During the summer days, the problem of nutrient loss through sweat becomes enormously important. What is the “healthy measure” in fluid intake and what to look for in sports drinks?
With few exceptions, sport – as a strained umbrella term for physical activity – is movement, made possible by the approximately 600 muscles of the human body. Whether this will always be the case is another question, considering the rapid development of e-sports, which recently even has Olympic ambitions. Currently, however, it still takes the efficient work of a large portion of these muscles to perform the precise actions involved in athletic activity.
A prerequisite for movement is the coordinated interaction of a certain number of muscles. Muscle contraction, triggered by impulses from the central nervous system, requires energy, which is mainly obtained from the breakdown (i.e. burning) of sugar and fat. The efficiency of this “human machine” is about 25%, which means that out of 100% of the energy used, only about a quarter is converted into mechanical energy or electricity. movement is converted; the efficiency of the human organism is thus not much lower than that of a gasoline engine, but it is significantly lower than that of an electric car (80%).
During the combustion process, therefore, almost three quarters of the fuel consumed is lost in the form of heat – with the consequence that body temperature can rise to 41°C during intense exertion (e.g. during a marathon). Without a well-functioning protection system, this temperature rise could continue to harmful levels. Think of protein that gets onto a hotplate while it is still lukewarm: after a short time, it has changed in such a way that it is functionally denatured. In the case of overheating of the body caused by athletic performance, it would look similar inside the body, which is why this emergency must be prevented by appropriate cooling.
Thermoregulation through transpiration
The human cooling system consists of four main mechanisms: radiation (about 40%), convection (about 20%), conduction (about 10%), and evaporation, which is responsible for a good 25% of heat removal. From the point of view of sports medicine, the latter mechanism is the most important, since athletic performance is closely related to the body’s water balance. Sweat secretion and the associated elimination of excess heat are thus of vital importance. However, through sweating, the body not only releases heat, but also loses water, which makes up 99% of sweat composition, and various other important substances, because useful for muscle work, such as salts (Na, Cl, K, Ca, Mg, Zn), urea, uric acid, amino acids, fatty acids, ammonia, sugar, lactic acid and ascorbic acid (Vit C). The specific composition of sweat differs depending on whether it is eccrine or apocrine sweat. Sweat, which is responsible for thermoregulation, is predominantly eccrine. With every liter of sweat, 500 kcal of heat can be removed. Previous research found that any fluid loss greater than 2% of body weight results in a reduction in physical performance [1]; at a fluid loss of 4% of body weight (2.8 L for a 70 kg athlete), the decrease in performance is already approximately 20%. Such an amount is not unusual, as studies have shown that even in our climatic conditions fluid losses through sweat of up to 5 liters are possible [2].
The volume of fluid lost depends on the outside temperature, the intensity of performance and the type of sport; when cycling, the airstream cools significantly, which reduces the amount of sweat. In this complex system of energy production → heat → heat accumulation → power loss, thirst plays an essential regulatory role. Thirst is the organism’s alarm signal of fluid loss. It limits the effectiveness of the system and thus saves it from serious damage. However, drinking – “quenching thirst” – cannot completely prevent the drop in performance. It is assumed that losses up to 200 g per hour are 95% replaced by drinking. With a loss of 750 g, only 50% is compensated spontaneously. However, even if the organism were able to replace the missing fluid losses at once, there would be an overload of the stomach and the digestive tract and thus a delay in absorption with the corresponding consequences for performance. In addition, a full stomach during a continuous performance is highly undesirable. Therefore, it is important to tackle endurance competitions in hot weather with filled fluid reserves, i.e. to drink enough before the start of the test and to consume sufficient fluids during the competition and depending on the outside temperature. It is important to drink small amounts at regular intervals (1-2 dl every 15 minutes).
Drinking during exercise
The logical conclusion is therefore: fluid losses must be compensated. However, this seemingly self-evident conclusion was far from always true. In the sixties it was even said: “If you want to win, give your opponent to drink”! It was only a few years later, under the impetus of American physiologists such as David Costill, that the first sports drinks appeared on the market: Gatorade, XL-1, Isostar, Perform4, to name just the first. They were offered in powder form, water had to be mixed with them.
In addition to various salts, sports drinks also contain carbohydrates in the form of sugar, which is intended to cover the increased energy requirements during the performance phase. However, sugar absorption can only take place in limited quantities, otherwise the stomach will rebel: Too high a sugar concentration is compensated for in the stomach by water reabsorption, i.e. the loss of fluid occurs not only through sweat to the outside, but additionally through compensatory mechanisms in the stomach, quasi to the inside. The administration of sugar through sports drinks is crucial, because who does not know the famous “hole in the stomach” of the runner, which makes the best-trained legs slacken?
However, creating the optimal sports drink is not easy, because the requirements are different. Whether a specific sports drink is preferable to apple spritzer depends largely on the type and duration of the exertion and the ambient conditions: For units under one hour, it is sufficient to replenish the stores after the activity. Furthermore, individual factors such as taste and tolerance play a not unimportant role. Incidentally, sports drinks can easily be made by the user; Table 1provides an overview of the corresponding proportions. Simplified, sports drinks always have the combination of water + salts + carbohydrates to address the two main deficiencies: the loss of fluid due to sweat and the glycogen depletion in the muscle due to the higher consumption related to performance. The aim is to compensate for deficiencies as quickly and permanently as possible without placing too much strain on the reduced gastrointestinal tract.
In the search for an appropriate compromise, the stomach plays an important role, because it determines how quickly the ingested beverage reaches the intestine, where nutrient absorption occurs. Gastric emptying is a complex phenomenon controlled, among others, by various gastric wall receptors that react either to the distension of the ingested substance (mechanoreceptors), to its composition (chemoreceptors) or to its concentration (osmoreceptors). These receptors, together with other regulators from the intestine and the autonomic nervous system, define when the alimentary bolus is transported further into the duodenum. On an empty stomach, taking a glass of water only stimulates the mechanoreceptors, which makes the stomach open without any problems. However, a glass of a sports drink activates all types of receptors, stimulates the intestines and greatly delays the emptying of excess substances. Current research is trying to circumvent this unfavorable situation, and it seems that hydrogels are able to fool the stomach: As with water, only the mechanoreceptors are stimulated. Whether the intestine is capable of absorbing the amount of nutrients contained in hydrogels is another question. Affaire à suivre!
The healthy measure
For years, the scientific recommendation to endurance athletes was to actively address the problem of fluid loss: drink, drink and drink again! However, some recent incidents have shown, sometimes tragically, that it is possible to drink too much during endurance sports. Overcompensation can lead to hyponatremia, a decreased concentration of sodium ions in the blood serum. This more than serious situation is not unique in the marathon scene, and studies on the occasion of famous races like the Boston Marathon have shown that up to 13% of those tested had crossed the finish line with pathologically low blood sodium levels, fortunately far from all with fatal consequences [3]. This diagnosis already has a name: exercise-associated hyponatremia (EAH). Dilutional hyponatremia promotes a transfer of water into the intracellular space, provoking cell edema and possibly an increase in intracranial pressure in the brain. Fortunately, EAH can also be asymptomatic or cause only mild, nonspecific symptoms such as nausea and lightheadedness. The symptomatic form manifests with headache, vomiting, and mental changes such as confusion to seizures secondary to cerebral edema. Pulmonary edema is also possible. The therapeutic measure for pulmonary edema is emergency intravenous administration of hypertonic saline solutions without waiting for laboratory results beforehand. This sport-specific clinical picture must be absolutely known to supervising sports physicians!
In the search for permissible performance-enhancing measures, the efficient management of physical fluid balance occupies a central position. What is efficient in an individual case, however, must be tested and practiced – not only in competition, but already in training. In this sense: Cheers!
Literature:
- Armstrong LE, Epstein Y: Fluid-electrolyte balance during labor and exercise: concepts and misconceptions. Int J Sport Nutr 1999; 9(1): 1-12.
- Römer K, Boano CA: Core body temperature in marathon runners: Wireless monitoring by infrared sensor. Medical Sports Network, Succidia 2013; 02.13: 28-29.
- Almond C, et al: Hyponatremia among Runners in the Boston Marathon. N Engl J Med 2005; 352: 1550-1556.
- Mettler S, Colombani P: Guide to sports drinks. Swiss Forum for Sport Nutrition. 2013.
HAUSARZT PRAXIS 2018; 13(8): 4-5
