Most of us have our private ways of assessing how fat we are. We feel our pants getting snug — or loose, if we’re lucky. We take a glance in the mirror or at our reflection in the shopfront window.
Of course, there are more objective ways of answering the question. Plain old weight is a good clue, but it’s a total that includes bones, muscles, organs, hair — not just fat. The tried-and-true way of measuring just fat involves getting weighed while fully submerged in water. The difference between your weight in water and your regular weight is used to calculate body density, and from that, the proportion of the body made up of fat. But few of us are going to subject ourselves to regular dunking.
There are other, easier tests: bioelectric impedance, skinfold testing with calipers, dual-energy X-ray absorptiometry (the same technology used to measure bone density). Gyms and fitness centers are beginning to offer some of these. They’ll satisfy the curious, but they’re neither necessary nor practical for routine use.
That leaves us with three more common options. By now, most people are familiar with the calculation known as body mass index. Waist circumference is a hot topic as it becomes clear that it’s the fat we carry inside our abdomens that’s most metabolically active and harmful. And waist-to-hip ratio is getting a second look because of research showing that the fat under our skin — subcutaneous fat — may have some benefits. Here is a guide of these three measures of our fatness, or adiposity.
Body mass index, or BMI, is computed by taking your weight in kilograms and dividing it by the square of your height in meters. The BMI is easy to calculate, and in most people, it correlates reasonably well with overall body fat. It’s also a good measure of health risk: as a rule, when BMIs go up, so do deaths, particularly from cardiovascular disease. But BMI doesn’t distinguish whether the pounds are from fat or from fat-free tissue like muscle and bone. BMI also doesn’t tell us about the type of fat we’re carrying—a significant shortcoming, as the type of fat that builds up in the abdomen is believed to be particularly unhealthful.
Cutoffs and categories are another problem. People with BMIs of 25 to 29.9 are classified as being overweight and those with BMIs of 30 or over as obese. But risk accrues more gradually than those sharp distinctions might suggest. There’s also a question whether the cutoffs ought to be different for some ethnic groups. Researchers have found, for example, that Asians develop cardiovascular risk factors at lower BMIs than whites, so the overweight category for Asians might start at a BMI of 23 instead of 25.
Waist measurement puts a different spin on obesity: it’s no longer about weight or total body fat, but about the metabolically active fat that collects around the organs in our abdomens. Waist circumference is a better predictor of diabetes than BMI and a good indicator of heart disease risk. Measuring it identifies the sizable group of people who pass muster when it comes to BMI but whose large waists put them at higher risk. Still, waist measurement hasn’t become part of routine medical practice for several reasons. For one thing, there’s some uncertainty about exactly where the waist should be measured, although navel-level is widely accepted. Moreover, the definition of too large a waist may need revision: some studies show that health risks start well before the current cutoffs of 40 inches for men and 35 inches for women. Finally, given all the other information that’s collected on patients—blood pressure, cholesterol levels, BMI—it’s not certain that adding a waist measurement to the mix would affect treatment decisions.
The waist-to-hip ratio (WHR) is a simple calculation: waist circumference divided by hip circumference. A small waist combined with big hips yields a smaller number than a big waist with small hips—and smaller is better when it comes to WHR. For women, the risk for heart disease, stroke, and other health problems starts to climb at a ratio of about 0.85, so that is often set as the cutoff for a “good” ratio. For men, the cutoff seems to be about 0.90. Waist circumference has eclipsed WHR, but the WHR may be ready for a comeback. Research shows that WHR is more strongly associated with heart disease than waist circumference alone.
It would be great if there were a magic bullet for instant weight loss. But, the truth is that watching what you eat, reducing calories, and exercising more is the only tried and true way to change your weight and reduce the health risks associated with abdominal obesity.