Determining a state of water deficiency in a human body during daily life activities is done by measuring an impedance of a portion of the human body at a low frequency and at a high frequency. An amount of fluid in the body tissues in the volume under study is assessed based on the impedance at the high frequency. A current amount of extracellular fluid in the tissues in the volume is assessed based on the impedance at the low frequency. A reference value of the assessed amount of extracellular fluid in the tissues is preselected at the start of measurements. A correction value is determined based on the changes in blood amount in the volume at the current measurement time. A corrected assessment value is the determined. Using the corrected assessment value, the onset of water deficiency is determined.
