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What’s the Difference Between BMI and Body Composition?

Our experts explain.

By Leslie Pepper

If you’ve ever been told that your BMI is on the high side, you’ve probably felt some sort of cross between embarrassment, anger and a bit of confusion. That’s because BMI — or body mass index — doesn’t tell the whole story.

If you’re not familiar with the term BMI, it’s a person’s weight in kilograms divided by height in meters squared. Simply put, it measures weight in relation to height. But because it’s based on a number on the scale and height alone, BMI doesn’t take into account the relative proportions of bone, muscle and fat in the body. And since bone and muscle are denser than fat, a person who has solid bones and a healthy dose of muscle may have a high BMI.

“BMI doesn’t measure fat mass, so even someone with a ‘normal’ BMI could have a high percentage of fat around their organs, which is concerning when you consider someone’s health,” says Brittany Masteller, Ph.D., a research scientist at Orangetheory.

That’s not to say that BMI serves no purpose. “BMI is meant to track population weight trends. In general, BMI has been shown to correlate with body fat and health risks at a population level; however BMI should only be used as an initial screening tool for individuals,” says Kimberly Plessel, M.S., RDN, LD, a member of Orangetheory’s Medical Advisory Board. The formula was invented in the 1800s by statistician Adolphe Quetelet when he was looking for a way to analyze the population.

“BMI does a good job at placing people into categories based on population averages,” says Masteller. “However, I don’t typically recommend BMI to be used alone as a diagnostic tool nor prescription for anything,” she adds.

A more beneficial measure is body composition, which is the ratio of fat-free mass (bones, water and muscle) to fat mass. “When we look at body composition, we're looking at all of those components,” says Masteller. “We’re going to be able to analyze not just what the weight is, but what it’s composed of — how many pounds is muscle? How many pounds is fat?”

This offers much better insight into a person’s health and their risk for disease. A high percentage of body fat, especially when carrying weight in the abdominal area, may categorize you at higher risk for metabolic syndrome, a cluster of conditions such as high blood pressure and abnormal cholesterol levels that raises your risk of heart attack and stroke, says Plessel.

There are several methods of determining body composition, some more accurate — and accessible — than others. Many gyms offer skin-fold measurements, which use calipers to pinch the skin and measure specific parts of the body, which are then compared to a chart to estimate fat percentage. While this can give you a good idea of body composition, the accuracy really depends on how precise the measurements are, which varies according to who’s performing the assessment.

In contrast, exercise science research labs may utilize technology like dual X-ray absorptiometery (DXA), which uses X-ray technology to better estimate lean mass and fat tissue, but it is expensive and requires special training to administer.

Another reliable — and more accessible — option is a bioelectric impedance analysis (BIA) device, like the ones used at Orangetheory. The device sends several low-level electrical currents throughout the body, which are affected by the amount of water in the body. The machine then uses that rate to estimate lean mass and fat mass.

Events like Transformation Challenge and Back At Challenge are great opportunities to see how your body composition changes when committing to a fitness program for an extended period of time. “We estimate that members may realize somewhere around a 1 to 3% reduction in body fat over a 8-to-12-week period,” says Plessel. "And having realistic expectations is key, as slow and steady improvements are much more sustainable over the long-term," she adds. 

Keep in mind that getting hung up on a single metric can be counterproductive. Consider all the factors that go into your health and fitness goals: Are you running at a faster pace than you were a few weeks ago? Are you able to lift more weight at the studio? Answering questions like these, along with measurements like body composition, will give you a better picture of your health, and whether or not it’s shifting in the right direction.