Rehydration Solutions

Rehydration solutions for athletes are generally designed to replace fluid and minerals lost by sweating and also limited amounts of energy in the form of CHO. All three substances are either lost or used during endurance exercise. Higher exercise intensities require a higher degree of energy production for which CHO as energy source is most suitable. Accordingly, with higher exercise intensities, more metabolic heat will be produced. Consequently sweat rate will be increased, as will the excretion of electrolytes. The longer the exercise lasts, the larger the amount of fluid, electrolytes and CHO needed to replace the losses.

There are large differences between individuals in sweat rate, sweat electrolyte content, degree of CHO utilization, etc. These differences can be further influenced by climatological circumstances. As a result, it is impossible to recommend a general rehydration solution that will exactly compensate for the losses of any individual in any situation. Commercial rehydration solutions are generally designed to cover the needs of a large exercising population under different circumstances. This is necessarily a compromise that has to be made by any producer.

General guidelines for the composition of rehydration solutions have been obtained from a large number of studies in the field of gastric emptying, intestinal absorption, fluid balance regulatory factors and fatigue/performance and have been summarized in a number of excellent reviews (30, 32, 40, 47, 69,113,114,126,130,153,155). The general outcome from these studies is that addition of small to moderate amounts of CHO to a drink does not delay gastric emptying and improves absorption, compared to plain water. The scientific rationale behind these findings is the fact that coupled glucose-sodium transport across the gut membrane is very fast and stimulates water absorption due to the osmotic action of these solutes when being absorbed (69, 114, 131).

The addition of electrolytes, in small quantities as lost by the whole body sweat, will influence neither gastric emptying nor absorption (153,154). The CHO fraction will contribute to the maintenance of a normal blood glucose level and will lead to a sparing of the endogenous CHO reserves (49, 75,

Time (min)

Figure 26 Gastric emptying rate after ingestion of a single bolus (600 ml) of an isotonic carbohydrate (7%)-electrolyte solution or with repeated drinking as usual in endurance events. Repeated drinking with 70 g CHO/litre does not lead to fluid accumulation in the stomach. Taken from data of Rehrer et al. (215)

Time (min)

Figure 26 Gastric emptying rate after ingestion of a single bolus (600 ml) of an isotonic carbohydrate (7%)-electrolyte solution or with repeated drinking as usual in endurance events. Repeated drinking with 70 g CHO/litre does not lead to fluid accumulation in the stomach. Taken from data of Rehrer et al. (215)

Figure 28 A triple lumen catheter, which is used to study fluid and substrate fluxes in the jejunum

T1 CD

O us

H CD

124, 148). The latter may influence protein breakdown, delay fatigue and thus influence performance (25, 43-45, 113, 127, 188, 189).

A large body of scientific evidence shows that different types of CHO in amounts of 30-80 g/l and sodium in amounts of 400-1100 mg/l induce a high rate of gastric emptying and fluid absorption (69, 114). A maximal fluid absorption rate seems to be a prerequisite only in the event that the quantity of fluid ingested balances or exceeds the quantity that can be absorbed at the same time (for example in the case of massive fluid loss in watery diarrhoea). However, fluid intake during exercise generally does not exceed 600 ml/h in runners or 800 ml/h in cyclists (143). This seems less than the amount that could be absorbed maximally. Therefore, it is still open for discussion whether a maximal rate of gastric emptying and absorption is always necessary for the exercising individual. Thus, slightly more concentrated CHO-electrolyte solutions (up to 100 g CHO per litre) are known to reduce the rate of net fluid absorption, but enhance CHO availability. In the case of submaximal fluid intake, more concentrated

Table 4 Oral rehydration solutions for combined fluid carbohydrate-electrolyte supply in sports

Recommended

Sodiuma

Osmolality

30-100 g/lb max. 1100 mg/l <500 mosmol/lc favourable < isotonicity

Chloride" max. 1500 mg/l Potassium0 max. 225 mg/l Magnesium" max. 100 mg/l max. 225 mg/l

Carbohydrate sources:

Fructose

Glucose

Sucrose

Maltose

Maltodextrins

Dispersible starch

Maximal amount of CHO (to avoid hypertonicity and/or a too high concentration6) 35 gd 55 g 100 g 100 g 100 g 100 g

a Quantities taken from Table 3.

b Water absorption becomes maximized with approximately 30 g CHO/litre. This is also about the minimum amount of CHO needed to achieve measurable effects on glucose/energy metabolism. The upper level (100 g) is given because gastric emptying rates and therefore fluid availability will be reduced too much at higher concentrations. Additionally the osmotic load of drinks containing more than 100 g will be increasingly effective in reducing the net fluid absorption. More concentrated solutions cannot be considered as rehydration drinks, but are energy (CHO) supplements.

c Net water absorption in the gut, after gastric emptying, is mainly determined by substrate absorption— which pulls water along, and by osmotic gradients. An increase in solute (carbohydrate) concentration will lead to a higher solute absorption and with it water absorption. An increase in osmotic load, however, enhances osmotic fluid secretion into the gut. Net fluid absorption results from two opposite water fluxes (absorption-secretion). Thus, hyperosmolality will counterbalance water absorption benefits achieved by solute transport. Osmolalities of >500 mosmol/l should be avoided.

d Fructose as sole CHO source may induce gastrointestinal distress at concentrations of >35 g/l. This is not the case in combination with other CHO (e.g. sucrose).

a Quantities taken from Table 3.

b Water absorption becomes maximized with approximately 30 g CHO/litre. This is also about the minimum amount of CHO needed to achieve measurable effects on glucose/energy metabolism. The upper level (100 g) is given because gastric emptying rates and therefore fluid availability will be reduced too much at higher concentrations. Additionally the osmotic load of drinks containing more than 100 g will be increasingly effective in reducing the net fluid absorption. More concentrated solutions cannot be considered as rehydration drinks, but are energy (CHO) supplements.

c Net water absorption in the gut, after gastric emptying, is mainly determined by substrate absorption— which pulls water along, and by osmotic gradients. An increase in solute (carbohydrate) concentration will lead to a higher solute absorption and with it water absorption. An increase in osmotic load, however, enhances osmotic fluid secretion into the gut. Net fluid absorption results from two opposite water fluxes (absorption-secretion). Thus, hyperosmolality will counterbalance water absorption benefits achieved by solute transport. Osmolalities of >500 mosmol/l should be avoided.

d Fructose as sole CHO source may induce gastrointestinal distress at concentrations of >35 g/l. This is not the case in combination with other CHO (e.g. sucrose).

Figure 30 A schematic representation of osmotic effects in the gut. Plain water perfusion will induce electrolyte secretion and water-electrolyte absorption. Hypertonic perfusion will induce water secretion and water-substrate absorption. Isotonic perfusion will induce substrate-water absorption. (Net absorption = absorption — secretion.) O, water; C, electrolyte; S, solute

Figure 30 A schematic representation of osmotic effects in the gut. Plain water perfusion will induce electrolyte secretion and water-electrolyte absorption. Hypertonic perfusion will induce water secretion and water-substrate absorption. Isotonic perfusion will induce substrate-water absorption. (Net absorption = absorption — secretion.) O, water; C, electrolyte; S, solute

Figure 31 Low amounts of CHO stimulate water absorption (left part of figure, 'A'). High amounts of CHO in a beverage reduce gastric emptying and induce fluid secretion, leading to a reduced net fluid absorption (right part, 'B'). 'A' leads to high fluid-low CHO availability. 'B' induces high CHO-low fluid availability. Maximal CHO availability, without impairing fluid homeostasis is found with beverages containing 60-80 g of CHO/litre. Optimal choice of a drink depends on climatological circumstances and physiological characteristics of the sports event. Reproduced from Brouns (32) with permission from Chapman & Hall, London

Figure 31 Low amounts of CHO stimulate water absorption (left part of figure, 'A'). High amounts of CHO in a beverage reduce gastric emptying and induce fluid secretion, leading to a reduced net fluid absorption (right part, 'B'). 'A' leads to high fluid-low CHO availability. 'B' induces high CHO-low fluid availability. Maximal CHO availability, without impairing fluid homeostasis is found with beverages containing 60-80 g of CHO/litre. Optimal choice of a drink depends on climatological circumstances and physiological characteristics of the sports event. Reproduced from Brouns (32) with permission from Chapman & Hall, London drinks have similar effects on fluid homeostasis to water or very dilute CHO solutions (32, 40, 114, 126, 127).

Flavoured drinks are preferred by athletes compared to plain water. Consequently such drinks are ingested in larger volumes (88). A general guideline should be that rehydration solutions should not be strongly hypertonic (i.e. <500 mosmol and preferably <300 mosmol). Drinks in the low hypertonic range (414 mOsm) do not differ significantly in rate of fluid absorption, urine production and plasma volume, from isotonic (297 mOsm) or hypotonic drinks (197 mOsm) (491). Hypertonic solutions have been shown to reduce the rate of net fluid absorption by inducing fluid secretion into the gastrointestinal tract. Additionally, they may also reduce the rate of gastric emptying. The latter may lead to feelings of fullness and influence/limit quantitative fluid consumption (30, 32, 114, 115, 155).

FLUIDS AND ELECTROLYTES 79

Table 5 Carbohydrate content and osmolality of selected drinks

Sport energy drinks Dextro Energy Fruit Exceed

Extran Orange Isostar Long Energy Leppin enduro booster Perform Energy drink Rehydration drinks AA Drink Aquarius Athlon

Enervit Tropical Extran Citron Rivella activ Gladiators Lucozade low cal Gatorade Isostar XL-1

Soft drinks and 'designer' drinks

Apple juice

104

695

Coca-Cola

105

650

Fanta

108

478

Orange juice

94

662

Sprite

110

591

Red Bull***

107

686

Taurus***

S119

795

Guarana Jones ***

100

613

Flying Horse***

107

862

CHO content as labelled on the product. Osmolality was measured by using a freezing point depression osmometer (227).

  • An osmolality of this magnitude may result in gastrointestinal distress during exercise. It is recommended to dilute these drinks by >100% to obtain an osmolality of <400.
  • The carbohydrate content is too low to result in significant energy support during exercise. *** Caffeine content of 320 mg/l.

CHO content as labelled on the product. Osmolality was measured by using a freezing point depression osmometer (227).

  • An osmolality of this magnitude may result in gastrointestinal distress during exercise. It is recommended to dilute these drinks by >100% to obtain an osmolality of <400.
  • The carbohydrate content is too low to result in significant energy support during exercise. *** Caffeine content of 320 mg/l.

110 72 145 152 97 165

68 63 62 70 75

330 400 274 320 332 96 392 148 378 281 291

The source of CHO will influence fluid osmolality. Therefore, so as not to result in very high osmolalities, the quantity of monosaccharides dissolved is recommended to be smaller than that of disaccharides or polysaccharides. Based on current knowledge and evidence, a general recommendation for the composition of oral rehydration beverages for sport is given in Table 4. Table 5 gives examples of the composition of commercially available sport drinks and other drinks.

Key points

  • Fluids and electrolytes are important for the maintenance of fluid balance during prolonged physical exercise, especially in the heat.
  • Progressive fluid loss from the body, by means of sweating and breathing, is associated with a decreased blood volume and blood flow through the extremities. Also a reduction in sweating and heat dissipation may result from this. Under circumstances of high intensity work in the heat, it may lead to heat stroke and collapse.
  • Dehydration of >1.5 litres is known to reduce the oxygen transport capacity of the body and to induce fatigue and gastrointestinal disturbances.
  • Appropriate rehydration is known to counter these effects and to delay fatigue. In contrast to plain water, the addition of CHO to rehydration drinks is known to stimulate drinking and water absorption and additionally to have a positive effect on water balance.
  • The carbohydrate supplied with the drink will also be of benefit for maintaining a high CHO availability, to help reduce fatigue and maintain performance capacity.
  • Addition of sodium to drinks will have a positive effect on postexercise rehydration by reducing urine loss and stimulating water retention. Other electrolytes may be added but should not exceed the levels of loss with whole body sweat; they have not been shown to have a beneficial effect on performance.
  • Sport rehydration drinks should in principle not be hypertonic.

III Nutritional Aspects of Micronutrients in Sport

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