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  Last updated: 5/26/2014


Although water does not provide Calories to power the muscles in cycling activities, attention to adequate fluid intake and hydration is an important part of any program to maximize one's performance. A common error of many competitive athletes is the failure to replace fluid losses during training and competitive events. This is especially true in cycling where evaporative losses - sweat, exhaled air from the lungs - are significant and can easily exceed 2 quarts per hour. (Up to 60% of overall fluid loss can be via the lungs - which means that even swimmers can get dehydrated.)

To maximize performance, just as with carbohydrates attention to fluid replacement should begin even before you get on the bike, and continue throughout a ride.

Fluid losses during exercise decrease circulating blood volume as well as the water content of individual muscle cells. Early dehydration is defined as a >1% loss of body weight as a result of fluid loss. Unreplaced water losses equal to 2% of body weight (about 3 pounds for the average rider) will begin to impact heat regulation, at 3% there is a measurable decrease in muscle cell contraction times in the laboratory setting, and when fluid losses reach 4% of body weight, there is a drop in endurance performance (which can persist for up to 4 hours after rehydration takes place).

Based on these numbers, it is reasonable to assume that anticipating and regularly replacing fluid losses would be beneficial. However, as one reviews the literature, it appears that the impact of fluid losses may be less tha predicted, and depend on both the sport and the level/length of exertion. For example, in endurance cycling and running events there appears to be a threshhold water loss of 2% of body weight before a detectable difference in performance. But another study questioned this assumption for endurance athletic events when it demonstrated that ultramarathoners (100 km) appeared to tolerrate, and even benefit from water weight losses of up to 4%.

In time trial or sprint events there is the ability to tolerate losses of up to 3% in one study, and perhaps as much as 4% based on another without an impact on performance.

My bottom line? Attention to maintaining plasma volume remains an important part of an overall strategy to optimize physical performance, with a goal of a maximum weight loss of 2 to 3 % of body weight during a ride as the target.

Interestingly, for most athletes thirst IS a reliable indicator of the need to replace fluid losses. However, there are a few athletes where thirst is not a reliable indicator of fluid needs, thus active attention does need to be paid to fluid replacement.

How much water do you need to maintain a normal state of hydration without a daily exercise)? For a 70 kilogram adult, about 2500 to 3000 cc per day. This equates to about 4% of your body weight If your diet is well balanced, approximately 1000 cc (4 cups) is water in fruits, vegetables, and other foods you eat. Another 1 cup is produced when your body metabolizes carbohydrates, and the balance - about 7 cups - needs to be fluids you drink.

If you then exercise for an hour or two, add in replacement for the losses from sweat and respiration. Under normal environmental circumstances, you will lose 1 - 2 liters of sweat per hour, and if the ambient temperature is high, this can be as high as 4 - 6 liters per hour.

What are other factors, besides exercise, that can influence your fluid needs (and exacerbate dehydration)?

Under normal conditions, while riding you should be taking in 4 to 5 ounces of fluid every 15 minutes or 1 to 2 standard water bottles per hour. When extreme conditions of heat and humidity are anticipated, and the risks of dehydration are higher, the following strategy of maximizing hydration before you start the activity can be a good preventative measure:

If you want a simple measure of the effectiveness of your personal hydration program, weigh yourself before and after a long rides (without clothes to avoid inaccurate weights from sweat soaked clothing). A pound of weight lost equals 16 ounces (1 pint or 2 cups) of fluid; a quart (4 cups) is 2 pounds. For the purposes of calculating your replacement needs, a standard water bottle (20 ounces) weighs about 1 1/4 pounds. With this information, you can tailor YOUR OWN fluid replacement program.

For those who practice the philosophy of "if a little is good, a lot is better", it should be mentioned that there are risks associated with over correcting fluid losses of exercise. There have been reports of hyponatremia (low blood sodium concentration) resulting in seizures in marathon runners who over replaced sweat losses (which contain both salt and water) with water alone. This is rarely a problem for cycling events less than several hours in duration (except under extreme environmental conditions of heat or humidity) and becomes a potential problem only for events lasting more than 5 hours.


Q. Do electrolyte drinks (those containing minerals such as sodium and potassium) provide an advantage over pure water alone?

A. Not for rides of 1 to 2 hours. When two groups exercised for 2 hours at 67% VO2 max (with average fluid losses of 2300 ml) there was no advantage to rehydrating with electrolyte drinks versus water alone. For longer rides, especially over 5 hours in duration (100 miles) or in conditions of extreme heat and humidity, using electrolyte containing drinks for sodium replacement will decrease the risk of dilutional hyponatremia. With the large volumes needed for rehydration in long events, palatability and digestive tract tolerance are important in the selection of a replacement fluid.

In extreme conditions you might consider adding a pinch of salt to each water bottle of electrolyte replacement drink. For example, Gatorade doesn't contain much sodium. This added salt will help to prevent hyponatremia. In the same way, salting your food liberally the day before a hot-weather ride may help and there are personal stories that this prevents cramps in some individuals. A word of caution, if you are on a sodium restricted diet, check with your physician to make sure that adding salt won't be a health hazard for you.

Additional thoughts on drinks for those longer rides (and keeping hydrated):


In summary, drinking 1 to 2 quarts per hour of plain water is adequate for rides of 1 1/2 to 2 hours. For longer rides, where the body's glycogen stores will be depleted, carbohydrate containing fluids take on 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 to 10 % concentration is the optimal. Glucose polymers provide the ability to increase total Calories per quart without risking the side effect of an unpalatable, sweet taste. Aside from palatability, there is no proven advantage of polymer containing drinks over simple sugar (glucose) drinks. Although there are many commercial drinks available, the old standbys of apple juice and cola drinks are probably the least expensive per Calorie provided. In the pre and post ride period, the high Calorie, easily absorbed, glucose polymer sports drinks offer an advantage for taking in large amounts of carbohydrate in the sweet spot of 30 - 60 minutes post ride aiding rapid rebuilding (or restocking) glycogen stores. For those of you interested in saving a few $$, take a look at this site for some ideas on homemade energy drinks.

For longer rides, don't forget the risks of overdoing rehydration with pure carbohydrate (electrolyte free) drinks alone. If you plan to ride more than two or three hours, it's worth considering a commercial electrolyte containing drink, and if you are going to be riding 5 hours or more, it is essential to pace your fluid replacement rate (and keep an eye on your weight during training rides to be certain you are not overcompensating).

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