Bioelectrical impedance analysis: measurement of body composition via electrical conductivity in the body
Bioimpedance analysis (BIA) measures body composition by passing a weak electrical signal through the body. Because muscles and water conduct current differently from fat, the device calculates an approximate ratio of fat, lean mass and water.
The method is quick, inexpensive and non-invasive, so it is widely used both in sports clubs and in home scales. However, it is very sensitive to the body's water content, so the result can fluctuate even within the same day.
The result is an approximate estimate, so evaluate it with caution and according to age- and sex-specific tables. Because of its strong dependence on hydration, a single measurement should not be considered accurate.
The most reliable approach is to monitor the trend: measure with the same device, at a similar time of day and under the same conditions. Long-term change says much more than a single number, which can jump due to water fluctuations.
The result is misleading after intense sweating, alcohol or a heavy meal, and also for women during the cycle when fluid levels change. Compare accuracy with other methods (Bod Pod, DEXA) or read body fat calculator's explanations.
Are home scales with BIA accurate? They are suitable for tracking trends, but absolute values can have a large error, so it is important to measure consistently.
Why does the result differ from day to day? The body's water content fluctuates due to diet, sleep, training and other factors, so daily changes are not a change in fat.
How to improve accuracy? Measure in the morning fasting, with the same device and under similar conditions.
Can I measure every day? Yes, but daily results fluctuate greatly, so it is better to track the weekly trend rather than individual numbers.
What should I do if the results do not match other methods? This is normal — different methods measure differently, so stick to one method consistently.
If you measure to monitor changes in body composition, do not worry about fluctuations on individual days — look at the trend over several weeks, which more reliably shows the true direction.
To reduce error, avoid intense exertion and a heavy meal before measurement, and take the measurement in the morning before breakfast — this will help obtain more stable and comparable results.