Over the past three decades childhood overweight and obesity have reached epidemic proportions in most industrialized countries [1]. Although the Dutch prevalence rates of childhood overweight and obesity are still low compared to other developed countries [2], [3], both rates were higher than in 1980. In 2009, fourteen percent of Dutch children were overweight and two percent were obese [3]. This is a two to three fold higher prevalence in overweight and four to six fold increase in obesity since 1980 [4]. Upward trends were also found in the two largest ethnic minorities in The Netherlands originating from Turkey and Morocco.

Obesity is a leading preventable cause of chronic illnesses and death and has emerged as one of the most serious public health concerns in the 21st century [5]. Excessive body weight is associated with various diseases like cardiovascular diseases, diabetes mellitus type 2, and certain types of cancer [6]. The risks increase with higher BMI levels at the upper end of the distribution [7]. Obesity has been found to reduce life expectancy by two to four years, while morbid obesity even reduces life expectancy by ten years [7]. Therefore, in adults obesity has been broken down into three classes; class I obesity (30.0–34.9 kg/m²), class II obesity (35.0–39.9 kg/m²) and class III obesity (≥40.0 kg/m²). Only recently, in 2012, extended international (IOTF) cut-offs for childhood obesity classes II and III were presented [8]. Also in children, morbid obesity can be a life threatening condition [9]. Childhood morbid obesity often causes hypertension, dyslipidemia, insulin resistance/diabetes, fatty liver disease, and psychosocial complications [10]. In a study in 2005–2007 in The Netherlands among 307 children with severely obesity, 56% had hypertension and 67% had at least one cardiovascular risk factor [11]. A study among 230,000 Norwegian adolescents showed that morbid obesity was associated with increased mortality in middle age from a wide variety of systemic diseases [12].

The aim of our study is to apply the extended international (IOTF) cut-offs on the national Dutch growth studies to assess the trend in morbid obesity (class II and III combined) in The Netherlands among children of Dutch origin since 1980, and among children of Turkish and Moroccan origin since 1997. The Dutch growth studies offer a unique opportunity to study trends covering the time period before which the worldwide obesity epidemic emerged.

This study is the first that presents trends of morbid obesity based on the extended international (IOTF) cut-offs in Dutch children, and also in Turkish and Moroccan children living in The Netherlands. We showed that the prevalence of morbid obesity in children of Dutch origin was 0.59% in boys and 0.53% in girls in 2009. Significant upward trends occurred since 1980 and 1997. The prevalence rates were much higher in children of Turkish (2.53% in boys and 1.71% in girls) and Moroccan (1.07% in boys and 1.65% in girls) origin in 2009. Turkish children showed an upward trend before twelve years of age, while the results of the Moroccan children showed the opposite; an upward trend aged twelve years onwards. The morbidity index for overweight was highest among the Turkish and Moroccan children, which reveals that overweight is more severe in children of Turkish and Moroccan origin compared to children of Dutch origin. The severity of overweight increased between 1980 and 2009 in Dutch children, and between 1997 and 2009 in Turkish children, while a decrease between 1997 and 2009 was observed in Moroccan children. Part of the ethnic differences in morbid obesity rates can be explained by the educational level of the parents, as more Turkish and Moroccan children have low educated parents and morbid obesity is more prevalent among children of low educated parents.

The extended (IOTF) cut-offs encourage direct comparison of trends in child morbid obesity worldwide. The cut-offs are less arbitrary and more internationally applicable than other definitions, such as centiles on the United States (US) growth charts [17]. Until now, the prevalence of morbid obesity by the extended international (IOTF) cut-offs has only been reported in one study in New Zealand [18]. This study was limited to secondary school students and did not report information on trends. They showed that among 9,107 secondary school students in 2007, the prevalence of morbid obesity was 2.5%. Our study showed that the prevalence of morbid obesity in secondary school children of Dutch, Turkish and Moroccan origin was much lower in 2009 than the New Zealand secondary school children in 2007. Other studies applied as cut-off for extreme or severe obesity the 99^th percentile or a BMI≥120% of the 95^th percentile [19], [20] for age and sex according to the growth charts of the 2000 Center for Disease Control (CDC) [17], which complicates a fair comparison. At eighteen years, a BMI≥120% of the 95^th CDC growth chart corresponds to a BMI of 34.8 kg/m² in boys and 36.4 kg/m² in girls. A study in millions of US pre-school children living in low-income families showed that the prevalence of extreme obesity (≥120% of the P95 CDC) increased from 1.75% in 1998 to 2.22% in 2003 and decreased to 2.07% in 2010 [19]. Another study among 42,559 US children aged 3–5 years between 2007 and 2010 reported a prevalence of extreme obesity (≥120% of the P95 CDC) of 1.6% [20]. Among US children aged 2 to 19 years, the prevalence of extreme obesity (≥P99 CDC) was 3.8% [21] and in 6.9% of U.S. sixth graders in the HEALTHY cohort [22]. A study in New Zealand among 3,275 schoolchildren aged 4–15 year showed that the prevalence of extreme obesity (≥P99 CDC) was 2.7% in 2002 [23].

In agreement with our study, high differences in prevalence rates among ethnic groups within countries were reported [18]–[23]. In our study part of the ethnic differences in morbid obesity rates can be explained by the educational level of the parents. Families with low educated parents may lack resources, both economical and knowledge, which can reduce their ability to control weight gain by making healthy food choices and taking opportunities for physical activity [24]–[26]. Ethnic differences in morbid obesity may also be explained by differences in cultural opinions about body shape and acceptable weight gain [27]–[29]. Also, cultural and religious norms influencing dietary and physical activity patterns may play a role [27]–[29]. It is reported that children of Turkish origin in The Netherlands watch more TV [30], play less outdoor [31], have a higher snack intake [31], and less often have breakfast [30] compared to children of Dutch origin. All these results show that interventions or prevention programs should pay attention to the cultural aspects and educational level of the targeted population.

A particular strength of our study is the consistent methodology and inclusion/exclusion criteria, and objective measurements of height and weight in the large national growth studies between 1980 and 2009. The baseline growth study was performed in 1980, before the worldwide obesity epidemic started. Therefore, the growth studies offer a unique opportunity to monitor trends in a period of increasing levels of BMI. The data were collected from all regions in The Netherlands and are, therefore, assumed to be generalizable for the results of The Netherlands. A limitation of our study is that we only present the prevalence rates of morbid obesity in children of Turkish and Moroccan origin in 1997 and 2009, since the 1980 study did not provide a sufficiently large sample of children of Turkish and Moroccan origins. Another limitation is the limited sample size within the three age categories of the Turkish and Moroccan children that complicates a good insight into age-related trends. Further research is needed to evaluate the trends of morbid obesity within age categories of children of non-Dutch origin.

An upward trend in overweight and obesity between 1980 and 2009 was already reported in The Netherlands [3]. Our findings show upward trends in morbid obesity as well. Monitoring and reducing the prevalence of childhood morbid obesity is of high importance for these children, health care and the community. Childhood obesity leads to chronic illnesses [10], [11] and increased risk of mortality in middle age [12]. Therefore, an upward trend in morbid obesity and morbidity index for overweight may have a higher impact on health costs, than an upward trend in overweight. For this reason, it is important to monitor not only overweight, but also morbid obesity and the morbidity index for overweight. The Partnership Overweight Netherlands (PON) provides an integrated health care standard for obesity involving strategies for diagnosis and early detection of high-risk individuals as well as appropriate combined lifestyle interventions for those who are overweight and obese and, when appropriate, additional medical therapies [32].

Upward trends of morbid obesity in Dutch and Turkish children in The Netherlands occurred since respectively 1980 and 1997, while this prevalence remained almost stable in Moroccan children since 1997. The Dutch and Turkish children also showed an upward trend in morbidity index for overweight since respectively 1980 and 1997, while the Moroccan children showed a downward trend since 1997. Monitoring trends of morbid obesity according to international comparable cut-offs for BMI is important, because of the great impact on health care and costs.