Written by: Matt Fitzgerald
Fluid loading is like carbohydrate loading, except with water instead of carbohydrate. Carbo-loading entails gobbling carbohydrate before a long race to maximize muscle and liver glycogen stores. Fluid loading entails drinking extra fluid, usually with a hydrophilic compound such as glycerol, to achieve a “hyperhydrated” state before a race to minimize the effects of dehydration on thermoregulation and performance.
A number of studies have shown beneficial effects of fluid loading on performance in some circumstances, but the vast majority of studies have found that it has no effect on thermoregulation. The latter finding is only surprising to exercise scientists who cling to outdated beliefs about the relationship between hydration status and core body temperature during exercise. The best and latest research has clearly demonstrated that dehydration has a very small effect on core body temperature during exercise. The environmental temperature and the exercise intensity are the primary determinants of core body temperature during exercise. When the environmental temperature and the desired exercise intensity are high enough to make heat illness a possibility, the brain will enforce a pacing strategy that ensures the core body temperature rises to the maximum safe level and then plateaus there. Dehydration will not affect the maximum allowed core body temperature in such circumstances but will at most reduce the maximum pace that can be sustained without exceeding the same temperature limit.
Yet this is precisely how fluid loading could enhance performance-by slightly increasing the pace that the athlete can sustain at the maximum allowable core body temperature during races. But how relevant are the studies showing performance benefits of fluid loading to real-world competitive circumstances? The main questions are whether fluid loading provides any benefit when adequate fluid is consumed during the race and whether it provides any benefit in temperate conditions.
A recent study by researchers at the University of Sherbrooke, Canada, appears to provide some evidence that fluid loading could aid performance even when athletes drink during race-intensity exercise in warm-not-hot weather. Specifically, the researchers investigated the effects of hyperhydration on cycling performance in subjects who consumed small amounts of fluid while riding in a temperate environment. Hyperhydration was achieved with glycerol, a natural compound that is similar in chemical structure to alcohol. It is present in the body in stored fat and in fluids and can also be purchased as a supplement. One effect of ingesting glycerol is an increase in blood plasma volume, which is potentially beneficial to athletes because it could slow the dehydration process during exercise.
In the study, on two occasions, six trained cyclists completed a two-hour ride at 65 percent of VO2max interspersed with five two-minute intervals at 80 percent of VO2max and followed by an incremental ride to exhaustion. They began one trial in a normally hydrated state and the other in a hyperhydrated state and consumed fluid at a rate equaling only 33 percent of the rate of sweat loss throughout both rides. On average, the cyclists finished the post-fluid loading ride only half as dehydrated as they finished the other ride. Most significantly, they also achieved a higher peak power level and longer time to exhaustion in the incremental ride to exhaustion at the end of the post-fluid loading trial. (There was no difference in core body temperature between the two trials.)
The practical implications of this study are not entirely clear. Under what circumstances would cyclists be able to hyperhydrate before an exhaustive ride yet unable to replace more than 33 percent of fluid losses during the ride? When given access to adequate amounts of fluid and encouraged to drink according to their thirst, cyclists normally replace roughly 70 percent of fluid losses during race-intensity exercise in the heat, a rate that appears adequate to optimize performance. For example, in a 2006 study Goulet et al performed at study in which cyclists completed a 2.5-hour ride in the heat followed by an incremental ride to exhaustion. Subjects began in either a hyperhydrated state or a normally hydrated state and drank 2.5 ml of fluid per kg of bodyweight throughout both trials. There was no difference in performance.
In theory, hyperhydration might make more sense before running competitions, because it is not possible to drink as much during running as during cycling. To the degree that dehydration does hamper performance in the latter part of longer running events, beginning the race in a more than fully hydrated state could conceivably reduce this effect. In essence, runners could enjoy the benefit of drinking more during the race without actually drinking more, hence without the risks associated with doing more (namely, GI distress). But just how much does dehydration affect performance in the typical marathon, and does it hinder performance more than the weight gain resulting from pre-race hyperhydration would?
These questions have not been directly addressed by research, by relevant past studies suggest that runners optimize performance through ad libitum fluid intake. For example, a 2000 study from the University of Cape Town, South Africa, found that an enforced high drinking rate had no effect on performance in a two-hour run in the heat compared to ad libitum drinking. So the preponderance of evidence suggests that fluid loading is pointless. On the other hand, there is little evidence to suggest that it does any harm, except that there are some side effects associated with excessive glycerol intake (nausea, blurred vision). If you want to try it, note that, although the suggested dosage of glycerol depends on body size and varies between manufacturers, 1 g/kg body weight with an additional 1.5 L fluid taken 60 to 120 minutes before competition is standard.
A version of this article originally appeared on Poweringmuscles.com.