Bioelectrical Impedance Analysis (BIA)

Bioelectrical impedance analysis (BIA) estimates body composition from the body's resistance to a small, safe alternating current. Lean tissue (high water/electrolyte content) conducts well (low impedance); fat conducts poorly (high impedance). Devices range from handheld and foot-to-foot scales to multi-frequency segmental analyzers (e.g., InBody) that are more accurate.

Accuracy & error sources: convenient and non-invasive but highly sensitive to hydration status — eating, drinking, exercising, alcohol, caffeine, menstrual-cycle fluid shifts, and skin temperature all change the reading. A single glass of water can move the result. Consumer scales are typically less accurate than skinfolds; research-grade segmental devices are better but still trail DEXA/hydrostatic.

Standardize for valid tracking: test at the same time of day (ideally morning, fasted), euhydrated, no exercise/alcohol/caffeine in the prior hours, empty bladder, same device. Under those conditions, BIA tracks trends reasonably even if the absolute %fat is imperfect.

• Estimates %fat from impedance: lean tissue conducts, fat resists.
• Forms: handheld, foot-to-foot scales, multi-frequency segmental analyzers (more accurate).
• Very hydration-sensitive — food, fluids, exercise, alcohol, caffeine, cycle all bias it.
• Consumer scales generally less accurate than skinfolds; standardize conditions for trends.

• skinfold-assessment — alternative field method, often more accurate.
• dexa / hydrostatic-bodpod — higher-accuracy references.
• body-composition-overview — method comparison.
• dehydration-performance — why hydration swings BIA.