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Dehydration and efficiency

Dehydration affects both physical and mental performance in all sports and at all levels, but it can be avoided (or at least minimized) by appropriate drinking strategies.

Many athletes dehydrate in competitions, especially long ones, even when it's not particularly hot. You can not rely on the thirst for rememberance to replace fluid lost through sweating - because one of nature's dirty tricks is that the activity quenches your thirst.

Background Physiology

Sporting activity produces heat. Preventing overheating by transferring heat from the skin through an expansion of the blood vessels of the skin circulation and by the cooling effect of perspiration. Physical activity causes loss of body fluid, both from moisture in the exhaled air and from sweating. Although you sweat the most under the conditions of high intensity activity in heat and high humidity, fluid loss can be significant even during very long sporting events - no matter what the conditions. If the fluid losses are not compensated by drinks, sweating causes a gradual depletion of the circulating blood volume, resulting in hypohydration (commonly called dehydration or dehydration) and thickening of the blood. This burdens the cardiovascular system and raises the heart rate to maintain adequate blood flow to the active muscles and vital organs. Because the blood volume is consumed, the blood flow to the skin is reduced. The result is less sweating and less heat dissipation from the skin. As a result, body temperature rises, potentially leading to heat stress, collapse or even death.

Even a low level of dehydration has physiological consequences. A loss of just 2% of body weight (just 1kg for a 50kg person) causes an increase in perceived effort and is expected to reduce performance by 10-20%. A fluid loss of more than 3 - 5% of body weight noticeably reduces aerobic exercise performance and affects response time, judgment, concentration and decision making - vital elements in all sports, from pole vaulting to football. Especially important for boxers is that dehydration also increases the risk of brain injury.

The two major factors that cause early fatigue and impaired performance (both physical and mental) in all sports and activities are the depletion of carbohydrate reserves in the body and / or its fluid reserves. It optimizes the benefits of exercise when you consume adequate amounts of carbohydrates and fluid because this will enable you to work harder and longer. That can make the difference between victory and defeat. The opportunity and ability to eat and drink during exercise and competition depends on the sport - sports drinks are one way of doing so: they provide adequate carbohydrate and sufficient fluid at the same time, are well tolerated when used correctly, and can under certain circumstances Individual needs.

There are guidelines for fluid and carbohydrate needs in various activities, but deciding what, when and how much an individual athlete should consume is not always straightforward. Especially the amount of sweat varies for people who are exposed to the same exercise conditions. In addition, the tastes are different - and the most important feature of any sports drink is its enjoyability (in large quantities, not just a sip)! Even the perfect drink has no value if it tastes awful.

sweat losses

Most of the research has focused on high intensity or long duration activities in the heat. The amount of sweat increases with the intensity of the activity and is exacerbated by a hot or humid environment and heavy clothing (especially dark colored garments that do not let air through), both of which hinder heat dissipation and perspiration.

Prediction of fluid and sodium losses with sweat is complicated because sweat intensity and sodium levels in sweat are widely different in individuals who are physically active under the same conditions. For example: Measurements of sweat levels during one hour of activity at 70% VO2max at an ambient temperature of 23 ° C were reportedly between 426 g and 1665 g. A study of tennis players reported 0.7 - 1.4 liters per hour in women and 1.2 - 2.5 liters per hour in men in hot humid conditions (32 ° C with a relative humidity of 60%)., The absolute volume of sweat is relatively low for a small, slender athlete. The training status and degree of heat acclimation affect the composition and volume of sweat by inducing the onset of sweating, increased sweat, and more diluted perspiration, thereby retaining salt in the body. The sodium concentration in sweat was reported to vary between 40 and 140 mmol / L in amateur runners, but only slightly higher than 20 mmol / L in tennis players acclimated to heat.

No matter what the environmental conditions, sweat losses are likely to be greater than some athletes estimate. The highest reported sweat volume is 3.7 liters / hour, that was Alberto Salazar during the 1984 Olympic marathon. Sweats of 2-3 liters / hour can be expected in short periods of high activity in the heat, and during endurance activities slightly above 1.5 - 2 liters / hour. Even under cooler conditions, the losses are significant. During a football game on a chilly day (10 ° C), players can lose up to 2 liters of sweat, and runners are estimated to hover about 1.2 at a speed of 6 km / h on a cool, dry day every hour Lose liters (double this amount on a hot, wet day). In addition, fluid is lost through the exhaled air.

Except in extreme circumstances, the level of activity in the blood is maintained at activity. In fact, sweat is mainly water, and the sodium losses in sweat are a small fraction of the body's total content and are then quickly replaced by normal food. There have been rare cases of hyponatraemia (salt deficiency in the blood), usually at events that last 8 hours or more, such as the Hawaii Ironman. This potentially lethal condition results from "water poisoning" - consuming large volumes of water or drinks that contain little or no sodium, or too little sodium to counteract the sweat losses that accumulate over many hours in hot conditions.

Carbohydrate Electrolyte Drinks and Performance

Since 1984, when American College of Sports Medicine claimed that water was the optimal drink for endurance activities, many studies have found the performance-enhancing benefits of electrolyte additives (of which the only useful is sodium, which accelerates fluid intake) and carbohydrates ( to deliver fuel).

Drinking only water is not considered a healthy diet in this case: it causes bloating, suppresses thirst (and thus further drinking) and stimulates urine output (it is therefore inefficiently retained) - a poor choice when high fluid intake is needed, Sports drinks generally contain 10 - 25 mmol / liter of sodium as a salt. Although this is second below the optimal concentration for stimulating fluid intake, the ideal concentration would taste like seawater and would be inedible.

The optimum carbohydrate concentration depends on the physiological requirements of the sport, the environmental conditions and the compatibility of the athlete. In endurance sports, carbohydrate degradation is a factor in early fatigue, but if the sweat level is high and the dehydration is fast, replacing the liquid has priority over the carbohydrates. Powdered sports drink formulas are convenient because they can be dissolved to suit the climate and individual athletes.

Carbohydrate isotonic electrolyte drinks generally contain 4 - 8% carbohydrates, are absorbed quickly (as fast or faster than water) and deliver fuel. The benefits of carbohydrate drinks in reducing fatigue are well documented: endurance cyclists and marathon runners can achieve significantly faster times by drinking carbohydrate-electrolyte drinks instead of water. And also in strength training, more repetitions of a given weight can be lifted by consuming carbohydrates.

Not surprisingly, the higher the intensity of the exercise, the faster carbohydrates are consumed and the sooner these glycogen reserves are consumed. A recent study was designed to mimic the physiological requirements of sports with many sprints, such as football, tennis and hockey. Their findings suggest that carbohydrate supply is beneficial for both physical and mental performance. In other words, carbohydrates not only delay fatigue, but also help maintain mental alertness and judgment.

(Hydrogenation in training and competition)

How many carbohydrates?

According to these results, it seems logical to advise that carbohydrate-electrolyte drinks should be consumed during endurance competitions as well as in team sports whenever it is necessary to abstain from eating. But what about the carbohydrate concentration? EF Coyle (see References) advises a 68 kg man during endurance sports to consume between 30 and 60 g carbohydrates per hour in the form of between 625 and 1250 ml / h of a 4-8% carbohydrate electrolyte drink (the Quantities should of course be adjusted for other body weights). In fact, some athletes find a 4-8% carbohydrate drink "heavy" and choose a lighter solution to better tolerate it - although carbohydrate drinks are better tolerated in sports activity with increasing practice. At ultra-high heat events, where high volumes (typically 1-1.4 liters / hour) are recommended, the fact that you are tired of the taste of your drink can be a problem. In that case, a variety of drinks and the ability to eat something carbohydrate-based (if you can tolerate it) can be helpful.

In most sports situations, drinks with more than 10% carbohydrate concentration are not advisable. But during long activities (more than 60-90 minutes) with high intensity, in very cold conditions and with low sweat levels, the breakdown of glycogen is a more serious factor than the dehydration - and then a drink can be up to 15% more useful if you can handle it.

Many endurance athletes suffer deficiencies in fluid and carbohydrate during the competition. Sandwiches and orange drinks would help replenish body reserves - but generally, you do not have an appetite immediately after a sporting event. Carbohydrate electrolytic drinks may be easier to handle, are absorbed quickly, are easy to keep and relatively easy to drink in quantity because their sodium content drives them to thirst. For these reasons, carbohydrate electrolyte drinks can be particularly valuable at multi-stage events such as the Tour de France, large tennis tournaments and two-day mountain marathons. Not only is the need for carbohydrates and fluids high at these events, but there is also a limited amount of time to recover and replenish them before the next day. In these situations, even athletes with a huge appetite have difficulty refilling glycogen reserves in their time with food alone. Carbohydrate electrolytic drinks can then be useful for replenishment.

Why do athletes dehydrate?

Dehydration is the progressive depletion of body fluid caused by fluid losses exceeding fluid intake (for whatever reason). This leads to hyperhydration (or dehydration). Athletes can already arrive hyperhydrated at the start of their event - for example, after a long, hot journey or "weight-bearing" strategies, eg. As in boxing, lightweight rowing and horse racing. Possible reasons for inadequate fluid intake during an event include:

- poor understanding of the fluid requirement. Because sweat levels vary so much and are likely to be larger than most athletes appreciate, a worthwhile plan is to record the fluid intake during exercise or event (also note the weather conditions) and weigh yourself before and after without clothing. This should serve to evaluate how well the fluid intake corresponds to the losses.

- Limited option to drink or limited availability of drinks. Orienteers and mountain runners both take little or no drinks with them and rely instead on running waters. If the availability of flowing waters can not be guaranteed for events lasting one hour or more, consuming 500 ml of a (carbohydrate) electrolytic drink 10 minutes before take-off is beneficial for later performance.

- bad drinking strategies. As I said before, the activity quenches the thirst, and the event itself can keep the athlete from drinking. The resulting hyperhydrogenation impairs gastric emptying and may cause stomach upset, nausea, or vomiting, which in turn restricts the ability to drink.

- poor compatibility of drinks during the competition. The nausea that many athletes attribute to sports drinks could be caused by dehydration itself or too concentrated drinks. Dehydration can be misunderstood as carbohydrate degradation. Drinks containing more than 10% carbohydrate interfere with gastric emptying (and therefore the amount of fluid supply) and stimulate the secretion of body fluid into the intestine to dissolve the drink before it can be absorbed. This makes the dehydration even worse.

Highly concentrated carbohydrate drinks can actually cause nausea, even more so with a dehydrated athlete. Carbohydrate-fortified drinks are likely to cause flatulence or discomfort, and fructose (fructose) in high concentrations may cause stomach disorders and / or diarrhea. Although fructose is included in some sports drinks, it is inefficient as the sole source of carbohydrates because it is slowly absorbed and requires conversion to glucose by the liver before it is available for energy.

Inability to compensate for large amounts of sweat. Sporting activities that exceed 70% of VO2max will progressively suppress gastric emptying. The highest known amount of gastric emptying is 2400 ml / hour for resting subjects. During exercise, gastric emptying is rarely higher than 1 - 1.2 liters / hour, unless a large amount is left in the stomach. Runners in particular feel uncomfortable when they have large amounts in their stomach. And in racing situations, athletes are unlikely to drink more than 2 liters / hour, even if they tolerate it. A certain degree of dehydration is therefore inevitable if the fluid losses are higher than the acceptable hydration capacity.

In practice, endurance runners and canoeists generally consume about 500 ml / hour in the competition and dehydrate between 500-1000 ml / hour. Even triathletes, who are generally well-versed in nutrition and hydrogenation, have been shown to lose an average of 1.7-3.7% of their body weight during a 3-hour or 12-hour event. While cyclists are known to tolerate 1.2 liters / hour of a 6% carbohydrate-electrolyte drink, runners are more likely to feel uncomfortable and perhaps feel the time to slow down by drinking lost, could not be recovered. The amount most athletes drink during exercise replaces less than 50% of their fluid loss. Keeping this in mind, effective post-event hydrogenation is vital, such as successive heating or events that last several days.


1, Foods, Nutrition and Sports Performance, ed. C. Williams and JT Devlin (1994), pp. 147-178

2, Clinical Sports Nutrition, Ed. L. Burke and V. Deakin (1994), pp. 333-364

3. EF Coyle (1994)., Fluid and carbohydrate replacement during exercise: how much and why ?. ' Sports Science Exchange, 50, Vol. 7, No. 3

4. Maughan, Leiper & Shirreffs (1996). 'Rehydration and recovery after exercise.' Sports Science Exchange, 62, Vol. 9, NR. 3

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