Influence of acute water ingestion and prolonged standing on raw bioimpedance and subsequent body fluid and composition estimates

Bioimpedance technology has been widely used to monitor a variety of biological components, including vascular function, hydration status and fluid accumulation, disease prognosis, wound healing, and body composition [1]. While physiological applications vary, bioimpedance technologies rely on core biophysical principles, such as the application of Ohm’s Law to describe the relationship between voltage changes, the flow of electrical current, and electrical impedance (Z) [1]. Z is composed of two primary components, resistance (R) and reactance (Xc). While R quantifies the opposition to current flow, Xc represents the capacitive properties of the cells and tissues [2]. R and Xc can also serve as input values for the calculation of phase angle (ϕ), a well-established clinical parameter. ϕ is frequently presented as a noninvasive metric of cellular health, membrane integrity, and the quantity and quality of soft tissue. Furthermore, numerous investigations have reported the potential utility of ϕ as a prognostic indicator of nutritional status, physiological function, and mortality [3, 4]. Despite a resurgent interest in the utilization of raw bioelectrical parameters, including ϕ [1, 5], the estimation of body fluids and body composition remain two frequent applications of bioimpedance technology [6, 7].

As with many methods of body composition assessment, pre-assessment standardization is critical for reducing biological error and improving the utility of output data [2, 8, 9]. Abstention from eating, exercise, moderate or vigorous physical activity, and medication or substance ingestion are common components of pre-assessment standardization. Several investigations have indicated that transient alterations of bioimpedance-derived body composition estimates are observed in response to acute food and fluid intake [10, 11, 12, 13]. Importantly, the magnitudes of these errors may be large enough to obfuscate true changes in body composition over time [14, 15]. Although it is generally believed that bioimpedance technologies may be particularly susceptible to errors caused by transitory alterations in hydration, many investigations utilizing bioimpedance technology either do not report a mandatory abstention from fluid ingestion prior to assessment or utilize durations of fluid abstention shorter than an overnight period. Limited direct information is available to inform the choice of fluid restriction or allowance prior to bioimpedance assessments, specifically whether it is preferable to implement an overnight dry fast (i.e., no foods or fluids ingested for ≥8 hours) as compared to an overnight fast that allows either ad libitum or prescribed water intake (i.e., water fasting). While preliminary information is available for select single-frequency bioelectrical impedance analysis (BIA) analyzers [16, 17], most contemporary devices remain unexamined.

In addition to questions regarding standardization of fluid ingestion in the hours preceding bioimpedance assessments, other methodological questions remain. The influence of body posture is particularly relevant due to current utilization of both traditional analyzers requiring a supine body position and newer vertical analyzers requiring an upright (i.e., standing) body position. Through radionuclide dilution techniques, it has been established that plasma volume decreases when standing relative to seated and supine body positions, largely due to fluid movement into the interstitial space [18]. While several reports have clarified the time course of bioelectrical responses to postural changes, these investigations have primarily focused on transitioning from standing to the supine position due to the traditional implementation of supine bioimpedance assessments [19, 20, 21, 22]. To our knowledge, investigations reporting serial bioimpedance assessments have not employed standing durations longer than 30 minutes nor used modern vertical bioimpedance analyzers [20, 22, 23, 24].

Based on these important methodological considerations, the purpose of the present investigation was to determine the effects of acute water ingestion on changes in raw bioelectrical variables and subsequent estimates of fluid compartments and body composition variables. Additionally, the present design allowed for examination of changes in these variables in response to maintaining the upright body position for a longer duration than previous investigations.

The present investigation determined the effects of acute water ingestion on raw bioelectrical variables and subsequent estimates of body fluids and composition from a modern vertical bioimpedance analyzer, along with clarifying the effects of remaining in an upright body position for ~65 minutes. The broad findings were: 1) acute water ingestion increased Z and Xc of the arms, particularly at measurement frequencies ≤50 kHz, without clearly influencing the legs or trunk; 2) change over time or “drift” in numerous variables, irrespective of water ingestion and presumably due to maintaining an upright posture, was observed; these variables included leg Z and Xc, body fluids (TBW and ICW) and LBM, among other outcomes; 3) the increase in BM after water ingestion was detected exclusively as FM rather than body fluid or LBM; FM also drifted higher throughout the measurement period. While not fully discussed within the manuscript text, the supplemental materials present additional detail regarding the effects of water ingestion and time on a variety of raw bioimpedance and body composition variables estimated by the bioimpedance analyzer.

Bioimpedance is used to describe a wide variety of biological components, track physiological adaptations to lifestyle interventions, and inform evaluation of health and disease risk [1, 39, 40]. Although prior investigations have indicated a detrimental influence of unstandardized participant presentation, including the intake of food and fluids [10, 11, 12, 13, 14, 41], relatively few have examined water intake alone. However, the influence of water ingestion prior to assessments is potentially relevant as many investigations implement overnight fasts without restrictions on water intake or even recommend water intake in the hours prior to BIA assessments to promote euhydration.

Dixon et al. [16, 17] performed two experiments to determine the influence of fluid consumption on BIA variables. It was first demonstrated that foot-to-foot single-frequency BIA (Tanita TBF-300A) did not detect changes in Z or TBW in the 60 minutes following ingestion of 591 mL of water or a carbohydrate/electrolyte drink, although increases in BM and %BF estimates were observed [16]. Subsequently, a similar experiment was performed using an 8-point, hand-to-foot single-frequency analyzer (Tanita BC-418) [17]. Following ingestion of 591 mL of water, a 12-Ω increase in Z was observed, along with an increase in %BF of ~1%. However, the control condition with no fluid ingestion demonstrated a similar 11-Ω increase in Z and slight elevation of %BF of ~0.5%. In both investigations, participants sat between baseline and subsequent BIA assessments. It is worth noting that both BIA analyzers used in the studies of Dixon et al. have since been discontinued.

In the present investigation, which employed a modern, commercially available multi-frequency analyzer (InBody 770) and required participants to stand for the entire visit, the elevation of FM variables could have been driven both by the clear increase in BM detected by the scale component of the BIA analyzer and the differences in raw bioelectrical variables relative to the control condition. On average, FM was ~1.3 kg higher than baseline at the final assessment in the water condition, with ~0.9 kg of this increase occurring immediately after water ingestion. Meaningful upward drift in FM also occurred in the control condition, with final values ~0.7 kg higher than baseline at the final time point. While the magnitude of these errors is likely unimportant for one-time assessments, such as those used in epidemiological investigations and other cross-sectional settings, the implications of the artificial change in outcome variables may be greater when performing serial assessments over time. In this scenario, the magnitude of error could be non-negligible and either inflate or deflate the estimation of true changes depending on whether this error was present for the baseline evaluation, subsequent assessment, or both [15]. It should also be noted that these biologically induced errors, due to water intake and body fluid shifts, are notably larger than the TEM values of the MFBIA analyzer. Importantly, the present results for fluid volumes and body composition variables derived from raw bioimpedance may differ from effects that would be observed with other analyzers due to the device-specific manner in which these estimates are produced.

The clear drift in many variables over the course of 65 minutes of standing was presumably due to progressive fluid redistribution. The occurrence of dynamic fluid shifts upon changing body position has been established independent of bioimpedance techniques. Maw et al. [18] performed an informative investigation of postural changes on fluid volumes using simultaneous radionuclide dilution. While TBW remained constant throughout supine, seated, and standing postures, employed for 30 minutes each, blood volume was increased by 89 mL on average when supine and decreased by 406 mL (~6%) on average when standing, both relative to the seated position. In the standing position, the majority of volume shift occurred within the first 15 minutes and was attributable to an increase in interstitial fluid rather than expansion of ICW. These findings are consistent with other descriptions of the hemodynamic responses to the upright posture, which include a rapid redistribution of thoracic blood volume upon standing and a slower decline in plasma volume due to gravity-induced transcapillary diffusion [42].

In the present investigation, the general trends of increasing Z of the arms and decreasing Z of the legs, with little variation in the trunk, are consistent with the biophysical relationship between these variables and body fluids, as well as the expectation of fluid redistribution in the upright posture. While not definitively demonstrated, the directionality of changes in segmental fluid compartments also generally supports the redistribution of fluids from the upper to lower appendages with prolonged standing. From a practical standpoint, while apparent drift in some variables was observed throughout the measurement duration even without water ingestion, an ~15-minute period of upright rest prior to BIA assessments using vertical analyzers may help reduce unwanted variation in bioimpedance and variables derived from these values. For example, the greatest change in whole body ϕ – arguably the most used raw bioimpedance metric – was observed in first 15 minutes upright rest in the control condition.

Depending on the variable, the drift over time either accentuated or combatted the influence of water ingestion. For example, whole body ϕ displayed a clear drift of ~-0.1° on average within the first 15 minutes of standing in the control condition that was maintained throughout the visit. However, the ingestion of water apparently exerted an opposing effect, as ϕ values in the water condition did not change throughout the measurement duration. While the magnitude of change in ϕ observed in the control condition was relatively small compared to population-level SDs of ~0.5° to 1.1° [43, 44], it could be meaningful for longitudinal investigations attempting to detect changes in ϕ [5].

Importantly, the use of plain water ingestion as the physiological perturbation in the present investigation has several implications for BIA technology. A sufficient intake of water could affect body geometry by increasing volume and could also potentially decrease resistivity due to temporary dilution of ion concentrations. It has previously been demonstrated that bioelectrical variables are sensitive to changes in ion status [45].

Several investigations have described the influence of postural changes on fluid shifts using bioimpedance-derived variables; however, few have included serial assessments in the upright position, and standing durations have been relatively short (≤ 30 min) [22, 23, 46]. Gibson et al. [23] reported that, over the course of 30 minutes supine, bioimpedance spectroscopy ECW estimates decreased by 2.8%, ICW estimates increased by 2.5%, and no change in TBW (≤ 0.3%) was observed due to the opposing compartmental shifts. Conversely, when participants stood for 30 minutes, ECW estimates increased by only 0.8% with a 0.9% decrease in ICW. Several other investigations have collectively demonstrated an increase in whole-body Z when moving to the supine body position, with effects observed immediately and progressively increasing over at least four hours [19, 20, 21, 22, 23, 24].

Interestingly, some data indicate that following 60 minutes in the supine position, R normalizes to baseline values after only five minutes of standing [20]. Five minutes is also the duration of standing recommended in the product manual of the analyzer used in the present investigation. However, other data indicate that 30 minutes may be insufficient to achieve a steady state in fluid compartments estimated by bioimpedance [22]. It is not fully apparent if a true steady state was reached in the present investigation. While we observed no whole-body change in ECW (~0.4%), segmental evaluation indicated progressive declines in arm ECW (~1.9%) and increases in leg ECW (~1.8%). These findings are consistent with the idea that postural changes influence bioimpedance-derived variables due to fluid shifts in the extremities and subsequent alteration of limb volume, cross-sectional area, and muscle hydration [19, 47]. In contrast to the lack of change in whole-body ECW, slight decreases in whole-body ICW (~1.4%) and TBW (~1%) estimates were observed in the present study.

In conclusion, the present report provides data concerning two important methodological considerations for bioimpedance assessments. Acute water consumption altered some raw bioelectrical values and consistently elevated FM estimates without differentially influencing TBW, ECW, ICW, or LBM. Notable drift in most variables occurred during 65 minutes of upright posture, with some changes being clearly evident in the first 15 minutes of upright rest. As such, a period of 15 minutes of upright rest prior to assessments using vertical BIA analyzers may help reduce error, if consistently implemented. These findings have implications for pre-assessment standardization, methodological planning and reporting, and interpretation of bioimpedance-based metrics.

Supplementary figures:

https://github.com/joeb-files/2022_Tinsley