The research here is much less certain towards the need to maintain full hydration status. My former department boss in my Ph.D. lab, Ira Jacobs, was one of the first to study this topic (2). Using the common anaerobic Wingate test on the cycle ergometer, consisting of 30 s of maximal sprinting, he tested trained athletes during a progressive dehydration to 5% of body weight. No significant impairment in peak power or mean power over the 30 s was observed throughout the dehydration.
The pattern continues with very recent studies. In a study just published in this month's issue of Med Sci Sports Exerc Cheuvront and colleagues (1) exposed their trained subjects to either: 1) passive heat exposure with hydration maintenance (EU, but elevated core temperature due to heat exposure), or 2) passive heat exposure with dehydration of 2.7% (DH, dehydration + core temperature elevation). They then had the subjects perform 15 s Wingate sprints before the heat exposure, and also at 0, 30, and 60 min afterwards while recovering in a temperate (22oC) room.
- No effect from hyperthermia. Core temperature was elevated significantly with both hydration programs, with 0.6 (EU) and 1.0oC (DH) core temperature rises after the 3 h passive rest in the heat. Core temperature dropped back to normal after a 60 minute rest in a thermo neutral room, though DH remained a bit higher than EU. Regardless, no differences were observed in any power output measures at any time point.
- No effect from hypohydration. Coupled with the above lack of significance with hyperthermia, no significant differences were observed between the two hydration conditions at any time point.
I've obviously only discussed two studies here, but the general view to date seems to be that mild to even moderate decreases in body weight do not seem to have a really huge performance impact when it comes to very brief, high-intensity exercise. Weight-cutting athletes in sports like wrestling, boxing, martial arts, and body-building have taken advantage of this for years, often dropping 5+ kg in the days leading up to a weigh-in to make a weight category. Most of this weight loss is primarily water. Then in the short time between weigh-in and competition, they would try to gain back as much weight as possible. Their sport fits the scientific profile quite well, consisting of brief 1to 3 minute bursts of supramaximal intensity with short recovery breaks.
Does this have any relevance to cycling? Well, almost all cycling disciplines are primarily aerobic and last much longer than the typical anaerobic test such as the Wingate. However, if you're a kilo specialist or maybe even a pursuiter, then hydration prior to your event, while still probably a good thing, probably isn't going to be the make or break for your performance.
The importance of not sacrificing hydration continues to be reinforced in cycling-specific studies. In one interesting twist on this idea, a group of scientists at the Australian Institute for Sport presented an abstract at the recent ACSM conference in Denver on whether mild hypohydration would really impact performance in a simulated road race. The course profile was roughly analogous to a Mont Ventoux stage, where there's a long flattish run-in leading to a big mountain-top finish, where climbing ability and power-to-weight ratio becomes critical.
Subjects lost weight over the initial part of the test, then rode at a set pace to exhaustion on a simulated hill climb. Despite a lighter weight and therefore theoretically less power output required to maintain climbing speed, time to exhaustion was the same whether subjects were normally or de-hydrated prior to the hill climb. So the lighter weight did not have any performance benefits, but may lead to problems with thermoregulation and cardiovascular dynamics.
So what is the ultimate moral of these stories? Unless your name is Chris Hoy and your quads measure near my waist size, do not toss the bottle away. Make sure you stay hydrated!
- Cheuvront, S. N., R. Carter, 3rd, E. M. Haymes, and M. N. Sawka. No effect of moderate hypohydration or hyperthermia on anaerobic exercise performance. Med Sci Sports Exerc. 38:1093-1097, 2006.
- Jacobs, I. The effects of thermal dehydration on performance of the Wingate anaerobic test. Int. J. Sports Med. 1:21-24, 1980.