Comparing Ad Libitum and Prescribed Fluid Replacement to Fluid Balance Following Exercise‐Induced Dehydration
Authors: Adams William M , Vandermark Lesley W , Belval Luke N , Lee Elaine C , Armstrong Lawrence E , DiStefano Lindsay J , Casa Douglas J 
Published: November 2023

ObjectiveTo test the hypothesis that prescribed fluid replacement is more effective at replacing fluid losses than ad libitum replacement following exercise‐induced dehydration.MethodsEleven healthy, recreationally active males (mean±SD; age, 22±3 y; height, 178±6 cm; VO2max, 54.3±5.4 ml•kg−1•min−1; body fat, 11.6±3.9%) completed three exercise trials on a motorized treadmill in hot conditions (ambient temperature, 35.3±0.6°C and relative humidity, 31.3±2.0%): euhydrated + fluid replacement during exercise (EUR), euhydrated + no fluid during exercise (EUD) and hypohydrated (22 h fluid restriction) + no fluid during exercise (HYD). Following the exercise trials participants were randomly assigned to either ad libitum (n=6) or prescribed (n=5) fluid replacement groups, given appropriate instructions for rehydration and returned to the laboratory 24h later for hydration assessment. Before (PREEX) and after (POSTEX) exercise, nude body mass (NBM) was measured to calculate body mass loss (%BML). 24h before (PRE24) and after (POST24) exercise trials participants measured NBM, collected urine for 24h, and completed diet records. Urine specific gravity (USG), urine osmolality (UOSMO) and urine volume (UVOL) were measured from PRE24 and POST24 samples to assess 24h hydration status. Fluid consumed (beverages and water content in food) was assessed from the diet record. Mixed design (rehydration group × exercise trial × time) repeated measures ANOVAs with a Tukey post hoc test was used to evaluate differences in continuous dependent variables (α, p≤0.05).ResultsAt PRE24, there were no differences in UVOL (p=0.182), UOSMO (p=0.188), or USG (p=0.120) among EUD (3.1±1.6L, 404±176 mOsm•kg−1, 1.011±0.004), EUR (3.0±1.5L, 395±219 mOsm•kg−1, 1.011±0.006) and HYD (2.0±1.5L, 544±226 mOsm•kg−1, 1.015±0.006) (p>0.05). %BML was significantly greater in HYD at PREEX (1.8±0.6%) and POSTEX (5.6±0.6%) than EUD (−0.8±1.2%, 3.8±1.2%) and EUR (−0.7±1.1%, 0.4±1.0%) (p<0.001). %BML in EUD was significantly greater that EUR only at POSTEX, p<0.001 Independent of trial, there were no differences in fluid consumption between ad‐libitum (3.2±0.4L) or prescribed (3.6±0.5L) replacement at POST24, p=0.592. For EUD, EUR, and HYD at POST24 there were no differences in UVOL (p=0.337), UOSMO (p=0.073), USG (p=0.081), or %BML (p=0.595) among ad‐libitum (EUD, 1.7±1.0L, 691±276 mOsm•kg−1, 1.020±0.008, −0.8±1.1%; EUR, 2.3±1.4L, 503±184 mOsm•kg−1, 1.015±0.005, −0.2±1.1%; HYD, 1.1±0.7L, 804±249 mOsm•kg−1, 1.023±0.006, −0.9±1.3%) or prescribed (EUD, 2.4±2.1L, 564±274 mOsm•kg−1, 1.016±0.008, −0.3±0.8%; EUR, 2.1±1.8L, 451±170 mOsm•kg−1, 1.013±0.005, −0.1±0.6%; HYD, 2.2±2.1L, 539±166 mOsm•kg−1, 1.016±0.004, −0.2±0.7%) groups respectively.ConclusionIn this study, we observed body mass losses of up to 5.6% of body mass, however, there were no differences in the ability of our participants to rehydrate within 1% of PRE24 body mass when given an ad libitum or prescribed fluid replacement plan. Given these experimental conditions, ad libitum fluid replacement is as effective in returning to a state of euhydration 24 hours following exercise‐induced dehydration.Support or Funding InformationThis study was funded by Halo Wearables, LLC