October 2007
Body-fat scales: Will they help?
Two-thirds of Americans are overweight, according to a study released in May 2007. And if you have too much fat—especially
around the waist—you're at greater risk for health problems such as diabetes, heart disease, and stroke.
The body-mass index, or BMI, has been the standard for gauging obesity for years. The formula for body-mass index considers height and weight, but not body-fat percentage. BMI is your weight in pounds divided by the square of your height in inches and multiplied by 703. To calculate your BMI, go to www.nhlbisupport.com/bmi/bmicalc.htm.
But concerns about fat and obesity have spawned the body-fat bathroom scale; besides giving your weight, it uses a weak electric current to measure the percentage of fat on your frame. Some manufacturers maintain that body-fat percentage is the key to health and fitness. But our tests in 2003 suggested that such scales are not the most reliable measure of body fat. What's more, our consultants say it's a mistake to make too strong a connection between health and the amount of body fat you have.
"The connection between body fat and health outcome has defied a simple answer," says Steven B. Heymsfield, M.D., who leads obesity-drug development at Merck's metabolism division and has done extensive research on body composition. "That's why there is not a standard for health based on body fat."
MEASURING BODY FAT
Large long-term studies of people in many countries have shown that those with a BMI below 25 have lower rates of diabetes, heart disease, high blood pressure, and overall mortality than do people with a BMI of 25 or more. A BMI of 30 or more means you're obese, and your risk of disease and death rises sharply. Researchers have begun to relate measures of body fat to BMI calculations. However, no one knows what a healthful body-fat amount is, despite claims from the manufacturers of some body-fat scales.
When you step barefoot on a body-fat scale's metal footpads, a minuscule electrical current—much too low for you to feel—passes up one leg, through your pelvis, and down the other leg. The amount of resistance that the current encounters depends on the proportion of fat to muscle tissue. From this information, the scale's built-in formulas estimate fat content.
In practice, the scales we tested in 2003 were mediocre. When compared with an accurate laboratory measure of body fat, the best scale hit the mark for only about 80 percent of the volunteers who tried it. The scales sometimes understated and sometimes overstated a volunteer's body fat. There was no way to predict which way the scales would err. At least they were consistent, always over- or understating by the same amount.
HOW TO CHOOSE
Decide how much motivation you need. A body-fat scale is unlikely to give you more useful information than your BMI. But a scale's readings, even if inaccurate, may provide needed motivation—especially as you build weighty muscle while shedding fat.
Look for important features. If you're going to buy one of these scales, these features matter the most:
The body-mass index, or BMI, has been the standard for gauging obesity for years. The formula for body-mass index considers height and weight, but not body-fat percentage. BMI is your weight in pounds divided by the square of your height in inches and multiplied by 703. To calculate your BMI, go to www.nhlbisupport.com/bmi/bmicalc.htm.
But concerns about fat and obesity have spawned the body-fat bathroom scale; besides giving your weight, it uses a weak electric current to measure the percentage of fat on your frame. Some manufacturers maintain that body-fat percentage is the key to health and fitness. But our tests in 2003 suggested that such scales are not the most reliable measure of body fat. What's more, our consultants say it's a mistake to make too strong a connection between health and the amount of body fat you have.
"The connection between body fat and health outcome has defied a simple answer," says Steven B. Heymsfield, M.D., who leads obesity-drug development at Merck's metabolism division and has done extensive research on body composition. "That's why there is not a standard for health based on body fat."
MEASURING BODY FAT
Large long-term studies of people in many countries have shown that those with a BMI below 25 have lower rates of diabetes, heart disease, high blood pressure, and overall mortality than do people with a BMI of 25 or more. A BMI of 30 or more means you're obese, and your risk of disease and death rises sharply. Researchers have begun to relate measures of body fat to BMI calculations. However, no one knows what a healthful body-fat amount is, despite claims from the manufacturers of some body-fat scales.
When you step barefoot on a body-fat scale's metal footpads, a minuscule electrical current—much too low for you to feel—passes up one leg, through your pelvis, and down the other leg. The amount of resistance that the current encounters depends on the proportion of fat to muscle tissue. From this information, the scale's built-in formulas estimate fat content.
In practice, the scales we tested in 2003 were mediocre. When compared with an accurate laboratory measure of body fat, the best scale hit the mark for only about 80 percent of the volunteers who tried it. The scales sometimes understated and sometimes overstated a volunteer's body fat. There was no way to predict which way the scales would err. At least they were consistent, always over- or understating by the same amount.
HOW TO CHOOSE
Decide how much motivation you need. A body-fat scale is unlikely to give you more useful information than your BMI. But a scale's readings, even if inaccurate, may provide needed motivation—especially as you build weighty muscle while shedding fat.
Look for important features. If you're going to buy one of these scales, these features matter the most:
- Multiple-user memory. Some can store personal data for one or two users. Others can store as many as 10 profiles.
- Special modes. Some few have a "guest" mode so that someone else can check body fat without using the memory feature.
