Fluid: the one thing your body can’t learn to do without

You can train yourself to cope with many situations: adjust to crossing time zones, adapt to climate changes and acclimatise to altitude; you can learn to use carbohydrates more efficiently and to make fats go further. But there’s one thing you can’t learn to do without – and that’s water.

The human body has a core temperature of 37C. Fluctuations can only be tolerated within a narrow range, and an increase of only 5C or a loss of just 10C can be fatal. During exercise, muscular activity adds a considerable amount to heat production; metabolic rate can be increased as much as 25-fold, producing heat at a rate of 80kJ min-1 (20kcal min-1) and leading to rapid increases in core temperature.

The body’s primary line of defence against a rising temperature is evaporation by means of increased sweating; the rate of heat loss by evaporation can be enough to regulate body temperature within 2-3oC of resting levels, even in a hot environment. For every litre of vaporised water, 2.4MJ or 580kcal of heat is removed from the body. With several million sweat glands covering the body’s surface skin, the potential for water loss is great. In response to heat stress, these glands are opened, allowing secretion of sweat onto the skin, which then evaporates into the environment. It is not the production of sweat but its evaporation from the skin that allows the skin to cool. If the surrounding air is cool and has a low humidity, sweating is much more efficient. If the environment is warm, sweat is still produced but is unable to evaporate so that the cooling effect is reduced.

Sweat rates can reach 2-3 litres per hour in extreme circumstances, and water loss will rapidly reach a significant level. If fluid losses are not matched by increased fluid intake, exercise performance will deteriorate as body temperature rises unchecked. In other words, this very useful mechanism of controlling core temperature through evaporation of sweat is lost if fluids are not replaced. Furthermore, if an athlete is already dehydrated before exercise, the ability to lower body temperature by evaporative sweat loss is rapidly reduced.

Fluid balance is a vital issue in many situations, including marathon or Ironman training. For athletes travelling overseas to compete or for warm weather training, it is particularly important to prevent the debilitating effects of dehydration and overheating by practising a carefully planned drinking strategy. But even indoor training sessions in the gym or at home pose risks to fluid balance because still indoor air is not conducive to sweat evaporation. In every situation, though, adequate and effective fluid replacement will allow maintenance of optimal performance.

When exercise duration exceeds an hour, athletes must also bear in mind the need for carbohydrate replacement. Carbohydrate is the main source of fuel for muscles during endurance exercise but, if stores are not frequently topped up, fatigue due to glycogen depletion occurs. By regularly ingesting small amounts of carbohydrate it is possible to delay fatigue by up to an hour.

Unfortunately, though, this is not simply a matter of adjusting the carbohydrate content of a drink. An ingested drink only becomes useful to an athlete after it has left the stomach, passed through the intestine and been absorbed into the blood stream, where it maintains blood glucose concentration. Typically, stomach emptying occurs at a rate of around 600ml per hour, and increasing the carbohydrate concentration of a drink actually reduces the rate of emptying. Therefore a high-carb drink will deliver less fluid than either plain water or a low-carb drink.

A drink with a 2-3% carbohydrate content will deliver fluid at almost the same rate as water; a 4-8% carbohydrate drink will reduce the rate of emptying slightly but insignificantly. Optimal for prolonged moderate-intensity exercise is a 6-8% carbohydrate content, which helps to replace carbohydrate losses without greatly hindering the emptying of fluid from the stomach. This is the range of concentration offered by most sports drinks, including Lucozade Sport, High5 and Isostar. If a greater rate of carbohydrate delivery is required, you need to choose a drink with carbohydrate concentration of 10% or more, eg Coca Cola.

Drinking strategies must be rehearsed

The volume of fluid in the stomach also affects the rate of emptying, in that a larger volume encourages more rapid emptying, and vice versa. Starting exercise with a moderate amount of fluid in the stomach (about 500ml) and topping it up every 15 minutes will allow you to maintain a high rate of emptying. But this is a strategy that must be rehearsed before competition and often takes some time to get used to.

A study I was involved with as an undergraduate student nicely demonstrated the two major factors affecting fluid uptake from the stomach. The study contrasted two drinks – one containing 2% glucose and the other 10% – delivered in two different ways: in one pair of trials, subjects ingested only a single 600ml bolus of each drink and in another they initially drank the same amount of either drink, ‘topping up’ with a further 100ml every 15 minutes for an hour.

The rates at which the fluids left the subjects’ stomachs were measured, and the results are summarised in table 1. The highest rate of emptying from the stomach occurred when the 2% solution was drunk and then topped up, but the greatest energy delivery occurred with the 10% top-up trial. Thus, increased fuel delivery comes at the expense of fluid, and vice versa.

Table 1

Trial Volume of fluid emptied (ml) Amount of CHO emptied (g)
600 ml 2% CHO 580 12
600ml 10% CHO 490 46.8*
600ml 2% + 100ml/15 mins 770* 16
600ml 10% + 100ml/15 mins 580 61*

*Significantly more than other trials

Sports drinks come in a range of carbohydrate concentrations and flavours, and every athlete should be able to find something to his or her taste. Whether or not you like the taste of a particular drink is as important as its content and volume, because the plain fact is that if you don’t like it you simply won’t drink enough. Table 2 shows some of the drinks currently available.

Table 2

Drink Carbohydrate content (%) Flavours available
Gatorade 6 Tangerine, orange, berry, lemon-lime
Gatorade Energy 20 Orange, wild berry, grape
Isostar 8 Lemon, orange, grapefruit
Lucozade Energy 17
Lucozade Sport 6 Orange, berry
PowerAde 8 Mountain Blast, Fruit Punch, Lemon-Lime, InfraRed Freeze,Arctic Shatter, Green Squall and Jagged Ice (not all available in UK)
Coca-Cola 10

For an athlete performing prolonged exercise on a very warm, humid day, fluid replacement is the prime need, with carbohydrate replacement being a secondary consideration. Therefore a low-carbohydrate content sports drink, such as Gatorade or Lucozade Sport, will be the best option. For a home-made alternative, mix 200ml of concentrated orange juice with 1 litre of water. Ingesting 200ml every 15 minutes can maintain sufficient volume in the stomach to promote emptying as well as providing enough water and carbohydrate to meet temperature regulation and muscle glycogen needs. If fluid needs are of even greater significance, a 4% carbohydrate concentration may be more suitable; to make this at home, reduce the concentration of orange juice to 100ml.

On a cooler day, when high-intensity exercise is planned, carbohydrate replacement becomes the primary need because fluid losses will be lower and carbohydrate usage higher. In this case, a sports drink with a higher carbohydrate content would be best, such as Coca-Cola or a drink made up of 400ml orange juice with 1 litre of water. In extreme cases, when optimal carbohydrate replenishment is required, Lucozade Energy contains 17% carbohydrate and Gatorade Energy a massive 20%. In longer events, it may be appropriate to choose a 2-3% drink for the first hour to promote fluid uptake, then move onto a 6-8% drink in the later stages of the race, when glycogen replacement is more important.

There is a considerable amount of individual variation in fluid loss, with some people sweating more profusely than others and some being able to empty fluids from their stomach more rapidly. This makes it difficult to be too prescriptive about how much an athlete should drink. One way to work out how much fluid you lose during exercise is to weigh yourself immediately before and afterwards: any loss in weight will be due to sweat loss, and indicates the amount you need to ingest to maintain fluid balance.

Finally, follow these tips to achieve adequate fluid intake during exercise:

  • Be aware of your fluid requirements;
  • Make fluids as readily available as possible;
  • Ensure you are well-hydrated before exercise (500ml of a 6% sports drink 1 hour beforehand will suffice);
  • Empty your bladder before exercise;
  • Drink small amounts regularly during exercise;
  • Start drinking before you feel thirsty;
  • Practise a drinking régime during training;
  • Aim to drink 150-200ml every 15 minutes.

It really boils down to establishing your body’s particular fluid requirements, finding out what you can tolerate and then making sure it is available when you really need it. Don’t blow up for want of a water bottle!

Clare Miller

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