Published online September 1, 2008
PEDIATRICS Vol. 122 No. 3 September 2008, pp. 683 (doi:10.1542/peds.2008-1120)

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LETTER TO THE EDITOR

Changes in BMI: An Important Metric for Obesity Prevention

Arnaud Chiolero, MD, MSc
Department of Epidemiology, Biostatistics, and Occupational Health,
Faculty of Medicine,
McGill University,
Montreal, Quebec, Canada H3A 1A2
Institute of Social and Preventive Medicine,
University of Lausanne,
CH-1005 Lausanne, Switzerland

Katerina Maximova, MA
Gilles Paradis, MD, MSc
Department of Epidemiology, Biostatistics, and Occupational Health,
Faculty of Medicine,
McGill University,
Montreal, Quebec, Canada H3A 1A2

To the Editor.—

Foster et al¹ must be congratulated for showing that policy-based school interventions can reduce both the incidence and prevalence of overweight over a short time frame (2 years). However, the average weight gain, assessed by the mean changes in BMI and BMI z score, was similar for the intervention and control groups—a counterintuitive finding the authors found "not surprising."¹ We suggest that the authors should consider changes in BMI z scores as an important outcome and also report changes in the overall BMI distribution curve when assessing the impact of their trial.

First, the incidence of overweight does not necessarily relate to the overall amount of weight gain. For example, an incident case of overweight might result from a change in BMI from the 84th to the 86th percentile (a z-score increase from 0.99 to 1.10; child 1 on the Fig 1). However, a child who had a greater increase in BMI from the 74th to the 84th percentile (z-score increase from 0.64 to 0.99; child 2 in Fig 1) could still be considered within the "normal" weight range.

View larger version (8K): [in this window] [in a new window]   FIGURE 1 A given increase of BMI z score does not always result in a corresponding change in BMI category.

 
Second, the lower incidence of overweight in the intervention group in the absence of a difference in mean BMI suggests that although few children had BMI increases that crossed the 85th percentile (hence, became overweight), the BMI of other children in this group might have increased but without crossing the 85th percentile. For example, children 2 and 3 in Fig 1 show increases in BMI but remain within their respective normal weight and overweight categories.

These 2 points illustrate how the incidence of overweight provides an incomplete picture of the distribution of weight changes and, in this case,¹ may not reflect the magnitude of weight gain across the entire BMI distribution. Because the mean level of a continuous variable such as BMI is strongly related to the proportion of cases with extreme value of this variable,² prevention of weight gain in childhood should prevent the occurrence of overweight and obesity.³ Changes in BMI and BMI z score, therefore, are essential metrics for trials of obesity treatment as well as for school-based and other community trials of obesity prevention.^4,5

ACKNOWLEDGMENTS

Dr Chiolero was supported by Swiss National Science Foundation (Berne, Switzerland) grant PASMA-115691/1.

REFERENCES

1. Foster GD, Sherman S, Borradaile KE, et al. A policy-based school intervention to prevent overweight and obesity. Pediat-rics. 2008;121 (4). Available at: www.pediatrics.org/cgi/content/full/121/4/e794
2. Rose G, Day S. The population mean predicts the number of deviant individuals. BMJ. 1990;301 (6759):1031 –1034[Abstract/Free Full Text]
3. Barlow SE; Expert Committee. Expert committee recommendations regarding the prevention, assessment, and treatment of child and adolescent overweight and obesity: summary report. Pediatrics. 2007;120 (suppl 4):S164 –S192[Abstract/Free Full Text]
4. Reilly JJ, Kelly L, Montgomery C, et al. Physical activity to prevent obesity in young children: cluster randomised controlled trial. BMJ. 2006;333 (7577):1041[Abstract/Free Full Text]
5. James J, Thomas P, Kerr D. Preventing childhood obesity: two year follow-up results from the Christchurch obesity prevention programme in schools (CHOPPS). BMJ. 2007;335 (7623):762[Abstract/Free Full Text]

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PEDIATRICS (ISSN 1098-4275). ©2008 by the American Academy of Pediatrics

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This Article Extract Full Text (PDF) Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Services E-mail this article to a friend Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Add to My File Cabinet Download to citation manager Request Permissions Citing Articles Citing Articles via CrossRef Citing Articles via Google Scholar Google Scholar Articles by Chiolero, A. Articles by Paradis, G. Search for Related Content PubMed PubMed Citation Articles by Chiolero, A. Articles by Paradis, G. Related Collections Nutrition & Metabolism Social Bookmarking                         What's this?