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 higher 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.