Revisiting BMI: Issues to Consider

A few months back, I talked about BMI, or Body Mass Index. Now I have done a little bit more research and found some interesting issues to consider:

The current definitions commonly in use establish the following values, agreed in 1997 and published in 2000:

A BMI below 18.5 is characterized as underweight
A BMI of 18.5 – 24.999 is characterized as normal weight
A BMI of 25.0 – 29.999 is characterized as overweight or pre-obese
A BMI of 30.0 – 34.999 is characterized as obese
A BMI of 35.0 or higher is characterized as severely (or morbidly) obese

BMI is most accurate for people who live a sedentary lifestyle. It thus cannot distinguish between weights from body fat, muscle mass, or bone mass, so the table above is inaccurate for example in athletes, children or the elderly. Because muscle is more dense than fat, most amateur athletes would be classified as "overweight" and most professional athletes have enough muscle mass to be classified as "obese" or even "severely obese", when in fact their body fat percentage is very low and they are in no danger of developing any health problems correlative to carriage of excess fat. Children, meanwhile, have higher bone density in the years before puberty because of their smaller size, and that also results in skewed BMI values. In the case of elderly people, muscular atrophy and/or osteoporosis can also decrease the value of a BMI calculation.

There is the additional problem of sex differences between men and women. White and East Asian women tend to have less muscle mass and bone density than men of the same height. The same is not true of black and Polynesian women, however. In the future, a healthy BMI for a given individual may be defined to some extent by their ethnic group, racial origin, or sex, but that is not the case now.

In 2000, the WHO was advised to consider lowering the BMI threshold for overweight in East Asians from BMI 25 to BMI 23, and for obesity in East Asians from BMI 30 to BMI 25, due to epidemiological studies indicating that East Asians suffer a greater number of obesity-related health conditions at lower BMI values. To date, the WHO has not made any changes pursuant to those recommendations.

Additionally, some clinicians suggest raising the BMI thresholds for people with sub-Saharan African and Polynesian ancestry, because members of these groups have a greater ratio of lean body mass to fat at all body weights. The proposed thresholds for these groups are BMI 26 for overweight, and BMI 32 for obesity. Again, to date, no major professional or medical organization has officially adopted this suggestion.

 
(Click on chart to enlarge) This chart compares obesity figures in the population of OECD countries; it shows the percentage of total population (aged 15 and above) with a body-mass index greater than 30. The year the figures used for this chart were published, collected or compiled range from 1996 to 2003