In a country crazy about fitness and health, we’ve all become obsessed with numbers. Women, especially, worry about the number of our jeans size, weight, waist size, cholesterol level, and dress size, just to name a few. At least those numbers are finite—regardless of our exact weight, we know that it is what it is. But there’s one number that always makes people feel anxious and ashamed … the dreaded BMI. Although I try not to know exactly how much I weigh, I still get confronted with a BMI at the doctor’s office, and even when playing Wii Fit. Part of my distaste for it is the sneaking suspicion that it’s not entirely accurate at gauging fitness. Turns out, I may not be alone in my distrust.
Weight / Height = Fitness?
The Body Mass Index became part of our lives in the late 1990s, but it’s actually been around for over a hundred years. It was devised by a Belgian statistician named Adolphe Quetelet as a quantitative measure of fitness. The formula takes a person’s weight in pounds, divides it by her height in inches squared, then multiplies by 703. For a person who weighs 150 pounds and is sixty-six inches tall, that works out to a BMI of 24.2. It has long been used as a benchmark to roughly determine physical fitness, especially to qualify individuals for jobs as firefighters and policemen. It’s also long been used by insurance companies, researchers, and weight-loss groups.
The National Institute of Health started using BMI to measure general physical fitness in 1998. According to the guidelines, a BMI of 20 to 25 is within the range of normal weight. Anything less than twenty is considered underweight, and 25 to 30 is considered overweight. BMIs of over 30 qualify a person as clinically obese. Although we would never consider a person who was 5'6" and 150 pounds to be overweight, a person of that height and weight has a BMI that teeters on the brink of being considered overweight.
The BMI is useful as a rough estimate of proportionality since it’s a simple ratio of weight to height. The problem is that it’s not always accurate in measuring actual fitness, since it does not consider body composition, including the ratio of muscle to fat or bone density. People with high percentages of muscle mass—which is denser and weighs more—have higher BMIs, sometimes pushing them into the “overweight” category because the measurement doesn’t discriminate between muscle and fat. If two people are the same height and weight, but one person has more muscle, that person is going to register as being unfit, even though it’s the other person who actually carries more fat, putting him at risk for obesity-related complications. Since body fat percentage and fat distribution are more important indicators of obesity, many doctors feel that BMI measurements don’t do enough to assess risk. Many athletes have BMIs that range from 25 to 29, even though their percentage of body fat is far less than the average person’s, and people with seemingly normal BMIs can still have unhealthy amounts of visceral (abdominal) fat.
Besides failing to take muscle mass into account, BMI has been criticized for failing to differentiate between races and the sexes. The original formula was calculated based on Caucasians and may not accurately reflect the physical fitness of African-American, Asian, or Hispanic people, especially women, who tend to have higher percentages of fat in their bodies. A study at Baylor College of Medicine measured test subjects’ body fat, bone density, and muscle mass using high-tech x-ray absorptiometry; they also calculated the subjects’ BMIs. The researchers found that the African-American women had lower fat percentages than their BMIs indicated, and that Hispanic and Asian women had more fat than their BMIs revealed. They theorized that African-American women should have higher standard thresholds for obesity, while Hispanic and Asian women should have lower thresholds, reflecting each race’s different propensity for body fat, lean muscle, and bone density.
BMI measurements aren’t always effective for the elderly, since as we age, we tend to lose muscle and bone density but gain fat, reflecting in an overall weight loss. It’s also notoriously inaccurate for measuring the fitness of children. Kids start out with high percentages of fat, but they develop and get leaner at different times. There are also marked differences between girls and boys, with girls often having higher percentages of fat. Doctors have devised a special BMI formula to measure children, which takes this into account, along with the fact that girls and boys hit their growth spurts at different times. Children’s BMIs are measured on a sliding scale, placing them on a continuum in a percentile, rather than just assigning a number.
A Better Way
Some doctors like using BMI data because it’s simple and inexpensive and people can calculate their number and monitor their weight on their own. However, many health professionals argue that a broader range of tests can give a more accurate picture of a person’s physical fitness. Body-fat measurements taken with calipers or a scale that measures fat percentages can both be very effective and accurate. Waist circumference is a better indicator, since a waist size of more than thirty-five inches for women and forty inches for men can indicate increased risk for hypertension, diabetes, heart disease, stroke, sleep apnea, and other health problems.
The National Heart, Lung, and Blood Institute recommends that doctors use BMI measurements in conjunction with waist circumference and an assessment of a person’s other risk factors for disease, such as physical activity, cholesterol level, and family history. On their own, these assessments paint a limited picture, but together they reflect a person’s health much more accurately.
Ultimately, there’s much more to health and fitness than just an arbitrary number. Thin people can have dangerous amounts of visceral fat and bigger people can be perfectly fit and healthy. BMI might be good for guesstimating fitness, but it’s not the only thing to pay attention to. It’s important not to get too caught up in any number, whether it’s BMI, weight, or your best pace for running a mile, no matter what your Wii says.