Past and recent trends in childhood obesity

The body mass index (BMI), defined as weight/(height)² is a surrogate parameter for body fat mass in adults and in children [11]. There are only a few datasets describing the cross-sectional development of BMI values in children over several decades before 1980. These data show a rather stable or slowly increasing prevalence of childhood obesity [12–15]. However, between 1980 and 2000, mean BMI values in children and the rates of childhood overweight and obesity increased dramatically in many countries [1]. This was paralleled by a steep increase in skinfold thickness, which is an anthropometric indicator of the amount of subcutaneous fat, and is widely used to assess body fat [16]. Skinfold thickness increased not only in overweight children, but also in normal and underweight children [17].

Along with the increase in obesity prevalence, the BMI distribution shifted in a skewed fashion, indicating that the heaviest children had become even heavier [18]. This matched the observation that the numbers of extremely obese children and adolescents (BMI >99th percentile) increased to a greater degree than those of individuals in other obesity categories (BMI 95th to 96.9th and BMI 97th to 98.9th percentile) [19, 20].

Starting at around the year 2000, childhood obesity rates apparently reached a plateaue or even declined in developed countries [2–10]. This was an unexpected finding, because, for example, in the USA, it has been suggested that the prevalence rate of obesity in children will reach 30% by 2030 [21]. Recent data from the National Health and Nutrition Examination Survey (NHANES) now show that the rapid increase in obesity prevalence rates seen in the 1980s and 1990s has not continued [5]. When sex differences were analyzed, the flattening was more marked for girls than for boys. Furthermore, there were age-related differences, with prevalence declining more in preschool children (aged 2 to 5 or 6 years) compared with primary school aged children (6 to 11 or 12 years) or adolescents (12 to 19 years) [6]. However, it should be noted that extreme obesity is still increasing, despite the declining rates for lower obesity categories [19, 20].

Possible causes and consequences of the observed trends in childhood obesity

Changes in BMI as a surrogate of body fat mass parallel changes in living conditions and energy intake over time. This has been shown in a study of children in Jena over a period of more than 100 years (Figure 1) [15]. This study used measurements based on 10 anthropological investigations carried out between 1880 and 2005/2006. It belongs to the longest running longitudinal studies of schoolchildren in a single community worldwide, and provides a unique dataset to analyze secular changes in the physical development of schoolchildren.

The dramatic increases in obesity prevalence rates and body fat after 1980 are related to changes in individual behaviors of children. Children nowadays have decreased physical activity, and increased screen time and consumption of energy-dense foods and snacks [1]. These behavioral changes are probably related to social and environmental changes that affect the whole population [18].

In return, the recent decline in obesity prevalence rates in children in developed countries may be the result of a cumulative effect of programs designed to prevent childhood obesity [23–25]. After 1980, the recognition by healthcare professionals, schools, community organizations, industry, and governments of obesity as a health problem increased. Both at national level and at state and local level, programs focusing on reducing the consumption of energy-dense foods and television viewing, as well as increasing the daily physical activity have been developed to reduce environmental factors contributing to inappropriate weight gain [23–25]. Although it is possible that a biological plateau for obesity has been reached, there are several hints indicating that these initiated public health efforts have contributed to the leveling off of obesity rates.

A very recent study showed significant increases in daily physical activities and consumption of fruits and vegetables between 2001 and 2010 in a nationally representative cohort of students in grades 6 to 10. Television viewing and the consumption of sweets and sweetened beverages decreased during the same time period [26]. Another supporting example is the recently observed decline in the consumption of sugar-sweetened soft drinks by children, which parallels the decline in obesity prevalence rates [27, 28]. These examples support the notion that in countries facing the childhood obesity epidemic over several years, the initiated public health programs have been able to stop the increasing obesity trends by influencing the lifestyle of children. If this can be confirmed in further analyses, such programs might even be improved in order to achieve a yet better success rate.

However, in contrast to these findings in developed countries, recent prevalence rates of overweight and obesity in children in developing low and middle income countries are still increasing at large [29]. This seems to be because the western lifestyle with easily available and low cost energy-dense food and increased motorization started to develop later in these countries, and had now become increasingly adopted [29]. These countries may be able to adapt public health programs from developed countries to their situation and launch them immediately in order to prevent further increases in childhood overweight and obesity rates, and possibly reach a plateau at a lower level than seen in developed countries.