Although the effect of caffeine on hydration status, usually assessed by a urine specific gravity test (USG) has been extensively studied no research has been conducted to analyze its effects on total body water and its compartments using reference methods. In addition some uncertainty still remains about the influence of caffeine ingestion on hydration due to methodological limitations, specifically the use of less valid techniques for total-body water assessment, small sample size, and the lack of control for potential confounding factors. The information above specifically the methodological gaps mentioned and an inadequate experimental design lead us to further understand the effect of a moderate dose of caffeine in total body water (TBW) and its intra (ICW) and extracellular (ECW) compartments in non-caffeine consumers during a short-term period (4 days). Additionally, though caffeine effects on energy expenditure has been studied, its influence on physical activity (PA) during free-living conditions using gold standard and objective measures of PA is limited. Therefore, we also investigated the impact of a moderate dose of caffeine on resting energy expenditure (REE), PA energy expenditure (PAEE), total energy expenditure (TEE), and daily time spent in sedentary (DTSS), light (DTSL), moderate (DTSM), and vigorous (DTSV) intensity activities in non-obese physically active males. To overcome the methodological gaps in previous studies we assessed dietary intake and objective measures of physical activity throughout the experimental trial to assure that the same diet and physical activity patterns were maintained. Also it was analyzed if the effect of caffeine was independent of body composition, assessed by dual-energy x-ray absorptiometry.

To perform this research study, a total of 30 non-smoker males, low caffeine users (\<100 mg/day), aged 20-39 yrs \[body mass (BM): 72.7 ± 8.8 kg; Height: 1.77 ± 0.07 m\] were followed in a double-blind crossover experimental design with two conditions in a random sequence: caffeine (5 mg per kg of BM/day) and malt-dextrine as placebo, both through capsules. Conditions lasted for 4 days with a 3-day washout period.

Evaluations were performed at baseline (visit-1), end of condition 1 (visit-2), and end of condition 2 (visit-3). Fat and fat-free masses (FFM) were assessed by dual energy x-ray absorptiometry. TBW and ECW were determined by dilution techniques (deuterium and bromide, specifically) while ICW was calculated as the difference between TBW and ECW (Schoeller et al., 1980. Total body water measurement in humans with 18O and 2H labeled water. Am J Clin Nutr 33:2686-2693). TEE was assessed using doubly labeled water technique by administrating two stable isotopes 2H (deuterium) and 18O (oxygen 18) with a respective dose of 0.1 g / kg and 1.8 g / kg of body water (Schoeller DA, van Santen E, 1982. Measurement of energy expenditure in humans by doubly labeled water method. J Appl Physiol 53:955-959). REE was assessed by indirect calorimetry and PAEE calculated as \[TEE-(REE+0.1TEE)\]. An accelerometer (ActiGraph, GT1M model, Fort Walton Beach, Florida) was used to estimate DTSS, DTSL, DTSM, and DTSV. The cutoff values used to define the intensity of physical activity and therefore to quantify the mean time in each intensity (sedentary, light, moderate or vigorous) for persons aged 18 years or older were: sedentary: \< 100 counts/min; light: 100-2019 counts/min; moderate: 2,020-5,998 counts/min (corresponding to 3-5.9 METs); vigorous: ≥ 5999 counts/min (corresponding to ≥ 6 METs) (Troiano et al. Physical activity in the United States measured by accelerometer. Med Sci Sports Exerc. 2008;40:181-8).