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 ± SD: age, 21 ± 3 yr; V˙O2max 50.9 ± 4.3 mL·kg−1·min−1). Control (CON) participants received their estimated energy (2903 ± 199 kcal·d−1) and water (3912 ± 500 mL·d−1) requirements. For FR, participants received their energy requirements and 193 ± 50 mL·d−1 water to drink, and for ER, participants received their water requirements and 290 ± 20 kcal·d−1. F + ER was a combination of FR and ER. After 48 h, participants performed a 30-min treadmill TT in temperate conditions (19.7 ± 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 ± 0.4% (CON), 3.2 ± 0.5% (FR), 3.4 ± 0.3% (ER), and 3.6 ± 0.3% (F + ER). Compared with CON (6295 ± 513 m), less distance was completed on ER (10.3%) and F + ER (15.0%: P < 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.