Although water does not provide a source of Calories, adequate hydration is at least as important to good athletic performance as the food you eat. Perhaps the single biggest mistake of many competitive athletes is the failure to replace the fluid losses associated with exercise. This is especially so in cycling because rapid skin evaporation decreases the sense of perspiring and gives a false sense of minimal fluid loss (even though sweat production and insensible loss through the lungs can easily exceed 2 quarts per hour). For a successful ride, it is essential that fluid replacement begin early and be continued on a regular basis. In fact, a South African report on two groups of cyclists (one rehydrating, the other not) exercising at 90% of their maximum demonstrated a measureable difference in physical performance as early as 15 minutes into the study.
Total body fluid losses during exercise result in a decrease in plasma volume (the fluid circulating within the blood vessels) as well as muscle water content. As fluid loss progresses, there is a direct effect on physiologic fuction and athletic performance. Unreplaced water loss of 2% of body weight begins to impact heat regulation, at 3% there is a measureable effect on muscle cell contaction times, and when fluid loss reaches 4% of body weight there is a measureable 5 to 10% drop in performance. And one study demonstrated that this effect on performance can persist for 4 hours after rehydration takes place emphasizing the need to anticipate and regularly replace fluid losses. Maintaining plasma volume is one of the hidden keys to optimal physical performance.
But for those who practice the philosophy of "if a little is good, a lot is better", there are also risks associated with overcorrecting the water losses of exercise. There have been multiple reports of hyponatremia (low blood sodium concentration) with seizures in marathon runners from the over replacement of sweat losses (salt and water) with pure water. This is a risk for longer events (more than 5 hours). If there is a question as to your hydration status, weighing yourself regularly on long rides will help you tailor YOUR replacement program. A weight gain of more that 1 or 2 pounds indicates that you are overcorrecting and may be placing yourself at risk for this unusual metabolic condition.
Under normal conditions, a minimum of 4 to 5 ounces of fluid should be taken every 15 minutes. When extreme conditions of heat and humidity are anticipated, the following strategies may be of benefit:
- Hydrate before, during, and after the ride - thirst is inadequate to stimulate complete rehydration, so learn to drink before you are thirsty. Using a CamelBak or similar device on long rides will let you drink without worrying about the need to stop and possibly lose your group.
- Don't skimp when using a sports drink - don't assume that because they contain electrolytes and carbohydrates you don't need to drink as much. And the sweet taste often keeps you from drinking, so take an extra bottle of plain water to alternate. And keeping liquids cool helps so add ice or freeze half a bottle the night before and top it off with water or extra sports drink just before the race.
- Weigh yourself before and after the ride - most of your weight loss will be fluid, and 2 pounds is equal to 1 quart. A drop of a pound or two won't impair performance, but a greater drop indicates the need to reassess your personal program. And for longer rides of 5 hours or more, beware of any gain of more than 1 or 2 pounds that indicates you have over compensated.
- Wear the right clothing - light colored to reflect heat; a loose weave jersey; shorts made of one of the new "wicking" materials.
Wear your helmet - modern well vented helmets funnel the wind onto your head and are actually cooler than your bare head, and the helmet material can act to insulate your head from the sun.
In one study conducted over 2 hours at 67% VO2 max, and resulting in average fluid losses of 2300 ml, there was no obvious advantage of electrolyte drinks over water alone. However, the fact that large fluid volumes needed to be replaced emphasized that palatability and digestive tract tolerance are important considerations in the selection of replacement fluids. And the longer the ride, the more important electrolyte replacement becomes in preventing dilutional hyponatremia.
Adding carbohydrates to your fluid replacement is an additional benefit for rides lasting longer than two hours. Cyclists are able to ingest large volumes of water during competitive events, and in extreme events such as the Tour de France for example, by using 20% carbohydrate solutions and drinking 2 to 4 quarts an hour, the competitors were able to replace 50% of their daily energy expenditure.
Because liquids are readily emptied from the stomach, any sugar they contain is quickly absorbed into the bloodstream and transported to the muscles where it is readily available as an alternative to muscle glycogen as an energy source. Following the same reasoning, drinks containing glucose polymers (increasing Calories while remaining iso-osmotic) are of additional benefit, delivering additional Calories per ounce of fluid.
NO studies have confirmed a benefit of fruit drinks (which contain the sugar fructose) over glucose drinks. Although fructose requires less insulin to enter muscle cells, this does not appear to provide a performance advantage for cycling performance. Taste alone is the only advantage.
For many years it was believed that a 2.5% concentration of glucose or glucose polymer molecules was the maximum tolerated without slowing stomach emptying and causing nausea. However a recent study in cyclists demonstrated normal gastric emptying with a 6 - 8% solution, and nausea occuring only when concentrations were pushed above 11%. Interestingly, the old standbys, such as apple juice and cola drinks have a sugar concentration of 10% and, although the glucose polymer sports drinks can provide more Calories per quart at the same overall concentration, in controlled studies there has been no demonstrated performance advantage of these complex carbohydrates over simple sugars such as glucose alone. It appears that the major benefit of the polymers is the absence of the sweet taste and nauseating properties of high concentration isocaloric glucose drinks, minimizing this barrier to maintaining a high fluid intake.
The stomach does have its limits, however, and it appears that 800 ml, or approximately 1 quart, is the near the upper limits of volume easily handled per hour, and this diminishes as exercise approaches 100% VO2 max and gastric emptying slows. If larger volumes are pushed, nausea and distention will result. A regular water bottle is 1/2 quart (16 ounces or 480ml) and the large ones are 3/4 quart, so you should be able to drink at least 2 bottles per hour in hot conditions.
In summary, drinking plain water at a rate of 1 quart per hour is adequate for rides of 1 1/2 to 2 hours. On longer rides, where the body's glycogen stores will be approaching exhaustion, liquid carbohydrate supplements assume increased importance (glucose containing liquids can deliver Calories from the mouth to the muscles in as little as 10 minutes as compared to solid foods and energy bars which empty more slowly from the stomach). In most individuals, an 8 - 10 % concentration appears to be the maximum tolerated. Glucose polymers offer the advantage of increasing the total concentration of Calories per quart without the side effect of an unpalatable, sweet taste, but otherwise don't have any proven advantage over simple sugar (glucose) drinks during a ride. Although there are many commercial drinks available, the old standbys such as apple juice and cola drinks are probably the least expensive per Calorie provided. However, in the pre and post ride period, the high Calorie, easily absorbed glucose polymer sports drinks are ideal for building (or restocking) glycogen stores.
For those longer rides, don't forget about the risks of overdoing rehydration with pure carbohydrate drinks alone. If you plan to ride greater than two or three hours, it's worth considering the use of an electrolyte containg carbohydrate drink, and if you are going to be riding 5 hours or more, pace your fluid replacement rate (and keep an eye on your weight during training rides to be certain you are not overcompensating).
There have been some encouraging studies on the use of glycerol to minimize the negative impact of dehydration on performance, but at this time there are still unresolved questions as to harmful effects and there are no commercial products available.
Except under extreme conditions, electrolytes (particularly sodium chloride or salt) do not need to be replaced along with fluids.