The purpose of this study was to determine if the consumption of caffeine influences BIA-derived BF% and body water values in habitual caffeine users. The primary findings of this study demonstrated that there was a significant interaction for BF%. Analysis revealed that BF% for CAF was lower than the CON and PLA conditions at PRE and 15-min but not statistically significant for the remaining time points (i.e., 30-, 45-, 60-, 75-, and 90-min). Although these mean values were statistically significant at PRE and 15-min, the ES of the mean differences were all trivial. As a result, these finding suggest that the significant BF% differences have minimal practical implications. There were also significant main effects for time when evaluating ICW values in all three conditions. Post hoc analysis revealed that the CON, PLA and CAF conditions had higher PRE ICW values than their associated post time points (i.e., 15-, 30-, 45-, 60-, 75-, and 90-min). However, similar to BF%, the effect sizes of the mean differences were all trivial, which once again demonstrates that these differences have minimal practical implications on changes in ICW. Collectively, these findings are suggestive that current restrictions for pre-testing guidelines involving caffeine and BIA may be too stringent.
BIA technology has advanced and is now capable of using high and low frequencies to estimate ICW and ECW, respectively. After determining these values, the estimation of TBW can be derived, which can then be used to estimate BF% based on the assumed hydration of FFM being 73.72% . This is the first study to identify the impact that caffeine consumption has on BF% and body water when using BIA. Research has previously examined the impact of altering BIA pre-testing guidelines such as avoidance of exercise and meal consumption [4, 5]. However, research has yet to emerge on whether the consumption of caffeine can have an influence on BIA-derived body water and BF%.
During the CON conditions, throughout the 15-min intervals, no observed changes in mean TBW were observed. Very few studies have examined fluid consumption and time course evaluation of BIA. When evaluating the TBW in PLA and CAF conditions, there were no significant changes in value despite consuming 8 fluid ounces of water between the PRE and 15-min time points. Our findings are consistent with those of Gibson et al. , who observed no significant changes of TBW when measured every five-minutes over a 30-min time period in both standing and supine positions. Importantly, in the aforementioned study, the participants did not consume any food or drink during the duration of testing, unlike the CAF and PLA conditions of the current study. Conversely, Androutsos et al.  examined the effects of consuming 750 mL of water with a meal and observed significant changes in TBW when measured every 30-min for up to 2-h. The results of the current study are likely different due to the consumption of a comparatively small amount of water while in a fasted state. A unique aspect of the current study is that throughout the ingestion period, participants were not allowed to void their bladder, therefore, preventing changes in TBW. This however does not explain the lack of TBW increase with the consumption of 8 fluid ounces of water, given that ample time was provided for absorption into the bloodstream, which can occur within five-minutes . It is possible that the 5 frequencies utilized by the BIA device in the current study are not sensitive to the 8 fluid ounces of water consumption. Therefore, future research should determine if higher frequencies and more sophisticated bioimpedance devices, such as bioimpedance spectroscopy, are more sensitive to small quantities of water absorption.
Similar to TBW, no changes were observed in ECW throughout the 90-min period in any of the conditions. Our findings were consistent with those of a study conducted by , in which measurement of ECW via the sodium bromide dilution method showed no changes after consumption of 2.5 mg·kg− 1·day− 1 of caffeine consumed twice per day for four days. Few studies have examined repeated measurements of ECW over a period of time, therefore the response to fluid and supplement ingestion is relatively unknown. It was an expected observation that the CON trial would elicit little to no change in ECW throughout the time points. Through impedance measurements, ECW is quantified using frequencies under 50 kHz, quantifying fluid outside of the cells. For example, the InBody 770 utilizes multiple low frequencies (i.e., 1 kHz, 5 kHz, and 50 kHz) in order to quantify ECW. A potential rationale for a failure to detect changes in ECW is the rate of extracellular fluid flow (i.e., cardiac output). When compared to ICW, which is relatively constant, ECW is more mobile throughout the body and may lead to interference with small transient changes in volume. Consequently, the measurement of TBW with lower impedance frequencies and higher rate of extracellular flow make it difficult to assess ECW.
There were significant decreases observed in ICW through out the 90-min time period. The observed changes occurred in all three conditions, suggesting that CAF was not the primary cause of the decrease in ICW. Although significant decreases were observed, the effect sizes of the differences were trivial, and therefore have minimal practical implications. The decrease in ICW was unexpected due to the stable conditions observed in TBW and ECW throughout the time period. A likely mechanism involved for the decrease in ICW among all conditions was the prolonged periods of rest. At the beginning of all conditions, HR was elevated, likely due to the activity of walking to the laboratory and acclimating to the new environment. The prolonged rest periods throughout the trial could have contributed to a lack of requirement of blood and fluid into large cellular tissue (i.e., skeletal muscle). An additional reason for the observed findings could be the ionic composition of ICW varies amongst different types of cells , and therefore, could be a limiting factor due to the potential inability to directly measure ICW.
A time-dependent increase in BF% was observed over the 90-min period. Importantly, the condition elicited little to no effect over BF%, which is an interesting finding due to the current pre-testing guidelines and recommendations requiring the abstinence of caffeine. Caffeine is known to stimulate sympathetic nervous system activity  and is also a known diuretic under resting conditions , both of which in theory, should influence homeostatic fluid balance. However, habitual caffeine users may demonstrate less sensitivity to caffeine consumption and therefore, have little to no influence over measurements of fluid compartments . Consequently, future research might seek to determine the influence of caffeine on BIA measurements (e.g., body water) at varying doses as well as in individuals who are not considered habitual caffeine users such as that of the current study sample.
Multi-frequency BIA may not be sensitive enough to detect smaller volumes of fluid added to the system. It was anticipated that 8 fluid ounces of water, which is approximately 5% of average systemic volume (i.e., 5-l) , would be detected in either TBW, ECW, or ICW. The absorption rate of water from the gastrointestinal tract typically occurs within five-minutes of consumption . Despite this, the only time dependent change within these variables was an observed decrease in ICW for all groups. This is a surprising result, especially considering that PLA and CAF conditions involved consumption of additional fluid, suggesting a lack of sensitivity in the multi-frequency BIA. However, the magnitudes of the significant differences for ICW values were minimal. A possible rationale, as suggested earlier, may be due to the circulation of fluid in the extracellular component.