Smart scales miss mark on fat, muscle mass measurements

Smart scales are not accurate for body composition measurements, consistently underestimating body fat mass and overestimating muscle mass, according to a recent study. For patient care, smart scale devices cannot replace the dual x-ray absorptiometry (DEXA) gold standard.

“To our knowledge, this is the first study to assess metrologic accuracy of commercially available smart scales,” the researchers said. “We show that all scales were reasonably accurate for body weight but not body composition.”

Three smart scales, each of different brands and models, were assessed in the current cross-sectional study. Consenting patients were weighed with one device right after undergoing DEXA, and measurements were compared between methods. Absolute errors relative to DEXA were used to express accuracy in terms of body mass, fat mass, and lean mass.

Smart scales, 1, 2, and 3 were able to collect measurements for 53, 52, and 48 patients, respectively. All three devices returned highly accurate weight measurements, deviating from DEXA by less than 1 kg. For instance, the mean absolute body weight errors were 0.3, 0, and 0.25 kg for scales 1, 2, and 3, respectively. [JMIR Mhealth Uhealth 2021;9:e22487]

In contrast, body composition measurements were less reliable. All three smart devices consistently underestimated fat mass, with absolute errors of –2.2, –4.4, and –3.7 kg for scales 1, 2, and 3, respectively, relative to DEXA.

Muscular mass, on the other hand, was more commonly overestimated. Scales 1 and 3, for instance, showed absolute errors of 4.50 and 4.0 kg, respectively, as compared with DEXA, while scale 2 underestimated muscle by an average of 6.6 kg.

“Smart scales combine a classic weight scale and a foot-to-foot impedance metre [FFI],” which tends to be more sensitive to morphological differences than body impedance, the researchers explained. “Since there is no precise guidance on the scales for subjects on where to put their feet during body composition estimation, this may partly explain the differences between DEXA and scales.”

Similarly, a person’s foot length and width may also lead to variance in measurements, suggesting that their height and weight may also play a role.

In subsequent analyses, the researchers did show that for scales 1 and 2, weight was significantly associated with fat mass measurement errors (p=0.03 and p<0.001, respectively), as was body mass index (p=0.034 and p<0.001, respectively).

“Since smart scales are widely available, it is possible that patients will follow their body composition at home,” the researchers said, especially since DEXA cannot be used to track body composition. “At home, subjects should try to follow directions given by the scale as closely as possible and keep the same position on the scale for follow-up.”

“In clinical settings, clinicians or technicians should pay attention to the subject’s position during measurement,” they added, further noting that they should be aware that there may be discrepancies in body composition measurements that could reach up to 1 kg relative to the initial DEXA. Moreover, follow-up measurements should be obtained using the same smart device to ensure consistency.