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  Last updated: 11/04/2015


Although water is not a source of Calories to power your physical activities, adequate fluid intake and hydration are at least as important as Calorie replacement to maximize one's athletic performance. The single biggest error of many competitive athletes is a failure to replace fluid losses during training and competitive events. This is especially true in cycling where evaporative losses are significant and can often go unnoticed even though fluid loss with sweating and loss through the lungs can easily exceed 2 quarts per hour. (Respiratory fluid losses are not insignificant. Up to 60% of overall fluid loss can be via the lungs - which means that even swimmers can get dehydrated.)

To maximize your performance, it is essential that fluid replacement begin early and continue throughout a ride. A South African study comparing two groups of cyclists (one focusing on staying hydrated, the other not) exercising at 90% of their personal maximums demonstrated a measurable difference in physical performance as early as 15 minutes into the ride.

Fluid losses during exercise decrease the circulating blood volume as well as the water content of individual muscle cells. The impact on performance is directly related to the level of dehydration. 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 impact heat regulation, at 3% there is a measurable decrease in muscle cell contraction times, and when fluid losses reach 4% of body weight, there is a 5 to 10% drop in overall performance (which can persist for up to 4 hours after rehydration takes place). Thus it is essential to anticipate and regularly replace fluid losses. Thirst is not a reliable indicator of your state of dehydration as it is often not triggered until one has lost 0.8 - 2% (of body weight). Maintaining an adequate plasma volume is an important piece of an overall strategy to optimize your physical performance.

Hydration and Performance - is total volume replacement necessary?

This article highlights how conclusions as to the benefits of optimal hydration on performance have been changing over the last half century. Work from the 1960s through the late 1980s supported complete fluid replacement as the goal to maximize endurance performance. This was codified in 1996 in the American College of Sports Medicine (ACSM) fluid replacement Position Stand that urged at least the full replacement of sweat loss during exercise.

But then things started to shift. Based on more in depth studies and observations of real life performance, the ACSM revised its recommendations on fluid replacement in 2007 proposing 400-800 mL/h, varying depending on the athlete's size, environmental conditions, and exercise intensity. It suggested that the goal be shifted from maintaining complete rehydration to limiting fluid loss to less than 2% of body weight for optimal health and performance.

About this same time, the theory of a central governor and its impact on performance was being refined. The theory is that sensory inputs to a central area (in the brain) of neural exercise coordination can limit neural outputs (stimuli) to the exercising muscles to limit muscle cell firing and as a result protect the athlete from the harm of excesses in endurance and sprint activity. This theory suggested that it was the sensation of thirst, rather than the absolute level of dehydration that was the critical factor leading to a decrement in performance.

I came to the conclusion (after a literature review in 2014) "...the recent trend has been away from an excessive focus on fluid replacement - with an increased acceptance of "thirst" as a reliable indicator of personal fluid needs (much akin to using perceived exertion as a measure of exercise limits in training)." But the pendulum continues to swing and we now have a study questioning even that more liberal approach to fluid replacement - the focus on thirst as a better measure of an indicator of dehydration (and limited performance) than using weight loss or total % BW dehydration.

The details below are based on this article as well as comments excerpted from an additional article by the lead author.

Although this result is intriguing, it is just a single study and as such will need confirmation before we can take the conclusions to the bank. Here are 3 examples that indicate to me things are still not crystal clear: So what do I conclude? What am I going to do? Although this article suggests some latitude in fluid replacement, and gives an easy out to those of us who are not good at fluid replacement, common sense says that the more you stay in physiologic balance (homeostasis) and replace fluid (as well as glycogen) losses during and after a ride, the more competitive you (and I) will be in endurance events.


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 a minimum of 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):


Is there a minimum fluid intake per 24 hours that is needed to "flush out toxins" and help us maintain health? The short answer in this nice summary is "no". There is:
  1. No evidence that increasing fluid intake decreases kidney disease.
  2. No evidence that increasing fluid intake decreases the risk of heart disease or stroke (but note: once again there is a correlation between increased coffee intake and decreased cardiovascular risk)
  3. No evidence that increasing fluid intake improves skin tone and luster.


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).

And drink when you are thirsty - not to meet a predetermined minimal daily fluid intake.

Questions on content or suggestions to improve this page are appreciated.

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