Figure 1: Images of eight women all having a BMI of 30. Notice the dramatic
differences in where each woman's weigh is distributed. These distribution
differences can correlate with large differences in risk for disease.
Body mass index or BMI is very old calculation that has become one of the most popular
and oft-used measures of human body weight. The very simple definition of BMI is:

BMI = (Body Weight in Kilograms) % {(Body Height in meters) x (Body Height In
Meters)}

Since we use imperial measurements in the United States, the formula is simpler as:

BMI = {(Body Weight in pounds) x 703} % {(height in inches) x (height in inches)}

So, for a woman weighing 230 pounds and standing five foot six inches tall, her BMI would
be about 37 which is considered dangerously obese. The chart below shows BMI ranges for
various heights and weights. People falling in the white area are considered underweight,
the yellow area; normal, the dusky yellow area; overweight, and the red area; obese.


The problem is that after all the fancy calculations are performed and the pretty graphs
are drawn, BMI is only based upon two measurements: height and weight. BMI does not
account for gender, for muscularity, for bone structure or for body fat measurements. In
the end, BMI is just another height versus weight chart like those that life insurance
companies have used since the 1930s. Here is an example of the problem with BMI: a 26
year-old male football player might weigh 230 pounds, stand five foot ten inches tall and
have a BMI of 33 and be in superb health because his weight is muscle not fat. On the
other hand, a sedentary woman of 52 who is five foot six and weights 145 pounds has a
BMI of 23.4 but might be quite unhealthy because she has very little muscle. So the
problem is that just knowing a person's height and weight doesn't tell us what we really
want to know, and that is how fat he or she is and how much risk that places upon the
person's health.

Consider the images below of real women all of who have a BMI of 30.
You can see substantial differences in where each woman carries her weight and these differences
correlate with wether the weight is fat or muscle and, if fat, how much risk it places upon the woman
carrying it. So, what we would really like to know is not just a person's height and weight, but many
other parameters as well. This brings us to the subject of body composition.
Body Mass Index (BMI)
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