Intracellular Water (ICW)

• Derived from measured impedance. The manufacturer presents intracellular water as a measured quantity rather than an estimate.
• Separating intracellular from extracellular water is possible thanks to multi-frequency measurement: low frequencies do not cross the cell membrane, high ones do. The difference between their responses carries the information about the water inside cells.
• According to the catalogue, InBody 970 measures at 1, 5, 50, 250 and 500 kHz and 1, 2 and 3 MHz across five segments, that is 40 impedance measurements in total.
• The manual does not publish the equation converting impedance into litres of intracellular water.
• The reference range in brackets is a comparison against a reference; the manual does not publish how it is calculated either.

• The manual states that intracellular water makes up roughly 62 percent of total body water in a healthy person. The remaining roughly 38 percent is extracellular water, which corresponds to a ratio of about 3 : 2.
• Read the absolute litres against the reference range in brackets, not against another person value. A taller and more muscular person naturally holds more intracellular water.
• More important than the value itself is its relationship to extracellular water. On the result sheet that is expressed by the ECW/TBW ratio, with a reference range of 0.360 to 0.390.
• Intracellular water on its own does not establish whether cells are in good shape. It describes quantity, not quality.

• A rise in intracellular water together with a rise in muscle and fat free mass is the typical picture during sustained strength training. The manual links a rise in intracellular water directly to a rise in body cell mass.
• When intracellular water falls while extracellular water holds, the ECW/TBW ratio rises. That is a different situation from both compartments falling together, which usually reflects an overall fluid loss.
• Short-term differences of a few tenths of a litre between two measurements sit within ordinary variation, especially when the time of day or fluid intake differed.
• For following muscle development the meaningful horizon is months, not days. Water itself responds fast; cell mass responds slowly.

General principles: how to read change between measurements →

• Extracellular water and the ECW/TBW ratio – only the relationship between the two compartments says whether distribution is balanced.
• Body Cell Mass (BCM) – the manual defines it as intracellular water plus protein, so the two move together.
• Protein – the other component of body cell mass.
• Skeletal Muscle Mass – muscle is the main tissue holding intracellular water.
• Phase angle – adds information about the electrical properties of cell membranes to information about their quantity.

• Intracellular water is not a measure of how well your muscles are hydrated after a workout. It describes volume, not the acute state after exertion.
• A higher value is not automatically better. It should always be interpreted against total body water and against the extracellular compartment.
• Intracellular water cannot be compared between people. It depends on body size and the amount of muscle.
• The value does not establish cell health or metabolic activity. It is a volume figure.

• Fluid and food intake before the measurement.
• Training shortly beforehand, which changes perfusion and fluid distribution.
• Sauna and hot or cold showers, which change circulation according to the manual.
• Time of day and body position before the measurement — the manual recommends standing for at least 5 minutes.
• The quality of contact with the electrodes. The manual recommends wiping hands and feet, because dry skin worsens electrical contact.
• The menstrual cycle, during which body water fluctuates.

• Always read intracellular water together with extracellular water and the ECW/TBW ratio. On its own it carries limited information.
• If you are following muscle development, treat intracellular water as a companion indicator to muscle mass and body cell mass.
• Measure under comparable conditions. The difference between a fasted morning measurement and an afternoon one after training can exceed the real change over a month.
• Do not try to influence the value by drinking before the measurement. The only thing you gain is worse comparability with the previous result.

• It does not establish the quality or function of cells, only the volume of water inside them.
• It does not diagnose any medical condition.
• The equation converting impedance into litres is not published by the manufacturer.
• Values are not interchangeable with results from other methods or other devices.