Original Article

International Journal of Obesity (2008) 32, 959–966; doi:10.1038/ijo.2008.11; published online 19 February 2008

Accuracy of body mass index in diagnosing obesity in the adult general population

A Romero-Corral1, V K Somers1, J Sierra-Johnson1,2, R J Thomas1, M L Collazo-Clavell3, J Korinek1, T G Allison1, J A Batsis4, F H Sert-Kuniyoshi1 and F Lopez-Jimenez1

  1. 1Division of Cardiovascular Diseases, Mayo Clinic College of Medicine, MN, USA
  2. 2Department of Medicine, Atherosclerosis Research Unit, Karolinska Institutet, Sweden
  3. 3Division of Endocrinology, Mayo Clinic College of Medicine, Rochester, MN, USA
  4. 4Division of Primary Care Internal Medicine, Mayo Foundation, Rochester, MN, USA

Correspondence: Dr F Lopez-Jimenez, Division of Cardiovascular Diseases, Mayo Clinic College of Medicine, Gonda 5-368, 200 First Street SW, Rochester, MN 55905, USA. E-mail: lopez@mayo.edu

Received 22 October 2007; Revised 4 January 2008; Accepted 7 January 2008; Published online 19 February 2008.

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Abstract

Background:

 

Body mass index (BMI) is the most widely used measure to diagnose obesity. However, the accuracy of BMI in detecting excess body adiposity in the adult general population is largely unknown.

Methods:

 

A cross-sectional design of 13 601 subjects (age 20–79.9 years; 49% men) from the Third National Health and Nutrition Examination Survey. Bioelectrical impedance analysis was used to estimate body fat percent (BF% ). We assessed the diagnostic performance of BMI using the World Health Organization reference standard for obesity of BF% >25% in men and>35% in women. We tested the correlation between BMI and both BF% and lean mass by sex and age groups adjusted for race.

Results:

 

BMI-defined obesity (greater than or equal to30 kg m- 2) was present in 19.1% of men and 24.7% of women, while BF% -defined obesity was present in 43.9% of men and 52.3% of women. A BMIgreater than or equal to30 had a high specificity (men=95% , 95% confidence interval (CI), 94–96 and women=99% , 95% CI, 98–100), but a poor sensitivity (men=36% , 95% CI, 35–37 and women=49% , 95% CI, 48–50) to detect BF% -defined obesity. The diagnostic performance of BMI diminished as age increased. In men, BMI had a better correlation with lean mass than with BF% , while in women BMI correlated better with BF% than with lean mass. However, in the intermediate range of BMI (25–29.9 kg m- 2), BMI failed to discriminate between BF% and lean mass in both sexes.

Conclusions:

 

The accuracy of BMI in diagnosing obesity is limited, particularly for individuals in the intermediate BMI ranges, in men and in the elderly. A BMI cutoff ofgreater than or equal to30 kg m- 2 has good specificity but misses more than half of people with excess fat. These results may help to explain the unexpected better survival in overweight/mild obese patients.

Keywords:

diagnosis, body mass index, body fat percent, lean mass

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