Comparison · Body
DEXA, bioimpedance, and the bathroom scale
Three readings of the same body can disagree by ten pounds of fat. What each method actually sees, and when the cheap one is enough.
Three instruments can describe the same body and disagree. The disagreement is not mystery; each one measures something different and infers the rest.
A scale measures mass. A bioimpedance device measures how easily a current crosses you, which mostly reflects water, and converts that to fat with population equations. DEXA measures fat, lean tissue, and bone directly from how each attenuates X-rays.
In research, multi-compartment laboratory models are the criterion, and DEXA is the accepted practical reference standard against them: group averages typically agree within one to two percentage points of body fat. Bioimpedance also looks good on averages. The difference appears at the level of one person.
Agreement studies put the error band for an individual bioimpedance reading at several percentage points of body fat, around plus or minus five points for good hand-to-foot devices and wider for foot-to-foot scales, which only pass current through the lower body and extrapolate the rest. Because the measurement rides on water, a salty dinner, a hard workout, or a different time of day shifts it. A study of three consumer smart scales against DEXA found weight accurate to 0.3 kg while fat mass missed by median errors of roughly two to four kilograms, with systematic underestimation; the authors concluded the scales should not substitute for DEXA on composition.
Weight also fails silently in the one scenario people train for. Gaining muscle while losing fat leaves mass unchanged, so recomposition is invisible to a scale and to BMI, while a DEXA scan resolves it directly with roughly 1% precision on lean mass. Two people at identical weight can differ by entire risk categories in how that weight is distributed.
The practical split is simple: weigh yourself at home under consistent conditions for direction, and scan a few times a year for position. The scale tells you something moved; the scan tells you what.
- 01Frija-Masson J, et al. Accuracy of smart scales on weight and body composition: observational study. JMIR mHealth and uHealth, 2021.
- 02Comparison of multifrequency bioelectrical impedance analysis with DXA for assessment of percentage body fat in a large, healthy population. American Journal of Clinical Nutrition.
- 03Nickerson BS, et al. Validity of DXA body volume equations in a four-compartment model. PLoS ONE, 2018.
- 04Smith-Ryan AE, et al. Validity and reliability of a 4-compartment body composition model using DXA-derived body volume. Clinical Nutrition, 2017.
