Caffeinated Energy Drink Formulations Differentially Impact Hydration Versus Water: Does Habitual Caffeine Intake or Biological Sex Matter?

<b>Background/Objectives</b>: 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. <b>Methods:</b> 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, <i>n</i> = 19) or regular users. <b>Results:</b> BHIs at 240 min for CAF (0.86 ± 0.16) and CAF + E (0.91 ± 0.16) were lower (<i>p</i> < 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 (<i>p</i> < 0.01) by 244 g and 162 g versus CAF + CE. The % fluid retained at 240 min was lower (<i>p</i> < 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 (<i>p</i> = 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 (<i>p</i> < 0.001) and drink x sex interaction (<i>p</i> = 0.042) indicated women had higher BHI than men (<i>p</i> = 0.03) for caffeinated drinks despite higher relative fluid and caffeine dosages. <b>Conclusions:</b> 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.