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Towards a simplified definition of childhood obesity? A focus on the extended IOTF references

To provide meaningful advice for the prevention of childhood obesity, studies are needed to assess obesity prevalence and to investigate risk factors for obesity. Such research requires international consensus on the definition of childhood obesity, in order to form a global perspective of the epidemic. Much progress has been made since researchers first started to investigate childhood obesity. Originally, the definition of obesity was derived from the corresponding definition of underweight, and since then, a variety of growth indices, cut-offs and weight-for-age or weight-for-height references have been proposed. A step towards harmonization occurred when the body mass index (BMI) was first applied to children (1) and the first BMI reference curves were published in the early 1980s (2). The earliest BMI reference curves were constructed using national samples with statistically based centile cut-offs. BMI has since emerged as the most practical, widely available and inexpensive indicator for classifying overweight and obesity in children. In 2000, the International Obesity Task Force (IOTF) proposed an international definition (3) for childhood obesity based on representative data from six countries, and cut-offs were constructed in reference to World Health Organization (WHO) health-related adult obesity cut-offs (4) rather than based on centile distributions. The WHO growth standards, released in 2006 for assessing the growth of children from birth to 5 years of age, were created from samples of healthy breastfed children from six countries around the world (5), and were intended to represent a ‘standard’ of physiological growth rather than a descriptive ‘reference’. To extend these growth curves to school-age children and adolescence, references for 5to 19-yearolds were developed using data from US surveys (6). In addition, various other countries have constructed their own references, primarily for clinical use, as they are more representative of the country's children. The adoption of BMI to define overweight and obesity and the availability of BMI reference curves both represent progress, but the multiplicity of references, and hence definitions, makes it difficult to choose between them (7). Thus, introducing yet another definition might be viewed as a further shift away from harmonization. However, the paper by Cole and Lobstein in this issue (8) does not propose a new definition of obesity – an important point to be stressed. On the contrary, the authors propose several simplifications and improvements to the existing IOTF (3) and thinness references (9). Reference values for overweight, obesity and thinness are now presented together. In addition, the article provides L (lambda), M (mu) and S (sigma) values allowing standard deviation (SD) scores to be calculated. A different approach as compared to the previous papers (3,9) was used to build the new cut-offs, which involved averaging the LMS curves for the six datasets rather than averaging their cut-offs. Differences between the original and revised cut-offs appear, but they are so small that they cannot really impact on estimates of overweight or obesity prevalence. For example, the prevalence of obesity in 2to 17.9-year-old boys from the NHANES 2005/6 is 10.2% using both previous IOTF and new definitions, while it is respectively 12.1% and 12.3% in girls. Differences are slightly larger for thinness in the youngest age groups. Thus, prevalence data based on the new references can be compared with previous data based on IOTF (3) and Cole et al. (9) definitions. There are some limitations of the original IOTF cutoffs (7): they provided only limited centile ranges, they did not allow calculation of SD scores and there was no high cut-off defining morbid obesity. These limitations are resolved in this new article (8), but other limitations remain. The references start at 2 years of age and refer only to BMI, with no references for weight, height or skin-fold measurements. All moves towards harmonization are good news, but there is still a long way to go to reach an internationally accepted definition. Even the most optimistic observers do not expect the adoption of a common international reference in the near future. As a compromise given the availability of numerous definitions, the European Childhood Obesity Group recently proposed (7) that prevalence data should be published using several of the main references (IOTF, WHO and additionally Centers for Disease Control and