What the scale is estimating
Many consumer composition scales use bioelectrical impedance: a small electrical signal, measured resistance, personal inputs, and a proprietary equation produce estimates such as body-fat percentage or fat-free mass. The display is precise to a decimal; the underlying inference is not equally exact.
Hydration can move the estimate
In a 2023 study of 39 participants, drinking two liters of water changed body-composition results from both single- and multi-frequency bioelectrical-impedance devices. The direction and size depended on the mode and group, illustrating why an estimate can respond to measurement conditions rather than a sudden tissue change.
Consistency is not validation
Similar timing, device, placement, and conditions can make a personal series easier to compare. That may improve trend usefulness, but it does not prove agreement with a clinical reference method or make two different scale models interchangeable.
Keep source context without resetting the story
A device replacement or firmware algorithm change can create a visible discontinuity. Retain the source label and treat the jump cautiously, but do not assume it proves an equipment or biological change. Trendline preserves the earlier history and does not create a new waiting period merely because the source changed.
How Trendline talks about it
Trendline labels composition values as estimates, avoids exact tissue attribution, and can use cautious language when the evidence does not support a stronger conclusion. The full boundaries live in the methodology — including what composition estimates can and cannot support. Using estimates during a deliberate gaining phase? See Build Lean.