet before and in early pregnancy is associated with alterations in fetal hemodynamics leading to greater hepatic blood flow and less shunting through the ductus venosus, and this is associated with greater adiposity at birth and age 4 years in the child (Godfrey et al. 2012). The improvements in socioeconomic status, migration, and urbanization lead to nutritional transitions which exacerbate this problem (Popkin et al. 2012).

Studies of the biology of fetal growth have now shown that the process of maternal constraint (evolved maternal and uteroplacental factors limiting fetal size to minimize risk of obstructed labor occurs in all pregnancies to a greater or lesser degree (Gluckman et al. 2004). Greater maternal constraint predisposes the offspring to mismatch. Such constraint is greater in primiparous pregnancies of great relevance to countries such as China where family size has been limited (Reynolds et al. 2010). Greater constraint is also associated with shorter maternal stature, multiple conception, at the extremes of maternal age, and with unbalanced diet, smoking, or some drugs. Each of these situations has been associated with a greater risk of childhood overweight or obesity.

Maternal Obesity and Gestational Diabetes Mellitus

There is now much concern about the effects of maternal adiposity and gestational diabetes mellitus (GDM) on risk of overweight or obesity in the offspring (Gaillard et al. 2014). Pregnancy is a state of modest insulin resistance, which favors hyperglycemia and adiposity in the offspring (Hanson et al. 2012; Ma et al. 2013). Maternal obesity is also associated with increased birthweight independent of GDM (Black et al. 2013) with fetal hyperinsulinemia and obesity from the neonatal period through to childhood and young adulthood, graded across the whole range of maternal BMI (Modi et al. 2011). Higher gestational weight gain is also correlated with BMI in early adulthood independently of maternal obesity. Once again these processes appear to operate across the range of BMI and gestational weight gain, at least in Western populations; there does not appear to be a threshold BMI or weight gain at which a pathological as opposed to a physiological process operates.

The mechanisms underlying these pathways are now increasingly recognized to involve epigenetic processes – work which was first demonstrated in animal studies (Lillycrop et al. 2005) but is now shown to be applicable to human development. For example, maternal diet in early pregnancy is associated with changes in the methylation of part of the genome associated with the RXRa gene and in turn with degree of fat mass in the child at age 6 or 9 years (Godfrey et al. 2011). Epigenetic changes in metabolically relevant genes are now also reported for the offspring of GDM pregnancies (Ruchat et al. 2013). Some epigenetic marks, even in blood, appear to be stable through childhood and to relate to childhood fat mass (Clarke-Harris et al. 2014). These may therefore be valuable biomarkers of risk and potentially useful for monitoring their efficacy of interventions .

Reviews on pathways linking overweight/obesity to NCDs show that inflammatory processes are particularly important (Singer et al. 2014). Wider social issues related to pathways involve ethnic differences (Taveras et al. 2013) and amplification by migration in childhood Schooling et al. 2004). The mechanisms by which these process occur, which involve in part mismatch, possibly some genetic factors, and other influences, are not known (Popkin 2001; Han et al. 2010).