Body Weight vs. BMI vs. Body Composition—Which Matters More?

When it comes to understanding body weight vs. body mass index (BMI) vs. body composition, things can get confusing. Even more confusing is when the numbers don’t seem quite right, which is exactly what happened when actress Sarah Hyland stepped on the scale to determine her body composition—AKA how much of her weight is fat versus lean mass.

“Yeah. My scale says I’m 48% fat. Soooo...how’s your Monday?” the Modern Family star captioned a photo she posted on Instagram stories.

Normal Ranges for Women

Your standard bathroom scale measures your physical body load. According to Houston Methodist Health System, "body composition scales...use bioelectric impedance analysis to determine how much of your total weight is fat-free mass and how much is body fat." While body mass index (BMI) is a calculation that uses your height and weight to determine body fat.

According to Hyland’s body composition scale, she weighs 92.8 pounds, with 49% body fat, 11% muscle, 37% water, and 3% bone. Hmm.

According to the American Council on Exercise, normal body fat percentages for women range from 10% to 31%, with a body fat percentage over 32% considered obese. At 5’2” and 92 pounds, Hyland is anything but obese. So we had some questions.

First off, is it even possible to be 92 pounds and have 49% body fat? “While it is possible to have that high of a percentage of body fat, it would be very abnormal for half of her body weight to be fat,” says Amy Rothberg, MD, director of the University of Michigan's Weight Management Clinic.

Samuel Klein, MD, director of the Center for Human Nutrition at the Washington University School of Medicine in St. Louis agrees: “Women tend to have more body fat than men, but it's highly unlikely that she would be 92 pounds and almost 50% body fat.”

Distribution of Body Fat

It’s not surprising that Hyland’s home scale provided questionable results. According to Dr. Klein, there's so much variability in home body composition devices that they aren’t always reliable sources. Plus, body composition scales—or the regular versions, for that matter—don’t account for the distribution of fat throughout the body. Body fat distribution is actually just as important as the amount of fat one carries.

Why? Belly fat is linked to an increased risk of metabolic diseases like diabetes and heart disease compared to fat stored in the buttocks and legs.

When assessing body fat, it's critical to consider distribution in addition to composition, says Dr. Klein: “Measuring your waist circumference, for example, will give you a better estimate of your fat distribution than stepping on a scale.”

Measuring Body Composition

According to Dr. Rothberg, the most reliable way to measure body composition is with a DEXA scan or Bod Pod device, two tools that are typically found at medical centers, fitness clinics, or in research settings—not your bathroom–and use high-tech methods to compare fat tissue to lean mass.

We weren’t the only ones who were skeptical of Hyland’s home scale reading. The actress captioned her body comp score, “Definition of #skinnyfat or my scale is broken.” We’re going to guess it’s broken.