Body Mass Index (BMI) and body fat percentage are two measurements that I take with all new clients. Fitness and health professionals feel differently about the usefulness of each value with some claiming BMI is superior and others stating that body fat is more beneficial to know. In order to determine which is “superior” first we need to know what each is, and how it can be useful in your quest to a healthier lifestyle.
BMI in its simplest form is merely a ratio of your weight to your height. Historically BMI has been a useful indicator of someone’s risk of certain diseases such as diabetes and heart disease.
Unlike body fat percentage, which takes special equipment and/or training to measure, to calculate BMI all you need is your height (in inches) and weight (in pounds). By using the following formula you can quickly determine your BMI.
BMI = [weight (in pounds) / (height in inches)^2] x 703
If math isn’t your thing you can also use the BMI calculator found here.
The following table shows the normative values for BMI.
Underweight - Less than 18.5 Recommended - 18.6 to 24.9 Overweight - 25.0 to 29.9 Obese - 30 or greater
Despite BMI being widely used as a health indicator, many fitness and health professionals (yours truly being one of them) feel that BMI falls short in determining true risk of disease and being an indicator of overall health. In fact, a recent study at UCLA, featured in the International Journal of Obesity, has shown how people with “normal” BMIs can be unhealthy and those with “overweight” or “obese” BMIs can in fact be healthy. This study claims that upwards of 54 million Americans are classified as overweight or obese. Since BMI is simply a ratio of weight to height it fails to take into account a person’s amount of body fat vs. fat-free mass. Simply put; if you have more muscle than average you will most likely measure as overweight. In cases such as this body fat percentage is a more useful measure.
BODY FAT %
Body fat percentage tells you the makeup of body fat vs. fat free mass on your frame. Although BMI has been widely researched and linked to certain disease, the studies which attempt to correlate body fat percentage to disease is slowly increasing.
The downside of body fat over BMI is that you need to have special equipment and a trained person to use the equipment in order to analyze it. The two methods I use most often to measure body fat are calipers which measure skin fold size and bioelectrical impedance which uses a handheld device. Of the two measures, bioelectrical impedance is the quicker measure and does not rely on tester skill like skin fold measurement with a caliper, however it is dependent on several factors including hydration levels, and depending on the machine used the error rate can be as high as 8%.
The following table is the ACSM standards on body fat percentages.
With body fat percentages being a more true measure of composition, it is a better metric to use when fitness and health professionals are dealing with people interested in losing weight. To illustrate how BMI and body fat differ refer to the below chart.
Although both men are the same height and the same weight, the man with lower body fat percentage has less fat mass, and more fat-free mass on his frame. This person will look leaner, than his counterpart, although both would be classified as overweight if relying on BMI alone.
Unfortunately it’s a draw. While BMI is the more studied metric regarding risk of certain diseases, it falls short of measuring true body composition. People with larger amounts of muscle will get a reading that may classify them as overweight or obese. For a true measurement of body composition body fat percentage is the winner. BMI however is more economical, easier to perform, does not rely on tester skill, and for the general population, may be a better overall indicator of health. BMI is not without its limitations and as future studies continue to assess it as a health indicator it may prove to be a worthless measure. In my opinion both are useful to know, however relying solely on BMI as an indicator of health is not the way to go. Body fat should be assessed and taken into account when trying to determine someone’s overall health as well as to assess the effectiveness of any diet and exercise program.
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