> Fitness > Body Mass Index (BMI)

It’s not enough to stand on the bathroom scale to measure your present physical (and therefore physiological) condition. Remember that muscle weighs significantly more than fat.

Many people remain at the same weight for years, even though their valuable lean muscle tissue has been replaced by fat.

Other ways of assessing your physical condition may be more accurate. Body mass index (BMI), waist circumference, and lean body analysis are alternative measures you have probably heard about. But what do they mean?

Body Mass Index

Your BMI is calculated using height and weight (see sidebar). The Nurses’ Health Study II, which followed 116,000 nurses over a 15-year period, indicated that the lowest mortality rates were found in women with a BMI of 19 or less. The risk of premature death increased by 60 percent for BMIs of 27 to 28.9, and doubled for those with a BMI of 29 and higher.

The problem with the BMI is that it places all weight in the same category. For instance, muscular people often have a high BMI even though they have little body fat; elderly people often have low BMIs even though they have little muscle mass and a high body fat percentage.

Waist Circumference

A more efficient (and simpler) means of measuring health risk associated with excess body fat is measuring your waist circumference. This is because fat located in the abdominal cavity is more associated with diseases such as diabetes and heart disease than fat located elsewhere.

A waist circumference of more than 40 inches in men and 35 inches in women signifies increased risk of disease, especially if your BMI is more than 25. To measure, place the measuring tape around the narrowest part of your waist. (If that isn’t easy to find, measure at your belly button.)

Lean Body Analysis

The hallmark of measurements - as it pertains to true body fat levels - belongs to lean body analysis. It is performed by numerous means (listed here from least to most reliable): skin-fold callipers (this method depends entirely on the expertise of the person performing the evaluation), bioimpedance testing (a harmless electrical measurement), near infrared technology (using near infrared light), and hydrostatic weighing (underwater weighing).

As you can see, many more effective ways can be used to measure your present physical condition than standing on the bathroom scale.

Calculate Your BMI

Divide your weight in pounds by your height in feet squared; then multiply by 4.89. For example, a 135-pound woman who stands 5 feet, 6 inches tall would calculate her BMI as follows:

135 lb X 4.89 = 22 BMI
(5.5 ft X 5.5 ft)

Here’s what your BMI calculation means:

• Under 18.5 - underweight
• 18.5 to 24.9 - normal
• 25 to 29.9 - overweight
• 30 to 39.9 - obese
• 40 and over - extremely obese

Brad J. King, MS, MFS, is a nutritional researcher and author of five books, including the international best-seller Fat Wars (Wiley, 2004). Subscribe to his free monthly newsletter at fatwars.com.

Source: alive #268, February 2005