Step aside BMI, body composition tests are on the rise. Here's what to know

The scale has never been a friend to Mana Mostatabi. Even back in high school, when she ran a quick 100m on the varsity track team, her BMI – a ratio of weight to height – put her in the overweight category.

“My dad always joked that I should be a wrestler,” Mostatabi says due to her build. Many professional athletes flunk BMI tests. Some are considered obese despite their fitness, and many doctors say it isn’t a helpful metric to focus on.

“BMI is a very crude measure,” says Dr. Richard Joseph, a physician at Brigham and Women’s Hospital who specializes in metabolic health. “It doesn’t tell me much about your underlying health,” he says. People can be a normal weight but have low muscle mass and high body fat, while others have higher body weight but are muscular and lean.

That’s why Mostatabi has found a new tool – a body composition scan – that measures her body fat and muscle mass, which are two key metrics of health. “It’s very affirming,” Mostatabi says. Over the last year, she has lost ten pounds of body fat and also gained several pounds of muscle. “This actually gives me information,” to track progress. “It really is motivating,” she says.

Body composition scans are becoming an increasingly popular way to gauge health and there are lots of different kinds.

An MRI (magnetic resonance imaging) is considered the gold standard, but it’s not practical for most people given the expense and access to medical imaging.

Dr. Joseph orders DEXA – dual-energy X-ray absorptiometry – scans for some of his patients. These scans measure bone mineral density, and also measure body composition and fat distribution. They typically cost more than $100, but prices have begun to drop in some areas as demand rises and more machines are available.

An option that’s taken off in gyms and workout studios, such as Anytime Fitness and Orangetheory Fitness is bioelectric impedance analysis, using devices such as the InBody or the Evolt 360. Depending on the studio, the scans are often free with membership or are available for a small fee. This test is not as precise as an MRI or DEXA, but is reliable at tracking changes over time, as long as people follow directions.

Mostatabi had her first InBody scan in January, when she signed up for a strength challenge at Orangetheory. “It’s a super simple process,” Mostatabi says, which takes less than a minute.

The device looks like a scale with two arms. “You step onto the machine,” which has a metal base, she explains and you hold onto the ends of the two arms which send a safe, low-level electrical current through the body, estimating fat and lean mass.

“How fast that current is returning to the electrodes gives a measurement of how much fat mass and muscle you have, because the current travels through those body tissues at different speeds,” explains Scott Brown, Vice President of fitness at Orangetheory. The technology is decades old, and has become increasingly popular with the advent of commercial devices and rising demand.

Mostatabi says you can’t feel anything during the test, and the results are sent directly to a smart phone app. Her first scan gave her a benchmark to improve upon.

She explains her fitness took a nosedive during the pandemic, and she was aiming for a fresh start this year. All winter she pushed herself through 60 minute classes that combined resistance training, weight lifting, cardio on a treadmill and rowing. “I was very diligent,” averaging about five to six classes per week.

“The InBody scan is the first tool I’ve ever used that shows gains,” she says. Mostatabi was accustomed to weighing herself, and recalls the frustration when the scale would not budge. Now, even if she hasn’t lost weight, she knows her body is healthier, with a lower percentage of body fat and an increase in muscle.

“It’s just incredibly empowering,” she says, knowing that women who weight train and build strength can expect to live longer and healthier lives.

On average, women gain less muscle at a slower rate compared to men. During Orangetheory’s eight week ‘transformation challenge’ the company found that males who were focused on muscle gains added about .83 lbs of muscle over eight weeks, on average, compared to a half pound of muscle gain for females. Though across all participants there was only about .1 lb of average muscle gain.

Eight weeks is not a lot of time to gain muscle, explains Brown. And he says it’s important to set “realistic goals and targets'' given the variability from person to person. The ability to build new muscle is influenced by gender, age and genetics.

Dr. Joseph says the reason it can be helpful to know your muscle mass is because studies show that strength is a predictor of longevity. Also, loss of muscle increases the risk of falling, which is a top cause of death from injury among older people. “A lot of people are under-muscled,” Joseph says.

When it comes to body fat, having too much can increase the risk of metabolic disease, especially visceral fat, which surrounds the abdominal organs including the stomach, liver and intestines. “It’s inflammatory,” Joseph says and drives up the risk of heart disease. The American College of Sport Medicine sets fitness categories for body fat based on age and gender, but there isn’t not an agreed upon target for what’s considered ideal. Using the ACSM standards, most Americans could be classified in the “poor fitness” category, as the average body fat among adults in the U.S. is 33%. The U.S. military considers the optimal body fat for military fitness to be between 10% and 20% for young men and up to 25% for middle-aged men. Women typically have more body fat, with an ideal range from 15% to 30% for young women and up to 38% for middle-aged women.

Joseph says the rule of thumb for fitness is that “it’s you versus you.” Rather than fixate on an external benchmark, “it’s most important to look at trends over time,” in your body composition, he says.

It’s possible to lose fat and gain muscle, without losing any weight. This is what happened to Karen White, who is 59, and a certified health coach in Alexandria, Virginia. She’s gained about three pounds of muscle over the last three years, and has shed body fat,too. Her body fat has dropped from 26% down to 22%. “Literally, I’m the exact same weight,” after three years of tracking, but the positive changes in her body composition are profound.

She lifts weights three times a week, for about 30 minutes and has progressively built up to lifting heavier weights. She still does cardio work-outs and stays active with daily walks with her dog, though she has shifted her focus to resistance training.

“The misperception is that older people can't gain muscle, and that's absolutely not true,” she says. She points to a new client she’s working with in her 60s. Already, after a few months, her client has lost body fat and increased her strength.

White agrees that it’s important to set realistic expectations, and recognize the changes in body composition may take time. She has gained about a pound of muscle per year, on average, and feels a lot stronger.

Given that muscle peaks in our 30s, it’s important to do strength-training to maintain muscle mass, especially as we age. “The risk of frailty really increases exponentially with age,” Dr. Joseph says and muscle-loss, also called sarcopenia, affects an estimated 45% of older adults, especially women. Weight training can help fend off this loss.

Find Allison Aubrey on Instagram at @allison.aubrey and on X @AubreyNPR.

This story was edited by Jane Greenhalgh