What Is Your Body Mass Index?

By Jude Buglewicz

It's no secret that the U.S. is one of the fattest nations in the world: 66.3 percent of Americans over 20 years old are overweight or obese (about 140 million); 32 percent are obese (67 million); and almost 5 percent (9 million) are morbidly obese. Among adolescents 12 to 19 years old, over 17 percent are overweight (over 12.5 million)—16 percent are girls and 18.2 percent are boys. But what exactly do the terms "overweight," "obese," and "morbidly obese" mean, and why should these distinctions matter to you?

The standard definitions as used by the National Institutes of Health (NIH), the Centers for Disease Control (CDC), and the World Health Organization (WHO) (and most social science and medical journals that rely on the data from those organizations) are based on body mass index (BMI) levels. This is a calculation using your height and your weight.

Calculate your BMI

Metric formula. Divide your weight (in kilograms) by your height (in meters) squared: weight (kg)/height squared (m²).

NIH method. If you prefer good ole American pounds and inches, multiply your weight (in pounds) by 704.5. Divide that by your height (in inches). Then divide that number again by your height (in inches): weight (lbs.) x 704.5/height (inches)/height (inches).

Which group are you in?

Normal weight – BMI of 18.5 to 24.9. Nonsmokers in this range have the lowest risk of disease and premature death.

Overweight – BMI of 25 or more. This group has an increased risk of weight-related medical problems, including diabetes, heart disease, high blood pressure, and high cholesterol.

Obese – BMI of 30 or more (at least 30 pounds overweight). Below are some statistics associated with this group.

- 67 million Americans (32 percent of adults)
- Women: 36 million (33 percent)
- Men: 32 million (31 percent)
- The number of obese American adults doubled in the last 20 years.
- Weight-related medical problems increase sharply for this group: type 2 diabetes, cardiovascular disease, breast and colon cancer, gall bladder disease, high blood pressure (twice as common as for people at a healthy weight), stroke, osteoarthritis, sleep apnea, respiratory problems, etc.
- This group has a 50-percent to 100-percent increased risk of premature death from all causes.

Morbidly obese – BMI of 40 or more (typically about 100 pounds overweight). Below are statistics associated with the morbidly obese group.

- 9 million Americans (almost 5 percent)
- The number of morbidly obese American adults quadrupled in the last 20 years.
- People in this group have an increased risk for a shorter life expectancy (it could be up to 20 years shorter). Death from diabetes or heart attack is 5 to 7 times greater than for non-obese people, heart disease is 6 times more common, and diabetes is 10 times more common.

Data from the National Center for Health Statistics, based on 2003–2004 estimates from National Health and Nutrition Examination Surveys (NHANES), and from the U.S. Department of Health & Human Services.

Problems with BMI

Although body mass index is the most commonly used measurement of obesity, it doesn't distinguish between fat and fat-free mass, like muscle and bone. Bodybuilders and other athletes with lots of muscle (which weighs more than fat) may have high BMIs, and so they would be classified as "overweight" or "obese," though they're more likely to be healthy and fit—not fat. And older people who lose muscle mass through the aging process and then replace muscle weight with fat may still have the same height and weight, and so the same BMI number, though they'd actually be "fatter."

Because of such concerns, some researchers are pressing for more accurate ways to assess body fat, including using body fat percentage, while others argue that it's the location of body fat that's most important, not simply how much of it you have. Excessive deep abdominal fat is far worse than fat around your hips and thighs, as it is linked to an increased risk of diabetes, heart disease, stroke, and other serious medical conditions. Your waist measurement, then, is also a gauge of your health (over 35 inches for women and over 40 inches for men are associated with higher disease risk).

But because BMI is so easy to determine, and because most of the research on the medical risks stemming from obesity is based on BMI data, your body mass index is a number worth knowing.

Reverse the trend!

If you're reading this, chances are you've bought a Beachbody® product—perhaps P90X® or Turbo Jam® or Hip Hop Abs®—and you are on your way to a long-term healthy and fit lifestyle. Good for you! And if right now you happen to be one of the 140 million Americans who are considered overweight or obese, just keep exercising, Keep Pushing Play, and keep eating right. Below are just three benefits that you can look forward to.

1. You'll lower your risk of heart disease or stroke by losing just 5 percent to 15 percent of your weight.
   
2. You'll lower your risk of type 2 diabetes (losing 10 to 15 pounds is enough for most people, according to the American Diabetes Association).
   
3. By losing 10 percent of your weight, you can have a 10-percent decrease in total cholesterol and a 40-percent decrease in obesity-related cancers.
   

And if you're one of the 9 million morbidly obese Americans, take heart. Have a look at the 2007 Million Dollar Body Game® Grand Prize Winners, a group of dedicated individuals who prove that regular exercise and a healthy diet can dramatically alter your life for the better. As Jenn B., 2006 Success Story, says, "I'm so much happier and healthier and I have more confidence, better posture...heck, I've got my life back! My medical conditions (constant back pain, leg pain, asthma) have fixed themselves." Amazing what you can do if you're committed.