Weighing In on Obesity—What BMI Can Tell You

December 16, 2014 | 4:09 pm
Lindsey Haynes-Maslow
Former contributor

In public health, one of the first steps to addressing a problem is defining it. The second, measuring it. Most Americans are aware that our country has an obesity problem—in adults as well as children—in part because of our unhealthy food system. But how do researchers gauge the extent of the problem, and what do we know about whether it is increasing?

I recently began thinking about this in my veterinarian’s exam room, when I heard words that no obesity-prevention researcher wants to hear: “Your cat is heavy for her size.” Embarrassed, I quickly countered, “Based on what? Her body mass index? Where does she rank on the chart?” My vet simply chuckled and shook her head, “Cats don’t have a body mass index chart.” It turns out that obesity in cats is determined by placing your hands around their sides and trying to feel their ribs. If you can’t feel them—the cat is overweight.

Defining obesity in humans is complex

Defining and measuring obesity in humans is a bit more complex. It requires both researchers and doctors deciding how to measure body fat and then establishing body fat thresholds—healthy weight, overweight, and obese. Just as I felt my cat’s “heavy” diagnosis was biased, we also deal with bias in human science. When measuring body fat in humans, several factors that may bias weight thresholds are age, sex, race/ethnicity, body fat distribution, height, and weight.

According to the World Health Organization obesity is excessive body fat that may impair health.

Figure 1: BMI and Mortality Ratio*

Currently, the most widely-accepted method to measure excess fat is body mass index (BMI). BMI is obtained by dividing weight (measured in kilograms) by height squared (measured in meters). Generally, most professional organizations (i.e., the Centers for Disease Control and Prevention) consider persons obese if their BMI is 30 or greater. This threshold is largely based on the positive relationship between BMI and mortality (see Figure 1). That is, as BMI increases, mortality rates increase. BMI is also positively associated with Type II diabetes and heart disease.

Options for measuring body fat

BMI is the most popular method for measuring obesity. It’s quickly measured, easily calculated, and cost-effective. Unfortunately, since BMI is based on height and weight, it can’t account for differences between fat mass and lean body mass (i.e., muscle). Additionally, it has been suggested that BMI threshold categories are based predominately on Caucasians living in Europe and the United States. This can be problematic since some races/ethnicities have higher percentages of body fat than do Caucasians. Therefore they might have a normal BMI, but still classified as overweight or obese. While BMI is one of the most widely-used methods for measuring body fat, there exist more precise methods. Three other options for measuring body fat are: bioelectrical, densitometry, and imaging techniques.

  1. Bioelectrical Impedance Analysis. Bioelectrical impedance analysis (BIA) measures body fat by sending low electrical currents through the body with the idea that a leaner person will have lower resistance to the electrical current. However, BIA’s precision is influenced by the amount of water in the body, which can be influenced by body structure (i.e., short person vs. tall) and hydration status. For example, someone who just drank a glass of water will test differently than someone who hasn’t had anything to drink in the past couple hours.
  1. Densitometry. Densitometry, also known as “underwater weighing” is based on the idea that fat is less dense than water. Therefore a person with lower body fat will have higher body density. Densitometry is often considered the “gold standard” for accurately measuring body composition. Unfortunately, the test is time consuming, expensive and not easily performed on children and older adults since it requires a person’s attention and cooperation. If you’ve ever experienced a child’s bath time, you know “attention” and “cooperation” aren’t usually words used to describe it.
  1. Imaging techniques, such as dual energy x-ray absorptiometry (DXA), CT scans, and MRIs are very accurate when measuring body composition. Unfortunately, they are also very time-consuming, complex, and expensive. The tests can take up to an hour and technicians operating the machines must be highly trained. Additionally, since DXA uses X-ray exposure, it is not suitable for pregnant women. And good luck getting a health insurance company to cover the cost!

Is BMI a good measure for obesity?

While BMI may not be as precise as BIA, densitometry, and imaging, it is easy and cost-effective. That makes BMI a great tool for doctors and researchers. It gives doctors the opportunity to discuss with patients how they might be at risk for becoming overweight or obese. And it gives researchers the ability to track obesity trends from year to year. In the latest report from America’s Health Rankings, released just last week, results show that adult obesity rates in the U.S. increased 7 percent from 27.6% to 29.4%. With almost 30 percent of adults being obese, America is in for a rude awakening. Increased healthcare costs due to obesity-related chronic diseases such as type 2 diabetes, heart disease, dementia, osteoarthritis, respiratory illness, cancer AND decreased employee productivity from missed work days due to illness.

So, can you really trust BMI as a measure of weight status? Yes!—BMI is a great indicator for understanding population trends in obesity and goal-setting for policymakers who want to reverse the current trend.

Special Note for Cat Owners: Prescription diet cat food is approximately three times more expensive than regular cat food.