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.

• Study subjects - 11 competitive cyclists and triathletes with experience with time trials
• Study protocol - a 90 min ride at a fixed rate (50% VO2max) followed by a 20 km TT. The only feedback received during the TT was time every 1 km. For all trials, participants had an IV in place. During both the 90 min steady ride and the 20 km TT, they either had 100% of their weight loss replaced with saline (warmed so they couldn't feel its infusion) versus the IV as a sham with no fluid replacement. This avoided a conscious awareness of fluid replacement.For some trials, participants were able to rinse their mouth with water, as much as desired, while in other trials no rinsing was permitted.
• Results were grouped -
  1. hydrated and not thirsty (rinsing allowed)
  2. hydrated but thirsty (no rinse)
  3. dehydrated and thirsty
  4. dehydrated but not thirsty.
• Study conclusion: Neither physical dehydration of up to 3% body weight, nor the associated sensation of thirst impaired performance of about 2 h in the heat.

• Study 1 - A 2 to 2.5% decrease in body weight from pre-exertion dehydration significantly decreased time to exhaustion and power output on a subsequent hill climb challenge.
• Study 2 - An additional 1% decrease in body weight (overall 2.2 % total decrease compared to 1.2% decrease in the group allowed ad lib fluids) led to a 6 % (1 minute) increase in total time for a 5 K hill climb.
• And finally study 3 that demonstrated that a 1% decrease in body weight affected speed and mean power output by approximately 1% in 3 cycles of a simulated 5 K hill climb.

• A relentless focus on total replacement of actual losses is not required in endurance activities.
• Drinking when thirsty will increase the rider's overall sense of well being and help the cyclist maintain a positive attitude (self confidence) as to their ability to maintain that level of exertion.
• There is little support for the idea that thirst alone has a subconscious impact (central governor theory) to limit performance.
• There is conflicting evidence on the impact of water losses of more than 1% BW on endurance performance.
• And finally, as pointed out in the review article, there is still an information void on the impact of dehydration on physiologic recovery, including such peripheral issues as glycogen replacement, which could be magnified in multi day (or stage) events.

Hydrating

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)?

• Caffeine - if you are a regular, daily caffeine user, your body will adapt to its diuretic (water losing) effects. On the other hand, if you have been caffeine free for 5 days, and then drink the equivalent of 6 cups of coffee (642 mgs of caffeine) over a 24 hour period, you will induce a negative fluid balance of nearly 0.8 kg (equivalent to 3 cups of water).
• Alcohol - this will decrease ADH secretion, a hormone that aids in water retention in the kidneys, and result in an increase loss of water via the kidneys. For every 2 drinks, you should take an additional 1 cup of fluid per day.
• Environmental factors - heat, humidity, and altitude

• drink 20 oz of cool water 2 hours before exercise
• 8 to 16 oz 30 minutes before
• and then 4 to 8 oz every 15 minutes on the bike

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.

HYDRATION TIPS

• Hydrate before, during, and after the ride - force yourself to drink as thirst alone will not reflect complete rehydration, so learn to drink before you are thirsty. Using a CamelBak or similar device on long rides will eliminate worries about stopping and possibly losing your group. Watch the color of your urine, if you are doing a good job on replacement it should be colorless.
• 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. As the sweet taste often keeps you from drinking, dilute it or take an extra bottle of plain water to alternate.
• Keeping liquids cool has been shown to increase intake on a ride - either add ice the day of the ride or freeze half a water bottle of fluid the night before and top it off with water from the tap or extra sports drink just before the ride.
• Weigh yourself before and after the ride - most of your weight loss will be fluid (2 pounds equals 1 quart or "a pint's a pound"). A drop of a pound or two won't impair performance, but any more and you need to reassess your personal hydration program. A gain of more than 1 or 2 pounds suggests you are over compensating. This is an especially important strategy in hot weather where fluid losses can easily exceed several quarts an hour.
• Wear the right clothing - light colored to reflect heat and a loose weave jersey to help keep you cool and lessen sweat losses.
• 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 will insulate your head from the heat of the sun's rays.

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):

• Carbohydrates. Two hours is the point at which carbohydrate supplements in a fluid replacement will consistently improve performance by supplementing your internal glycogen stores. Cyclists often drink large volumes while competing and in extreme events, such as the Tour de France for example, competitors have been able to replace up to 50% of their energy expenditures drinking 20% carbohydrate solutions at a rate of 2 to 4 quarts an hour. If you'd like, you can calculate your exact Caloric replacement needs based on the duration and average speed of you ride. For a rough estimate, you will need about 1/3 gram of carbohydrate per pound of body weight per hour to replace Calories expended.

  Certain carbohydrate containing liquids are more quickly emptied from the stomach and the sugar they contain more quickly absorbed into the bloodstream to be delivered to the muscles as an energy alternative to muscle glycogen. Drinks using glucose polymers can deliver additional Calories per ounce of fluid while remaining iso-osmotic) .

• Temperature of your drink. The temperature of replacement fluids MAY impact the rate of stomach emptying and absorption into your circulatory system - colder liquids empty more slowly and have the potential to produce nausea as well, thus delaying the delivery of electrolytes, water, and glucose into your system. On the other hand, in certain situations, cooler fluids may be more palatable and thus help to keep you cool (a positive for a ride in extreme conditions). The balance point for drink temperature depends on your personal physiology and the ride conditions, so no absolute recommendations as to the "best" temperature can be made. The same considerations apply to post ride drinks. If you are under time constraints to get back to a job after a noon workout, a cool fluid can help you cool down more quickly and cut down your "sweat time".

• Real fruit juices. No studies have confirmed a benefit of fruit drinks (which contain fructose) over glucose drinks. Although fructose requires less insulin to enter muscle cells, it does not appear to provide a performance advantage for cycling. Taste alone is the advantage.

• Concentration. For many years it was believed that a 2.5% concentration of glucose or glucose polymer molecules was the maximum tolerated without delaying gastric emptying and causing nausea. However a recent study, in cyclists, demonstrated normal gastric emptying with 6 to 8% solutions, and nausea occurred only when concentrations were pushed above 11%. The old standbys - apple juice and cola drinks - have a sugar concentration of around 10%. Although glucose polymer sports drinks can provide more Calories per quart (concentration being equal) studies have failed to demonstrate a performance advantage of complex carbohydrate drinks over simple sugar drinks alone (assuming the same total Calories were ingested). The advantage of the polymers was an absence of a sweet taste and the nauseating properties of high concentration glucose drinks - a potential barrier to maintaining an adequate fluid intake.

• Volumes. The stomach does have volume limits - for most riders this is around 800 ml (approximately 1 quart). This is particularly an issue when pushing aerobic limits (gastric emptying diminishes as exercise approaches 100% VO2 max). If larger volumes are forced, nausea and abdominal distension can result. For reference, a regular water bottle is 1/2 quart, 16 ounces, or 480 ml. and the large ones are 3/4 quart. Under normal conditions, you should be able to drink at least 2 bottles per hour.

ARE YOU HEALTHIER ON 8 GLASSES OF WATER A DAY?

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

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.