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With photos taken on your cellphone, Amazon's Halo wearable fitness device can accurately calculate your body fat percentage.

All sorts of electronic devices claim to assess your fitness. Do any of them actually work?

Pennington Biomedical Research Center is giving one of them a thumbs up.

The body composition feature of the Amazon Halo, a wearable fitness device, is able to calculate a person’s body fat percentage nearly as accurately as the best medical technology, according to a new Pennington study. That’s significant because body fat percentage is a more accurate measure of fitness than the widely used body mass index, a calculation based on height and weight.

“Physicians use BMI because it’s fast and costs next to nothing,” said Dr. Steven Heymsfield, professor and director of Pennington’s Metabolism and Body Composition Laboratory and co-primary investigator of the study. “The drawback to BMI is that it can’t distinguish between fat and muscle. Our study shows that the 'Body' feature offers consumers an easy, accurate way to measure body composition.”

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BMI’s drawbacks are no secret. When he was a researcher at Columbia University in the early 2000s, Heymsfield was asked to study 5-foot-10, 245-pound Ron Dayne, a New York Giants running back. Although his BMI indicated he was severely obese, Dayne had very little body fat. Conversely, senior adults who have lost muscle mass score better on BMI tests than they should.

But body fat percentage isn’t easy to measure precisely.

Bioimpedance analysis measures body composition based on how fast an electrical current travels through the body. Professional BIA devices are reasonably accurate, but the study found that scales sold to consumers that offer this analysis are less so, Heymsfield said. Other more accurate methods aren’t readily available to the public.

When Amazon approached Pennington about studying the Halo, Heymsfield was intrigued. Halo’s body composition function requires users to take cellphone photos of themselves from the front, back and both sides and Amazon uses the images to calculate the body’s percentage of fat.

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“We have comparable devices in our lab that cost up to $20,000,” Heymsfield said. “They’re called optical imaging devices, and I didn’t really believe that you could use a cellphone and take images of somebody and generate images that gave an accuracy of body fat that approached our gold standard methods. I have to say I have been surprised at how far along this technology has come.”

The Halo costs $99.99. The body composition information is among a group of features that are part of a membership that costs $3.99 a month after a free trial period.

The study evaluated 134 participants using the Amazon device, consumer and professional smart scales, air displacement plethysmography (also known as a “bod pod”) and dual-energy X-ray absorptiometry (DXA), Pennington’s most sophisticated device to determine body fat percentage. Only the DXA was more accurate than the Halo body composition function in the study.

So if you can fairly accurately learn your body fat percentage, how does that help you? There’s no one-percentage-fits-all number.

According to the American Council on Exercise, essential fat is 10% to 13% for women, 2% to 5% for men. It lists ranges for athletes (14% to 20% for women, 6% to 13% for men), fitness (21% to 24% for women, 14% to 17% for men), acceptable (25% to 31% for women, 18% to 24% for men) and obesity (over 32% for women, over 25% for men).

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Heymsfield expects fitness technology to continue to become more accurate and useful.

“These devices, including the Halo, can probably measure many more things — for example, your waist circumference or your chest circumference — with pretty good accuracy,” he said. “They haven’t put those as outputs, yet. Waist circumference is a very important metric undermeasured in doctors’ offices but widely recognized in my field of obesity. A large waist is a major risk factor independent of your BMI.

“I think these devices as they come down the line are going to generate more health information.”

Email George Morris at gmorris@theadvocate.com.