Samsung’s Galaxy Watch 4 calculates body fat from the wrist - The Verge

Samsung’s new Galaxy Watch 4 has a feature that can calculate body fat percentage and muscle mass, the company announced yesterday. It’s the latest tech company to offer a body composition feature, joining Amazon, which has a body fat percentage feature in its Halo Band.

Body fat is generally a better way to assess health than weight, but it’s often calculated using a metric called the body mass index (BMI), which is crude and inaccurate. If wearables prove accurate, they could give people a better resource to monitor their health, Diana Thomas, a mathematician who studies body weight regulation at West Point, told The Verge in June.

The Galaxy measures body composition using a technique called bioelectrical impedance analysis (BIA), which sends a weak electric current through the body. It’s calculating the amount of water in the body — the signal moves more quickly through tissue that has higher percentages of water. Since fat has a lower water content than muscle, the technique can estimate how much fat is present in a person’s body.

BIA can give an estimate of body fat, but it has limitations, said Thomas in an email to The Verge. There isn’t a perfect one-to-one ratio between the amount of water in the body and body fat, and the amount of water in the body changes over time, she said. It can go up and down with exercise, for example, or if someone is drinking a lot of water. One study found that BIA overestimated body fat if the measurement was taken right after someone drank around two cups of water.

Most available devices that use BIA to track body composition send the current through the soles of the feet (like a smart scale) or through the palms. Samsung shrunk the sensors down to fit on the back of the watch. The company’s Healthcare Sensor Lab published a study in January 2021 describing the technology. In the study, 203 people had their body composition analyzed by the watch, two other BIA devices, and a device that uses dual-energy X-ray absorptiometry (DXA) — an X-ray technique that shows fat distribution through the body, which is considered the gold standard for body composition measurement.

The smartwatch was accurate compared with the DXA measurement, the analysis found. Its calculations were also slightly closer to the DXA measurement than one of the two other BIA devices. Overall, the watch appears to work as well as a smart scale based on this study, Thomas said. She still wouldn’t use it in research studies based on the information available so far — the potential error rate as calculated in the study is too large for that. “For a personal touch point, it may be okay,” she said in the email to The Verge.

Samsung’s approach is different from the body composition analysis feature in Amazon’s Halo. First, the Halo only calculates body fat — not muscle. Its app directs users to take photos of their body, which it combines into a 3D image used to calculate body fat percentage. The system also gets an accurate measure of body fat compared with the DXA, according to an Amazon-funded study.

At the very least, both apps appear to work better than BMI at calculating body fat percentage. BMI is frequently used to guide medical decision-making, even though it’s often misleading — which can skew the diagnosis of some conditions and perpetuate weight-based stigma in healthcare. It’s used, though, because it’s cheap and easy. Most people don’t have access to the more accurate, expensive machines.

It’ll take more research to figure out how reliable the wearables can be in a real-world setting. But they’re easy to use and relatively cheap. If they work well enough to flag trends in people’s body composition that could affect their health, they could make these health metrics more accessible.

Correction August 12th, 6:06PM ET: A previous version of this article incorrectly stated that fat has higher water content than muscle. It has a lower water content. We regret the error.

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