What’s the Difference Between BMI and Body Composition?

Our experts explain.
What’s the Difference Between BMI and Body Composition?

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. Body Mass Index (BMI) is a commonly used metric, but it doesn’t provide a complete overview of your health.

What is BMI?

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.

Brittany Laboeuf, Ph.D., a research scientist at Orangetheory, emphasizes that BMI fails to measure fat mass accurately. Even someone with a ‘normal’ BMI might have a high percentage of fat around their organs, posing health concerns.

Kimberly Plessel, M.S., RDN, LD, a member of Orangetheory’s Medical Advisory Board, acknowledges that BMI serves a purpose in tracking population weight trends. However, she advises using it only as an initial screening tool for individuals. Laboeuf adds that while BMI categorizes people based on population averages, it should not be the sole diagnostic tool.

What is Body Composition?

Body Composition is the assessment of the ratio between fat-free mass (comprising bones, water and muscle) and fat mass. This measurement offers a more insightful perspective than simply considering overall weight. “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?”

A high percentage of body fat, especially concentrated in the abdominal area, increases the risk of metabolic syndrome, raising the chances of heart attack and stroke.

How to Measure Body Composition

Methods to determine body composition vary in accuracy and accessibility. Gyms often use skin-fold measurements, while advanced techniques include dual X-ray absorptiometry (DXA) and bioelectric impedance analysis (BIA).

Bioelectric impedance analysis (BIA), like the InBody devices used at Orangetheory, sends low-level electrical currents through the body, estimating lean and fat mass based on water content.

Track Your Body Composition with Orangetheory

An event like Orangetheory’s Transformation Challenge is a great opportunity 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 journey, and whether you’re progressing in the right direction.


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