From Start to Fitness: Body mass index vs. body fat percentage

Q: What is the difference between the body mass index and body fat percentage? And which is more important?

A: The body mass index and body fat percentage are two assessments designed to identify health risk factors. Each assessment has a purpose and each can be used to help you stay focused on your fitness goals.

The body mass index (BMI) is an equation that gives you a numerical rating of your health status based on your height and weight. Ratings are categorized as “Underweight,” “Recommended,” “Overweight” and “Obese.” Assessing BMI is as simple as looking at a BMI chart or using a computer BMI calculator. Just input your height and weight to get your rating.

BMI numbers in the overweight and obese ranges can identify the risk for diseases associated with being overweight such as heart disease, diabetes and even certain cancers.

Since BMI lumps all body mass (fat and lean) together, it is often not considered as valuable an assessment as body fat percentage because it does not take into account the weight of lean body mass.

Body fat percentage is an estimate of your total body mass, both essential fat mass and nonessential (storage) fat mass. Essential body fat is needed for the body to maintain life and reproductive functions. Nonessential body fat is storage fat that is an accumulation of fat tissue in excess of essential fat.

In the obese range (above 32 percent for women, above 26 percent for men), body fat percentage can also suggest potential health risk just as BMI does. However, body fat percentage determines the amount of your body’s fat mass, not just total body mass as with BMI. Many find this more useful because it can help you realize that although you may be at a “healthy” weight, your percentage of body fat may suggest that for your weight, too much of your total body mass is nonessential fat.

Body fat percentage is determined through many methods, with the most popular being skin-fold measurements (using skin-fold calipers) and bioelectric impedance. The accuracy of body fat testing can vary by up to 8 percent (plus or minus) depending on the testing form selected and the test facilitator.

Both BMI and body fat percentage have a place in assessing health risk factors and providing a benchmark to set fitness and lifestyle goals. Consider both as one part of your fitness assessment routine, remembering the purpose of each assessment.

Always consult a physician before beginning an exercise program.

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