Body mass index cutoffs being determined for the BMI calculator

By Steven B. Halls, MD FRCPC

This page continues the discussion about the "halls.md v2" thresholds for determining overweight, as used in the Body Mass Index BMI calculator. A previous page described the development of the slim changes proposal curve-shape, and this page describes how it is implemented and validated for separate curves for men and women.

Once the "slim changes proposal" curve is shifted upward to cross it’s key point at BMI=25 at age=18.5, the next step is to divide this threshold line into two lines, one for men, the other for women, separated by a gap of 2 kg/m².

 
 

The lines should divide smoothly and gradually, while preserving the feature that the average of men and women’s thresholds still equals the slim changes proposal curve ( which it does).

For additional validation, the threshold lines are compared to other important key points for older adults.

The curves are a good match for several key points shown in the graph above.

 
 

1. Triangles, from reference 1, show BMI=27 for men and BMI=25 for women, which at Age 31, seem to give the desired balance between sensitivity and specificity.
2. Diamonds, from reference 2, show BMI=28 for men and BMI=26 for women, which at Age 38, seems to give the desired balance between sensitivity and specificity.
3. Circles, from reference 3, show BMI=28.1 for men, and BMI=26.2 for women, which comes from the average age 50.6 years, in the study of "figural stimuli", for which figure=6 for men and figure=5 for women represent "overweight" figures.
4. Square, from reference 4, shows BMI=27.3, which is represents the healthiest BMI of older persons (men and women combined). It bothers me to label older people as "overweight", when a BMI around 27 has the lowest mortality rate, but at least it’s better than setting the threshold at BMI=25.

The next source of validation key points comes from reference 5, which contains five different formulas to estimate body fat percentage, based on Age, Gender and BMI.

 
 

By taking the average result from these five formulas, at age 35 and 45, for 25% body fat for men and 33% body fat for women, the BMI values are plotted on the graph below. The halls.md v2 overweight threshold lines cross nicely between these key points.

 

References

1. Wellens RI, Roche AF, Khamis HJ et al. Relationships between the Body Mass Index and body composition. Obes Res, Jan 1996;4(1):35-44
2. Curtin F, Morabia A, Pichard C, Slosman DO. Body mass index compared to dual-energy x-ray absorptiometry: evidence for a spectrum bias. J Clin Epidemiol, Jul 1997;50(7):837-43
3. Bulik CM,Wade TD, Heath AC et al. Relating body mass index to figural stimuli: population-based normative data for Caucasians. Int J Obes Relat Metab Disord, Oct 2001;25(10):1517-24
4. Allison DB, Zhu SK, Plankey M et al. Differential associations of body mass index and adiposity with all-cause mortality among men in the first and second National Health and Nutrition Examination Surveys (NHANES I and NHANES II) follow-up studies. Int J Obes Relat Metab Disord, Mar 2002;26(3):410-6
5. Jackson AS, Stanforth PR, Gagnon J, et al. The effect of sex, age and race on estimating percentage body fat from body mass index: the Heritage Family Study. Int J Obes Relat Metab Disord, Jun 2002;26(6):789-96

Back to the halls.md v2 criterion description.