The Significance of BMI & Body Fat Percentage

Tight jeans aren't the only way to tell when you've gained fat.
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You can quantify your health in many ways, such as by measuring your resting heart rate, blood pressure, cholesterol level or weight. The problem with trying to put a number on your health is that no single measurement provides the big picture, and these measurements can be misleading. Doctors and fitness professionals usually estimate body composition using body mass index and body fat percentage -- though these measures are imperfect.


Doctors and professionals use BMI to classify the overweight and obese. To calculate your BMI, multiply your height in inches squared by 703, then divide your weight in pounds by the product. A normal BMI ranges between 18.5 and 24.9. The major limitation of BMI is that by using weight, it fails to differentiate between body fat and muscle mass. For example, a 5 feet 10 inches, 230-pound bodybuilder with 5 percent body fat would be considered obese. BMI can also fail to identify obesity in those who are over-fat and have little muscle mass.

Body Fat Percentage

Body fat percentage indicates the portion of your weight that's fat. A healthy body fat percentage for women between the ages of 20 and 39 is 21 to 32 percent. This measurement can be helpful in predicting obesity-related problems like Type 2 diabetes, heart disease and stroke. Unfortunately, these measurements can be extremely inaccurate. Many fitness professionals measure with body fat calipers, but the accuracy of test results depends on the skill of the tester. Bioelectrical impedence is another common choice, but it is notoriously unreliable due to factors such as level of hydration and recent exercise. Hydrostatic weighing and the "Bod Pod" machine offer more accuracy, but expense and unavailability make these options a long shot.


The major drawback of BMI and body fat testing are that they don't provide an overall picture of health. They can also be misleading and even fail to identify the overweight and obese. If you suffer from poor body image or disordered eating, these measurements can become an unhealthy obsession. Because of their limitations, you should only consider BMI and body fat percentage to provide a partial picture of your overall health.


If BMI can be a lousy indication of body composition, and the accuracy of body fat measurements can be so far off, you may wonder why even bother with either of these. Both BMI and body fat percentage provide good baseline data for your fitness programs. Say you've put on a few pounds after a wedding and your BMI has inched up from 25 to 28 -- you can use this measurement as a basic gauge to indicate that you should probably lose some weight for health reasons, not just to look great. Similarly, body fat measurements should be used to help you estimate changes in your body composition to see if you're on the right track to your goals. The key is to not view either as the end-all of your health and fitness.

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