One of the main parameters to measure health in a person in the primary care office setting is a hundred-year-old calculation called the BMI, or Body Mass Index. This measure takes into account a person’s height and weight and categorizes them as underweight, normal weight, overweight, or obese.
The calculation is a simple formula: (Weight in Pounds / [Height in inches x Height in inches]) x 703. A score of 18.4 or lower indicates underweight; 18.5 to 24.9 indicates normal weight; 25 to 29.9 is overweight; and a BMI of 30 or higher indicates obesity.
Similar to our guidelines for what constitutes high blood pressure, these cutoffs are partially arbitrary, but based on evidence. I frequently review the BMI with patients, and always add that it is a rough estimate of health, and most useful when compared with other factors.
In 1832, Belgian statistician Adolphe Quetelet (1796-1874) came up with a formula he called the Quetelet Index of Obesity. Quetelet originally set out not to define body mass but to define what the “normal man” was. He based this on a series of measures, such as age of marriage and arm strength. He collected the data of a few hundred countrymen, and found a correlation between weight and height, noting that people 10 percent taller than average were also around 21 percent heavier. This formulated his weight-to-height equation. In the early 1900s, other scientists noted a linear correlation between increased body fat and rates of osteoarthritis, heart disease, diabetes, sleep apnea, and other chronic illnesses.
Quetelet’s formula had little impact until 1972, when researcher and physiology professor Ancel Keys (1904-2004) published a study called “Indices of Relative Weight and Obesity.” In this study, data was collected from over 7,400 men in five countries. Keys examined several height-weight formulas and compared them to the subjects’ body-fat percentages. The best predictor of body-fat turned out to be the Quetelet formula, renamed by Keys the Body Mass Index.
The BMI quickly caught on because it is a simple and inexpensive test. By 1985, the National Institute of Health (NIH) categorized a BMI greater than 30 as obesity. In 1998, the NIH added cutoffs to include an ‘overweight’ category.
There are setbacks with using the BMI. It does not take into account muscle mass or body fat differences between males and females. A fit athlete may fall into the overweight/obese category, because the BMI does not distinguish between muscle and fat. Age-related changes are alo not accounted for: as one ages, body fat increases and muscle mass decreases, yet their BMI may stay the same.
Why, then, do we so often use the BMI in practice?
On the whole, the BMI really does correlate well with adiposity (body fat) for most people and in population studies. The best answer at this time may be supplementing the BMI with waist circumference measurement and other individual assessments, which more accurately determine risks of being overweight.
We still care about the BMI, because science shows that increased body fat is an independent risk factor for many diseases.
Despite its controversy, the BMI is one of the best tools we have to assess obesity, a modifiable risk factor for several chronic diseases and conditions. Waist circumference is gaining in popularity, especially in assessing diseases such as metabolic syndrome, and can provide a more accurate assessment of abdominal body fat. For now, the BMI’s utility in health prevention is relevant and important.