The Confusion About BMI
David A. Dzewaltowski, Ph.D.
Distinguished Professor
Community Health Institute
Kansas State University
Over the last thirty years U.S. children have been measured on their weight status as part of the U.S. Health and Nutrition Examination Survey. Data from this study has revealed that the prevalence of overweight among children has increased from 5% to 19% in the last thirty years.
The tool used to measure the increase in the number of overweight children in the U.S. population is body mass index, BMI (kg/m2, see 2000 CDC Growth Charts: United States). Body mass index is a practical method to estimate body fat. There is consensus by several scientific panels that BMI provides a reasonable index of overweight in children (Dietz & Robinson, 1998: Barlow & Dietz, 1998; Himes & Dietz, 1994). Several studies have shown that BMI is adequately associated with body fat in boys and girls of different ages and ethnic groups (For a review of the issue, See Dietz & Beliizzi, 1999; Flegal et al., 2006). For example, Mei and colleagues (2002) showed that BMI was associated with both dual energy X-ray absorptiometry (DXA) measurement of body fat (approx. r =.80) and, in a sample of over 10,000 children, with skinfold measurements (approx. r =.80 to .90).
Confusion About BMI
Although there is consensus that BMI provides a reasonable measure of fat, there appears to be confusion among the validity of BMI among health professionals and the public at large. There may be confusion because BMI is widely recommended as a surveillance tool and a screening tool but not as a tool for clinical decision making.
BMI as a Surveillance Tool
BMI is used as a surveillance tool in the US by the Centers of Disease Control and internationally by several countries. Furthermore, the World Health Organization monitors the status of obesity world wide using BMI. For children BMI varies considerably with development so generally BMI is compared with children of the same sex and age. The CDC defines overweight as greater than the 95th percentile on age and gender adjusted norms and at risk for overweight as greater than the 85th percentile. Although a few children who are categorized as at risk for overweight and overweight may not be actually too fat, the measure is very reliable and provides a very valid indicator of trends over time.
BMI as a Screening Tool
The CDC and the American Academy of Pediatrics recommend the use of BMI to screen for overweight in children. A screening tool is used to identify children with possible weight problems. The American Academy of Pediatrics stated “early recognition of excessive weight gain relative to linear growth should become routine in pediatric ambulatory care settings. BMI (kg/m2, see 2000 CDC Growth Charts: United States) should be calculated and plotted periodically (American Academy of Pediatrics, 2002, pg. 427).”
Clinical Assessment of Overweight
BMI is not an accurate clinical assessment tool as some children may be categorized incorrectly. This is not a concern for surveillance or screening, but is a concern for clinical decision-making. Children with BMI values at or above the 95 percentile should undergo an in depth assessment to determine who are obese, to diagnose possible causes, and to provide a basis for treatment (Barlow & Dietz, 1998). Although it is less likely, children between the 85th and 95 percentiles may also be obese. Therefore, these children are also recommended for a second level screen to determine if further in-depth evaluation is warranted.
Conclusion
BMI is an effective tool used for surveillance and screening for overweight in children. BMI is NOT a clinical diagnostic tool used for treatment decisions.
References
American Academy of Pediatrics: Policy Statement (2003). Prevention of pediatric overweight and obesity, Pediatrics, 111, 424-430.
Bellizzi, M. C., & Dietz, W. H. (1999). Workshop on childhood obesity: summary of the discussion. Am J Clin Nutr, 70(1 Part 2), 173S-175S.
Barlow, S. E., & Dietz, W. H. (1998). Obesity evaluation and treatment: Expert Committee recommendations. The Maternal and Child Health Bureau, Health Resources and Services Administration and the Department of Health and Human Services. Pediatrics, 102(3), E29.
Barlow, S. E., & Dietz, W. H. (2002). Management of child and adolescent obesity: summary and recommendations based on reports from pediatricians, pediatric nurse practitioners, and registered dietitians. Pediatrics, 110(1 Pt 2), 236-238.
Cole, T. J., Bellizzi, M. C., Flegal, K. M., & Dietz, W. H. (2000). Establishing a standard definition for child overweight and obesity worldwide: international survey. Bmj, 320(7244), 1240-1243.
Dietz, W. H., & Bellizzi, M. C. (1999). Introduction: the use of body mass index to assess obesity in children. Am J Clin Nutr, 70(1 Part 2), 123S-125S.
Flegal, K. M., Tabak, C. J., & Ogden, C. L. (2006). Overweight in children: definitions and interpretation. Health Educ Res, 21(6), 755-760.
Himes, J. H., & Dietz, W. H. (1994). Guidelines for overweight in adolescent preventive services: recommendations from an expert committee. The Expert Committee on Clinical Guidelines for Overweight in Adolescent Preventive Services. Am J Clin Nutr, 59(2), 307-316.
Mei Z, Grummer-Strawn LM, Pietrobelli A, Goulding A, Goran MI, Dietz WH. Validity of body mass index compared with other body-composition screening indexes for the assessment of body fatness in children and adolescents. American Journal of Clinical Nutrition 2002;7597–985.
Ogden, C. L., Carroll, M. D., Curtin, L. R., McDowell, M. A., Tabak, C. J., & Flegal, K. M. (2006). Prevalence of overweight and obesity in the United States, 1999-2004. Jama, 295(13), 1549-1555.
Pietrobelli, A., Faith, M. S., Allison, D. B., Gallagher, D., Chiumello, G., & Heymsfield, S. B. (1998). Body mass index as a measure of adiposity among children and adolescents: a validation study. J Pediatr, 132(2), 204-210.