Guarana (GUA), a Brazilian seed extract, contains caffeine and other bioactive compounds that may have psychoactive effects. To assess the acute effects of GUA compared to a low dose of caffeine (CAF) on cognitive and mood parameters, twenty participants completed a double-blind, crossover experiment where they ingested capsules containing the following: (1) 100 mg CAF, (2) 500 mg GUA containing 130 mg caffeine, or (3) placebo (PLA). Cognitive tests (Simon and 2N-Back Task) were performed at the baseline (pre-ingestion) and 60 min after ingestion. The response time for the cognitive tests and heart rate variability were unaffected (p > 0.05) by treatment, although 2N-Back was overall faster (p = 0.001) across time. The accuracy in the 2N-Back Task showed a significant interaction effect (p = 0.029) due to higher post-ingestion versus pre-ingestion levels (p = 0.033), but only with the PLA. The supplements also had no effect on cognitive measures following physical fatigue (n = 11). There was an interaction effect on perceived mental energy, where the pre-ingestion of GUA had lower mental pep ratings compared to post-ingestion (p = 0.006) and post-exercise (p = 0.018) levels. Neither the acute ingestion of GUA nor low dose of CAF influenced cognitive performance or provided consistent benefit on mood or mental workload through vagal modulation. Additional investigations are beneficial to determining the lowest effective dose for CAF or GUA to influence mood and/or cognitive performance.

Background/Objectives: The beverage hydration index (BHI) assesses the relative hydration properties of beverages. Commercially available caffeinated energy drinks (with and without electrolytes or carbohydrates) were compared to water. Methods: Fourteen men and fourteen women completed four trials: 500 mL water plus either 500 mL water or caffeinated energy drink beverage (CAF) with 280 mg, CAF (280 mg) plus electrolytes (CAF + E), or CAF (106 mg) + E and carbohydrates (CAF + CE). Participants’ habitual caffeine intake (0–535 mg/day) was used to stratify users into naïve (<25 mg/day, n = 19) or regular users. Results: BHIs at 240 min for CAF (0.86 ± 0.16) and CAF + E (0.91 ± 0.16) were lower (p < 0.001) versus water (1.0 ± 0.0) and CAF + CE (1.01 ± 0.12). Urine mass with CAF and CAF + E was significantly greater (p < 0.01) by 244 g and 162 g versus CAF + CE. The % fluid retained at 240 min was lower (p < 0.001) for CAF (−71.2 ± 21.5%) and CAF + E (−65.1 ± 26.3%) versus CAF + CE (−46.4 ± 18.8%). Habitual intake status had no effect on the BHI (p = 0.827) between the naïve (0.92 ± 0.1) and habitual user group (0.93 ± 0.1) averaged across all three caffeinated beverages for 120 and 240 min. At 240 min, the drink (p < 0.001) and drink x sex interaction (p = 0.042) indicated women had higher BHI than men (p = 0.03) for caffeinated drinks despite higher relative fluid and caffeine dosages. Conclusions: A low-carbohydrate–electrolyte beverage with moderate caffeine had similar hydration properties compared to water; however, this differs from beverages with higher caffeine containing limited carbohydrate and/or electrolytes, which were inferior to water. Habitual caffeine intake had no apparent influence, but men and women exhibited differences in the diuretic response to energy drink consumption.

Background/Objectives: The Beverage Hydration Index (BHI) compares fluid retention of beverages relative to water based upon urine output over time. We examined the reliability of diuresis from water ingestion and its potential impact on the BHI of a sugar-free energy drink with electrolytes (CAF). Methods: Twenty-four adults completed two trials consuming either one liter of water or CAF over 30 min with urine output measured over 4 h. In a sub-group (n = 15), water trials were repeated twice (~6 months apart) to compare the impact on BHI of CAF. Results: Urine mass was not different (p = 0.46) between repeated water trials; however, intraclass correlations indicated poor reliability (r = 0.11, r = 0.31) at 2 and 4 h. Mean (±SD) BHI of CAF based upon two water trials did not differ (p = 0.44) at 2 h (0.90 ± 0.17 versus 0.97 ± 0.22) and 4 h (0.99 ± 0.18 versus 1.01 ± 0.21). The BHI for CAF at 60 min (0.75 ± 0.21; 0.83 ± 0.23) was the only time point lower (p < 0.001, p = 0.008, respectively) than a BHI of 1.0 (water control). The BHI of CAF for all participants had similar results: no difference between CAF and water from 2 to 4 h. Conclusions: Although the mean diuretic action of water ingestion appeared similar across repeated trials, the reproducibility of diuresis within individuals was poor. Despite low reliability, the impact on establishing the BHI of CAF was not significant. CAF provided similar hydration benefits as water from 2 to 4 h (varying by 1–7%) despite increased diuresis only in the first hour, suggesting that a sugar-free electrolyte beverage containing caffeine supports long-lasting hydration. Moreover, the establishment of BHI metrics is robust to interference from potential differences in individuals’ pre-test free-living conditions.