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

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