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

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&rsquo: habitual caffeine intake (0&ndash:535 mg/day) was used to stratify users into na&iuml:ve (&lt:25 mg/day, n = 19) or regular users. Results: BHIs at 240 min for CAF (0.86 &plusmn: 0.16) and CAF + E (0.91 &plusmn: 0.16) were lower (p &lt: 0.001) versus water (1.0 &plusmn: 0.0) and CAF + CE (1.01 &plusmn: 0.12). Urine mass with CAF and CAF + E was significantly greater (p &lt: 0.01) by 244 g and 162 g versus CAF + CE. The % fluid retained at 240 min was lower (p &lt: 0.001) for CAF (&minus:71.2 &plusmn: 21.5%) and CAF + E (&minus:65.1 &plusmn: 26.3%) versus CAF + CE (&minus:46.4 &plusmn: 18.8%). Habitual intake status had no effect on the BHI (p = 0.827) between the na&iuml:ve (0.92 &plusmn: 0.1) and habitual user group (0.93 &plusmn: 0.1) averaged across all three caffeinated beverages for 120 and 240 min. At 240 min, the drink (p &lt: 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&ndash: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.