Endurance Running Performance after 48 h of Restricted Fluid and/or Energy Intake SAMUEL J. OLIVER 1 , STEWART J. LAING 1 , SALLY WILSON 1 , JAMES L. J. BILZON 2 , and NEIL WALSH 1 1 School of Sport, Health and Exercise Sciences, University of Wales, Bangor, UNITED KINGDOM; and 2 Headquarters Army Training and Recruiting Agency, Upavon, UNITED KINGDOM ABSTRACT OLIVER, S. J., S. J. LAING, S. WILSON, J. L. BILZON, and N. WALSH. Endurance Running Performance after 48 h of Restricted Fluid and/or Energy Intake. Med. Sci. Sports Exerc., Vol. 39, No. 2, pp. 316–322, 2007. Purpose: To determine the effect of a 48-h period of either fluid restriction (FR), energy restriction (ER), or fluid and energy restriction (F + ER) on 30-min treadmill time trial (TT) performance in temperate conditions. Methods: Thirteen males participated in four randomized 48-h trials (mean T SD: age, 21 T 3 yr; V ˙ O 2max 50.9 T 4.3 mLIkg j1 Imin j1 ). Control (CON) participants received their estimated energy (2903 T 199 kcalId j1 ) and water (3912 T 500 mLId j1 ) requirements. For FR, participants received their energy requirements and 193 T 50 mLId j1 water to drink, and for ER, participants received their water requirements and 290 T 20 kcalId j1 . F + ER was a combination of FR and ER. After 48 h, participants performed a 30-min treadmill TT in temperate conditions (19.7 T 0.6-C). A separate investigation (N = 10) showed the TT to be highly reproducible (CV 1.6%). Results: Body mass loss (BML) was 0.6 T 0.4% (CON), 3.2 T 0.5% (FR), 3.4 T 0.3% (ER), and 3.6 T 0.3% (F + ER). Compared with CON (6295 T 513 m), less distance was completed on ER (10.3%) and F + ER (15.0%: P G 0.01). Although less distance was completed on FR (2.8%), this was not significantly different from CON. Conclusions: These results show a detrimental effect of a 48-h period of ER but no significant effect of FR on 30-min treadmill TT performance in temperate conditions. Therefore, these results do not support the popular contention that modest hypohydration (2–3% BML) significantly impairs endurance performance in temperate conditions. Key Words: DIET, DEHYDRATION, TIME TRIAL, REPRODUCIBILITY E pisodes of forced or voluntary fluid restriction (FR) and energy restriction (ER), often lasting for a number of days, frequently occur in occupational and athletic settings (e.g., military recruits on field exercise (5,22), athletes with eating disorders (3) and athletes making weight for competition (7)). It is commonly believed that modest hypohydration equal to 2–3% body mass loss (BML) has a detrimental effect on endurance performance (11,24). How- ever, the research investigating the effects of modest levels of hypohydration on endurance performance in temperate conditions remains equivocal. One widely cited paper reports significant 7% increases in time to complete 5000- and 10,000-m track races when athletes were hypohydrated to approximately 2% BML using the diuretic furosemide (2). Another study shows that a 3% BML evoked by heat exposure decreases work completed on a cycle ergometer by 8% in a 30-min period (10). In contrast, similar BML evoked using a combination of exercise and FR had no significant effect on work completed on a cycle ergometer in a 15-min period (20) or distance completed on a treadmill in a 30-min period (13). The different methods used to evoke hypohy- dration (process termed ‘‘dehydration’’) and distribution of losses from different fluid compartments may account for the equivocal findings regarding the effects of modest hypohydration on endurance performance (11). In line with this, endurance performance was compromised when hypo- hydration was evoked by diuretic administration (2) and heat exposure (10) but not when similar BML was achieved using a combination of exercise and FR (13,20). From a practical perspective, an advantage of the latter two studies (13,20) is that a combination of exercise and FR represents the type of dehydration commonly occurring in military personnel and athletes performing in temperate conditions. Unfortunately, all of the aforementioned studies induced hypohydration over a short time period (e5 h). Therefore, little information is available about the effects of more prolonged dehydration, similar to that encountered in many occupational and athletic settings, on endurance performance. Time to exhaustion (TTE) at intensities ranging from 50 to 100% V ˙ O 2max is widely reported to decrease after ER lasting 23–36 h (15,19,21,29). However, TTE protocols have been criticized for having poor test–retest reliability with mean CV ranging from 20 to 27% (17,18) compared with mean CV of G 5% for time trial (TT) protocols (17,25). Poor reproduci- bility in TTE protocols might be explained by psychological factors (e.g., motivation and boredom), which may be more variable in an open versus a known end-point test (17). Variability of this magnitude causes difficulty in identifying the proportion of change in individual performance attribut- able to the intervention and not to measurement error. Although numerous investigations have examined the effects Address for correspondence: Neil P. Walsh, Ph.D., School of Sport, Health and Exercise Sciences, University of Wales, Bangor, LL57 2PZ, UK; E-mail: n.walsh@bangor.ac.uk. Submitted for publication June 2006. Accepted for publication September 2006. 0195-9131/07/3902-0316/0 MEDICINE & SCIENCE IN SPORTS & EXERCISE Ò Copyright Ó 2007 by the American College of Sports Medicine DOI: 10.1249/01.mss.0000241656.22629.57 316 BASIC SCIENCES Copyright @ 2007 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.