According to the latest data provided by the Centers for Disease Control, nearly 4 out of 10 adults and 2 out of every 10 adolescents in the United States are now considered obese. While we certainly all know at this point that obesity is bad, what exactly does it mean to be obese? Hopefully this article, being the first of a four part series, will provide some clarification into this ever increasing weight epidemic.

To define obesity, we must first consider the Body Mass Index, more commonly referred to as BMI, and how it is used to assign an individual’s weight status. BMI is defined by dividing an individual’s weight in kilograms by their height in centimeters squared. The equation itself was derived in 1832 by Adolphe Quetelet, a brilliant mathematician who submitted it as part of a greater investigation into the normal physical attributes of man. Don’t fret however, it’s not important that we know how to do the calculation, just that we know BMI is a ratio of weight and height, and that any internet calculator will yield the same results

Once we have done the math we arrive at one number that represents an individual’s BMI. Since this BMI number accounts for an individual’s height and weight, their BMI can classify them as obese, overweight, normal or even underweight. The BMI to Weight Status categories are as follows:

BMI Weight Status
<18.5 Underweight
18.6-24.9 Normal
25.0-29.9 Overweight
>30 Obese
> 40 Morbidly Obese

Now that we can see where the number comes from and what it means, let’s take a closer look with a few examples. Using this calculator we can see that anyone who is 5’10 and 210 pounds would have a BMI of 30.1, and would thus fall into the Obese category. So the same individual would need to weigh about 174 pounds to fall into the more desirable “Normal” category.

While the BMI may be quick, convenient, and good at classifying an individual’s weight status, it very often does not tell the whole story. The biggest problem being that BMI only accounts for the ratio of height to weight, and says nothing of density. Take for example basketball star LeBron James, who reports as being 6’8, 250 pounds. Via the calculation, James’ BMI comes in at 27.5, placing him squarely in the overweight category, only 2.5 BMI points away from obesity! This is possible because, even with his immense height and lean frame, James has so much muscle mass that his BMI comes out too high; so clearly the BMI falls short this case.

The BMI measurement can also fall short for individuals with too little muscle mass. Take, for example, an elderly individual with a height of 5’10, a weight of 170, and a waist circumference of 40 inches. While this individual looks to be overweight, he or she could have so little muscle mass and so much fat mass, hence the 40 inch belly, that the BMI comes up in the normal range. Individuals like this often have difficulty with activities of daily living, as these activities require a certain amount of muscular strength and endurance. Long story short, BMI should only be taken definitively when other measures of body composition are known, like waist circumference and percent body fat.

To conclude, the Body Mass Index is a good tool for researchers, clinicians, and the public to classify an individual’s weight status. Yet, as we have seen from a few examples, the BMI should be taken along with other measures of body composition to get a better picture of an individual’s weight status. Please expect more from this topic from the Duke Center for Living at Fearrington over the following weeks.


Mike Clark is an Exercise Physiologist and Personal Trainer at Duke Center For Living’s Health and Fitness Center. He received his BA in Exercise and Sports Science from UNC.


image by Filimonas via flickr