Historically, an overweight child was presumed to be a healthy child, because it was capable to survive periods of undernourishment and infections.2 Natural selection probably favoured people with an thrifty energy metabolism (the thrifty gene hypothesis3). During millennia of frequent food scarcities this economical energy metabolism was favourable. However, with stable food supplies about 60 years ago society started to focus on consumers and became technologically advanced. This new society interacting with our evolutionary legacy probably led to what we now call the obesity epidemic.4
This epidemic is apparent with the worldwide prevalence of obesity at least doubling during the past three decades. More than half a billion adults worldwide were obese in 2008. The rising prevalence is also noticeable among the youngest; in 2010, more than 40 million children under the age of five were overweight or obese worldwide.5 In the Netherlands, 33% of adults were overweight in 1981 which increased to 48% in 2012. The prevalence of obesity more than doubled in that time from 5% to 12%.6 In 2009, 13’15% of the Dutch children were overweight and two percent were obese. Although these percentages are relatively low compared to other countries, rates are three to respectively six times the prevalence’s found in 1980.7 Most recent data show the prevalence of childhood overweight and obesity appears to be stabilizing in western countries.8
There has been debate on the question whether obesity is a disease or not. An expert panel concluded in 2008 that there is no clear agreed-on definition of disease, but that there are compelling reasons, related to both causes and consequences, to consider obesity a disease.9 The World Health Organization (WHO) defines overweight and obesity as ‘abnormal or excessive fat accumulation that may impair health’.5
To classify overweight and obesity the Body Mass Index (BMI) is commonly used. It is defined as a person’s weight in kilograms divided by the square of his height in meters (kg/m2). Among adults, obesity is generally defined as a BMI greater than 30 kg/m2, and overweight as a BMI between 25 and 30 kg/m2.5 This relates to an increased risk of comorbidities for a BMI of 25 to 29.9, and moderate to severe risk of comorbidities for a BMI greater than 30.10 For children there are difficulties in defining a single standard to classify overweight and obesity since growing children show significant fluctuations in the relationship between height and weight. Many countries use their own country-specific charts. Widely used thresholds for overweight or obesity in childhood are 110% or 120% of ideal weight for height and a BMI at the 85th, 90th , 95th and 97th percentiles of the country-specific reference population.11 The International Obesity Task Force (IOTF) developed an international standard growth chart to enable global comparisons. These age and gender specific cut-off values of the BMI correspond to the adult thresholds of 25 for overweight and 30 for obesity12 and are used in the present thesis to classify overweight and obesity.
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