Which Matters More: Body Fat or BMI?

Cara Jakob, MD

Both are ways of estimating how “fat” a person is. BMI calculation is a simple indirect measurement based on only weight and height.  BMI does not consider gender, age, or muscle mass.  Body fat percentage is a more useful tool because it distinguishes between the weight of fat and that of your lean body mass—bones, organs, muscle and connective tissue.

There are several different methods of assessing the percentage of fat and lean mass of an individual.  These methods are referred to a Body Composition Analysis.  Some of the most common measurements include skin fold thickness, underwater weighing, and bioelectrical impedance.

 Impedance is the method used by the body-fat scale you stand on at Total Family Wellness. Also known as bio-impedance analysis, it sends an alternating current through your body from one foot to the other. The faster the electrical signal travels, the more muscular you are. That's because water conducts electricity, and muscle contains significant amounts of water; fat contains virtually no water, so it impedes the signal. The scale uses the speed of the signal to calculate your body-fat percentage.

This method of testing is very sensitive: How well-hydrated you are when the test is done, and your temperature, prior exercise and meals, all can affect results. Also, impedance devices are typically less accurate for people who are very obese or very lean. For best results, get tested first thing in the morning, before you've eaten or exercised but about a half-hour to an hour after you've had a glass of water. Even then, take the results as an estimate.

 

*American Council on Exercise
Classification	Women (% fat)	Men (% fat)
Essential Fat	10-12%	2-4%
Athletes	14-20%	6-13%
Fitness	21-24%	14-17%
Acceptable	25-31%	18-25%
Obese	32% plus	25% plus

 

Don’t get confused when you see a BMI chart and try to insert your Body Fat Percentage.  You will be discouraged.  Body Mass Index and Body Fat Percentage are two different numbers!  A woman with a Body Fat Percentage of 30 is in the acceptable range.  A BMI of 30 is considered overweight!

 

BMI	Weight Status
Below 18.5	Underweight
18.5 – 24.9	Normal
25.0 – 29.9	Overweight
30.0 and Above	Obese