Effects of drinking on exercise

Fluids and Electrolytes

7.4 Effects of drinking on exercise

Pre-exercise hydration

For an individual undertaking regular exercise, any fluid deficit that is incurred during one exercise ses-sion can potentially compromise the next exercise session if adequate fluid replacement does not occur.

As such, fluid replacement after exercise can fre-quently be thought of as hydration prior to the next exercise bout. However, in addition to this, the issue of pre-exercise hyperhydration has been investigated.

In a healthy individual the kidneys excrete any excess body water, and therefore ingesting excess fluid before exercise is generally ineffective at inducing pre-exercise hyperhydration.

The practice of drinking in the hours before exer-cise is effective at ensuring a situation of euhydration prior to exercise if there is any possibility that slight hypohydration is present. The current American College of Sports Medicine (ACSM) position stand

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on exercise and fluid replacement suggests that when optimising hydration before exercise the individual should slowly drink a moderate amount (e.g. 5–7 ml/kg) at least 4 hours before exercise. If this does not result in urine production or the urine is dark or highly concentrated, then more drink should be slowly ingested (e.g. 3–5 ml/kg) about 2 hours before exer-cise. This practice of drinking several hours before exercise gives sufficient time for urine output to return to normal. The guidelines also suggest that con-suming beverages that contain some sodium and/or small amounts of salted snacks or sodium-containing foods at meals will help to stimulate thirst and retain the consumed fluids.

Hydration during exercise

The diversity of sport and exercise training and competition, including intensity, duration, frequency and environmental conditions, mean that providing specific recommendations in terms of drink volumes and compositions and patterns of ingestions is not sensible. The current ACSM position stand on exercise and fluid replacement highlights this and concludes that the goal of drinking during exercise should be to prevent excessive (>2%) body mass loss due to a water deficit and to prevent excessive changes in electrolyte balance. In doing this, any compromise in performance should be minimised. The ACSM also conclude that because there is considerable vari-ability in sweating rates and sweat electrolyte content between individuals, as highlighted above, custom-ised fluid replacement regimens are recommended to all. This is possible because individual sweat rates can be estimated by measuring body mass before and after exercise.

Post-exercise rehydration

The primary factors influencing the post-exercise rehydration process are the volume and composition of the fluid consumed and the rate with which it is absorbed into the body. The volume consumed will be influenced by many factors, including the palata-bility of the drink and its effects on the thirst mecha-nism, although with conscious effort some people can still drink large quantities of an unpalatable drink when they are not thirsty. The ingestion of solid food, and the composition of that food, are also important factors, but there are many situations

where solid food is avoided by some people between exercise sessions or immediately after exercise.

Beverage composition Sodium

Plain water is not the ideal post-exercise rehydration beverage when rapid and complete restoration of fluid balance is necessary and where all intake is in liquid form. This was established some time ago when a high urine flow following ingestion of large volumes of electrolyte-free drinks did not allow subjects to remain in positive fluid balance for more than a very short time. These studies also established that the plasma volume was better maintained when electrolytes were present in the fluid ingested, and this effect was attributed to the presence of sodium in the drinks.

The first studies to investigate the mechanisms of post-exercise rehydration showed that the ingestion of large volumes of plain water after exercise-induced dehydration resulted in a rapid fall in plasma osmo-lality and sodium concentration, leading to a prompt and marked diuresis caused by a rapid return to con-trol levels of plasma renin activity and aldosterone levels. Therefore, the replacement of sweat losses with plain water will, if the volume ingested is suffi-ciently large, lead to haemodilution. The fall in plasma osmolality and sodium concentration that occurs in this situation reduces the drive to drink and stimulates urine output and has potentially more serious consequences such as hyponatraemia.

As sodium is the major ion lost in sweat, it is intui-tive that sweat sodium losses should be replaced. It is not logical to think that the salty water we lose as sweat would be best replaced in the body by plain water. This area has been systematically investigated and studies show that, provided an adequate volume is consumed, euhydration is achieved when the sodium intake is greater than the sweat sodium loss.

The addition of sodium to a rehydration beverage is therefore justified on the basis that sodium is lost in sweat and must be replaced to achieve full restoration of fluid balance. It has been demonstrated that a drink’s sodium concentration is more important than its osmotic content for increasing plasma volume after dehydration. Sodium also stimulates glucose absorption in the small intestine via the active co-transport of glucose and sodium, which creates an osmotic gradient that acts to promote net water absorption. However, this sodium, to assist

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intestinal absorption, can either be consumed with the drink or be secreted by the intestine. Sodium has been recognised as an important ingredient in rehydration beverages by an inter-association task force on exertional heat illnesses because sodium plays a role in the aetiology of exertional heat cramps, exertional heat exhaustion and exertional hyponatraemia.

Potassium and magnesium

Potassium, as the major ion in the ICF, has been postulated to have a role in optimising post-exercise rehydration by aiding the retention of water in the intracellular space. Potassium is lost in sweat in concentrations of about 5–10 mmol/l. Initial work using dehydrated rats supported this idea, indicating that the role of potassium in restoring intracellular volume is more modest than sodium’s role in restoring extracellular volume. However, subsequent work in humans has proved to be less conclusive.

Potassium may therefore be important in enhancing rehydration by aiding intracellular rehydration, but further investigation is required to provide conclusive evidence. Importantly, however, no negative effect of including modest amounts of potassium in rehydration drinks has been demonstrated and indeed potassium, in small quantities, is an ingredient in most commercially available sports drinks that suggest they have a role in post-exercise rehydration.

The importance of including magnesium in sports drinks has been the subject of much discussion.

Magnesium is lost in sweat in small amounts (<0.2 mmol/l) and many believe that this causes a reduction in plasma magnesium levels that are impli-cated in muscle cramp. Even though there can be a decline in plasma magnesium concentrations during exercise, it is most likely due to compartmental fluid redistribution rather than to sweat loss. There does not therefore seem to be any good reason for includ-ing magnesium in post-exercise rehydration and recovery sports drinks.

Drink volume

Obligatory urine losses persist even in the dehydrated state, because of the need for elimination of metabolic waste products. The volume of fluid consumed after exercise-induced or thermal sweating must therefore be greater than the volume of sweat lost if effective rehydration is to be achieved. This contradicts earlier recommendations that after exercise athletes should

match fluid intake exactly to the measured body mass loss. Studies have investigated the effect of drink volumes equivalent to 50%, 100%, 150% and 200% of the sweat loss consumed after exercise-induced dehydration equivalent to appro ximately 2% of body mass. To investigate the possible interaction between beverage volume and its sodium content, a relatively low-sodium drink (23 mmol/l) and a moderately high-sodium drink (61 mmol/l) were compared.

Subjects could not return to euhydration when they consumed a volume equivalent to, or less than, their sweat loss, irrespective of the drink composition.

When a drink volume equal to 150% of the sweat loss was consumed, subjects were slightly hypohydrated 6 hours after drinking when the test drink had a low sodium concentration, and they were in a similar condition when they drank the same beverage in a volume of twice their sweat loss. With the high-sodium drink, enough fluid was retained to keep the subjects in a state of hyperhydration 6 hours after drink ingestion when they consumed either 150 or 200% of their sweat loss. The excess would eventually be lost by urine production or by further sweat loss if the individual resumed exercise or moved to a warm environment. Whilst other studies have also shown the importance of drinking a larger volume of drink than the sweat volume lost, an interaction between sodium intake, volume intake and whole-body rehydration has not always been reported. However, it seems likely that in these studies the length of subject observation after rehydration may not have been sufficient to observe the urine production response to the treatments. Additionally, evidence has recently emerged suggesting that the rate of drinking or the rate of delivery to the intestine for absorption of a large rehydration bolus can have important implications on the physiological handling of the drink. Drinking a large volume of fluid has the potential to induce a greater decline in plasma sodium concentration and osmolality, which in turn have the potential to induce a greater diuresis by the mechanism described above.

Food and fluid consumption

There may be opportunities to consume solid food between exercise bouts, and in many situations doing so should be encouraged to meet other nutritional goals unless it is likely to result in gastrointestinal disturbances. The role of solid food intake in

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promoting rehydration from a 2.1% body mass sweat loss with consumption of either a solid meal plus flavoured water or a commercially available sports drink has been investigated. The urine volume pro-duced following food and water ingestion was almost 300 ml less than that when the sports drink was con-sumed, resulting in a more favourable recovery and maintenance of hydration status. Subsequent studies have also highlighted a role for food products in post-exercise fluid balance restoration.

Beverage palatability and voluntary fluid intake

In the majority of scientific studies in the area, including those described above, a fixed volume of fluid was prescribed and consumed. However, in everyday situations that athletes find themselves in, intake is determined by the interaction of physiological and psychological factors. When the effect of palatability together with the solute content of beverages in promoting rehydration after sweat loss was studied, subjects drank 123% of their sweat volume losses with flavoured water and 163% and 133% when the solution had 25 and 50 mmol/l sodium respectively. Three hours after starting the rehydration process the subjects were in a better whole-body hydration status after drinking the sodium-containing beverages than the flavoured

water. Similar results have been reported in other research and together these studies demonstrate the importance of palatability for promoting con-sumption, but also confirm earlier results showing that a moderately high electrolyte content is essential if the ingested fluid is to be retained in the body. The benefits of the higher intake with the more palatable drinks were lost because of the higher urine output.

Other drink characteristics, including carbonation, influence drink palatability and therefore need to be considered when a beverage is being considered for effective post-exercise rehydration.

8.1 Introduction

The micronutrients (vitamins, minerals, electrolytes and trace elements) are required in small amounts (micrograms to milligrams) and do not provide any measurable energy or kilojoule content in foods.

Micronutrients work interactively to (i) regulate energy metabolism, nervous function and muscle contraction, (ii) regulate oxidative function, (iii) maintain bone and blood health, (iv) control fluid and electrolyte balance, and (v) assist with immune function.

Micronutrients do not work in isolation. Vitamins interact with each other or with other minerals to regulate biological and metabolic processes and are grouped into fat-soluble (A, D, E and K) and

water-soluble (B complex and C) categories based on their different solubility. Minerals are grouped into macrominerals (e.g. sodium, potassium, calcium, phosphorus and magnesium) and trace elements (e.g. iron, zinc, copper, chromium and selenium).

Dietary Reference Intakes (DRIs) for macrominerals are more than 100 mg/day, whereas trace elements are required in smaller quantities (<20 mg/day).

Minerals, whether they exist in large amounts (such as calcium and phosphorus in bones) or in trace amounts, also regulate metabolic functions.

Micronutrients are often considered separately, which can often encourage the concept that they work in isolation. This concept is fostered by the vast array of single vitamin and mineral supplements available in the market and the often exaggerated

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Micronutrients

In document Sport and Exercise Nutrition (the Nutrition Society Textbook) (Page 76-80)

Fluids and Electrolytes

7.4 Effects of drinking on exercise

Further reading

8.1 Introduction

8

Micronutrients