Bioelectrical Impedance Analysis

Bioelectrical impedance analysis (BIA) is a widely used body composition method that is easy, non-invasive, relatively inexpensive and portable. ^{[1] [2]} BIA allows the determination of fat-free mass and total body water in subjects without significant fluid and electrolyte abnormalities, provided appropriate population, age or pathology-specific equations and established procedures are used. ^[1] It is one of several body composition techniques, alongside anthropometry, tracer dilution, densitometry, dual-energy X-ray absorptiometry and air displacement plethysmography, which vary in their precision and accuracy. ^[3] Body composition assessment matters because weight and BMI do not reveal the relative contributions of fat mass and fat-free mass, while ageing brings a natural increase in fat mass and a gradual decline in lean mass, specifically bone and muscle mass. ^[4] Despite decades of widespread use, the accuracy and clinical value of BIA are still questioned. ^[5]

At low frequencies the current flows primarily through extracellular fluids, whereas at high frequencies it completely penetrates all body tissues. ^[6] Measurements at multiple frequencies can differentiate total and extracellular fluid compartments, while a single frequency captures only a small window of the impedance spectrum, which may explain its difficulty in discriminating among individuals. ^[6] Phase angle, a raw BIA parameter, is thought to be a proxy of water distribution and body cell mass and is associated with muscle strength. ^[7] Raw parameters such as phase angle and bioelectrical impedance vector analysis (BIVA), which plots resistance and reactance normalized per height, have gained attention as alternatives to the error-prone calculation of body composition in disease. ^[8] BIVA provides information about hydration and body cell mass and therefore allows assessment of patients in whom calculation of body composition fails because of altered hydration. ^[8] Estimating changes in body cell mass and in extracellular and intracellular water still requires further research using a model that ensures extracellular water changes do not corrupt the intracellular estimate. ^[1]

ESPEN guidelines state that BIA works well in healthy subjects and in patients with stable water and electrolyte balance when a validated equation appropriate for age, sex and race is used. ^[2] The same guidelines do not recommend BIA for routine assessment of patients at extremes of BMI or with abnormal hydration until its algorithms are validated in these conditions, although multi-frequency and segmental BIA may have advantages there. ^[2] Longitudinal follow-up of body composition by BIA is considered possible in subjects with a BMI of 16 to 34 kg/m2 without abnormal hydration, but must be interpreted with caution. ^[2] Criticisms of BIA focus on poor absolute accuracy and on different analysers or prediction equations failing to measure body composition identically, although a perspective review argues that its errors are not dissimilar to those of so-called gold standard methods. ^[5] BIA and DXA estimates are not interchangeable: in a meta-analysis of volleyball players, pooled body fat was 18.4% via BIA and 24.2% via DXA, although the method differences were not robust in sensitivity analysis. ^[9] In an Asian meta-analysis using the 2019 AWGS criteria, sarcopenia prevalence ranged from 7.5% with assessments based on bioelectrical impedance analysis to 20.8% with