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Fluid Intake

What Fluid Intake is Needed for Exercise?

One of the primary differences between the diet of the general population and that of an athlete is that athletes require additional fluid to cover sweat losses.

Exercise and competition performances are better when an athlete is properly hydrated.  Athletes perform less well if they are dehydrated.  Dehydration increases the risk of potentially fatal heat conditions such as heat stroke.  Athletes need to be aware of how to remain well hydrated before, during and after exercise.

THE FUNCTION OF WATER IN THE BODY

Water is essential in the body, and is needed in significant quantities to maintain life.  Water is important in the body because:

·         Water is a major component of body cells

·         Water cannot be compressed, and this makes it perfect for the protection of body tissues (eg: the brain)

·         Water maintains the body’s electrolyte balance

·         Blood is mostly water.  Blood is the major vehicle for transportation of substances, including oxygen around the body

·         For proper functioning of our senses including sight, hearing, smell, etc, significant water is needed

·         Water is involved in regulation of body temperature.

HOW MUCH FLUID IS NEEDED?

The amount of water needed varies with the weight of the athlete.  Under normal conditions, the average adult will need about 1 ml of water per Calorie of energy intake.  This amount will be higher if the person is exercising, using diuretics (including alcohol and caffeine), or is on a high protein diet (protein produces urea which needs to be excreted by the kidneys).

Water balance is maintained when the output of body fluids is equivalent to the input of fluids into the body.  A person experiencing a normal level of fluids in the body is in a state of normohydration or euhydration.  Dehydration or loss of body fluid results in hypohydration, or low body fluid levels.  In contrast, hyperhydration occurs when the body maintains excess fluids.

FLUID INTAKE BEFORE, DURING AND AFTER EXERCISE

Fluid loss during exercise is primarily in the form of sweat, which keeps the athlete’s body temperature from rising too much. In an ideal world, athletes would consume just the correct amount of fluid during exercise to maintain fluid balance. However, in reality the rate of fluid taken in by athletes will fall short of the amount needed to totally replenish lost fluids.

Gastric Emptying and Fluid Absorption

Fluids are initially consumed orally and taken into the stomach. From there, the fluid empties from the stomach and is absorbed into the bloodstream from the intestines.

 

A number of factors affect how well the fluid empties from the stomach:

  • The larger the volume of fluid taken in, the greater the rate of gastric emptying. Consuming large volumes of fluid can, however cause great discomfort to athletes.
  • Consuming fluids with high osmolalities (ie: those containing lots of solutes) inhibit gastric emptying. In order to get both water and carbohydrates into the body, a compromise is to drink solutions of 6-8% carbohydrates. Glucose polymers have been used instead of glucose, as they have less of an effect on osmolality. Their usefulness is still the subject of research.
  • Cold fluids empty rapidly and can help lower core temperature.
  • Moderate exercise helps gastric emptying, where very intense exercise can inhibit it.
  • Dehydration can inhibit gastric emptying.

 

Intestinal absorption of fluids may be helped by simultaneous absorption of glucose and sodium. Absorption of glucose and sodium into the gut tends to pull water into the gut as well. However, excess carbohydrates in the gut can actually reduce the ability of the intestine to absorb water. It can also cause discomfort such as diarrhea and abdominal cramping. Striking a balance is can be tricky, as both gastric emptying and intestinal absorption rates vary markedly with the individual.

 

Fluid Recommendations during exercise

Exercise alters the body’s levels of fluids and electrolytes. Severely altered levels can be life threatening. In order to avoid dehydration, hypohydration and hyponatremia (where blood sodium levels are dangerously low due to excessive loss of sodium in sweat), the following is advisable:

  • Athletes should be well hydrated when they start exercising. In addition to drinking plenty of fluids in the 24 hours before an exercise session, athletes should drink 400-600 ml of fluids 2-3 hours before exercise. This allows the athlete to be fully hydrated, but to have excreted excess fluids in the form of urine before exercise.
  • During exercise athletes should attempt to drink enough fluid to maintain fluid balance. Optimal hydration can be achieved by drinking 150-350 ml at 15-20 minute intervals throughout an exercise session.
  • Where intense exercise is to be maintained for more than an hour at a time, sports drinks with 4-8% carbohydrate can be very useful.
  • If an exercise session is less than 3-4 hours in duration, and the athlete’s last meal contained sodium, then there is little chance the athlete will develop a dangerously low electrolyte level. However, if the athlete is exercising for more than one hour. It is a good idea to include 0.5-0.7g of sodium per litre of water consumed. This increases the athletes desire to drink fluids, and can reduce the risk of hyponatremia.


Post Exercise Fluid Consumption

  • Despite all precautions, many athletes will finish exercise sessions somewhat dehydrated. Consuming up to 150% of the weight lost during a single exercise session may be necessary to replenish fluid lost via sweat and urine production. This should be consumed within 2-4 hours of competition ceasing. Including some sodium in fluid or food consumed post exercise will reduce the diuresis that occurs when lots of pure water is consumed.
  • Get informed about fluids and use knowledge, rather than just using thirst as a guide to fluid consumption.
  • Drinks containing carbohydrates may assist not only in fluid, but in general recovery from an event.
  • Remember that caffeine rich fluids such as coffee, alcohol and cola will increase fluid losses due to increased urination. Do not use this for fluid replenishment.
  • Fluids containing sodium can help to maximise fluid retention by minimising losses in the form of urine. Sports drinks, salty foods, rehydration solutions and even IV solutions will all include a certain amount of sodium.

LEARN MORE

Learn More about Nutrition for the Athlete.

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