Base Nutrition for Sport


Hydration

Also, as we become increasingly dehydrated, we increase our risk of negative health consequences (see table).

Level of dehydration

Effect

2-3% loss of body weight

  • Impaired performance

6% loss of body weight

  • Nausea
  • Difficulty concentrating
  • Failure to regulate temperature

10% loss of body weight

  • Dizziness
  • Laboured breathing
  • Muscle spasms
  • Delirium

15-20% loss of body weight

  • Inability to urinate
  • Deafness
  • Death

A fully hydrated athlete will contain about 60% fluids. A 70kg male athlete will contain about 42-45 litres of water.

Why do we sweat?

Sweating is a vital thermoregulatory response. Sweating provides an effective means of losing heat as fluid evaporates from the skin. High sweat rates only provide an advantage if fluid is replaced during exercise.

Environmental Conditions

Remember that evaporation of water from the body's surface provides the major physiological defence against overheating. Environmental conditions such as humidity are very important factors which determine the effectiveness of evaporative heat loss. When humidity is high evaporation is greatly reduced. This is because when humidity is high the ambient vapour pressure approaches that of the skin and so evaporation is reduced.

Consequences of dehydration

Dehydration leads to reduced blood flow and sweating rates. As a result there is increased heat storage and an increase in core temperature.

It is estimated that with each 1% loss in body weight, core temperature increases by around 0.1-0.4 degrees celsius. (see table for consequences of dehydration)

Monitoring sweat losses

A rough measure of sweat loss is to weigh yourself before and after exercise. For each kg of body weight that you lose, this is equivalent to about 1 litre of water. You also need to account for fluid you have consumed during this time and urination.

During very prolonged exercise, this may not provide an accurate measure of fluid loss as you may have also lost several hundred grams of glycogen (carbohydrate stored within the muscle) and fat.

Is thirst a good indicator of dehydration?

There is controversy as to whether the thirst mechanism is a sensitive indicator of dehydration. Some researchers believe that if we wait until we are thirsty to drink, we are already dehydrated to a certain extent. Others argue that the thirst mechanism is an effective means to prevent the over-consumption of fluid.

Factors which influence fluid bioavailability

There are 3 main factors that affect fluid bioavailability. These are fluid intake, gastric emptying (rate of which fluid is emptied from the stomach), and intestinal absorption (the rate at which fluid is absorbed).

Intake - factors influencing

  • Colour, flavour and smell
  • A cold drink is often preferred
  • Familiarity may enhance intake
  • A sweet flavour is preferred up to a point

Gastric Emptying - factors influencing

  • As carbohydrate concentration increases, gastric emptying is reduced (when the carbohydrate concentration is around 3-7% this effect is minimal)
  • As the volume of intake increases, gastric emptying increases
  • As osmolality* increases, gastric emptying decreases
  • Carbonated drinks may cause gastrointestinal discomfort which may reduce gastric emptying in some individuals
  • Dehydration and hyperthermia reduce gastric emptying
  • Mental stress, anxiety, fear, anger, can reduce gastric emptying
  • As exercise intensity increases above 70% VO2max, gastric emptying is reduced

Intestinal Absorption

The absorption of fluids, carbohydrate, fat, protein and electrolytes occur in the small intestine.

  • As osmolality* is increased, intestinal absorption decreases
  • As carbohydrate concentration increases, intestinal absorption decreases
  • Adding some sodium and carbohydrate will increase net water absorption when osmolality is kept low

*Osmolality is the number of particles from all solutes in a solution. Fluids which contain carbohydrates of different chain lengths vary in osmolality. The longer the chain length, the lower the osmolality.

E.g. 17% glucose beverage = 1200 mosmol/kg

17% glucose polymer = 300 mosmol/kg

Guidelines for optimal hydration

  • Aim to commence competition in a well-hydrated state
  • Fluids containing sodium will maintain thirst and reduce urine formation
  • As a guide before exercise:
    • Drink 500ml of fluid the night before competition
    • Drink 500ml first thing in the morning
    • Drink 250-800ml about an hour before the event
    • Some individuals may want to consume around 250ml, 20 minutes prior to competition
  • As a guide during exercise:
    • Trial replacement strategies during training
    • Try and consume as large a bolus of fluid as practical, to maximise gastric emptying
    • Depending on sweat rates and environmental conditions, consume around 250-800ml per hour during exercise
  • As a guide after exercise:
    • Consuming fluids which contains some sodium will reduce urination and promote thirst
    • Drink sufficient fluid to regain the body weight lost during exercise

It seems that most athletes drink less during exercise that they lose, meaning that they are at risk of dehydration. We know even a small degree of dehydration can impair performance and that severe dehydration can be life threatening.

Reasons why many of us may not drink sufficient amounts of fluid during exercise include the awkwardness of drink during exercise, a reduced sense of thirst after even just a few sips of fluid, the discomfort in our stomaches after ingestion of large amounts of fluid, as well as the taste of the beverage.

Hints

  • Monitor your sweat losses during training. If you seem to have a deficit of water intake, increase your intake so that your weight loss during exercise is minimised. Do not drink so much during exercise that you gain weight.
  • Your body can adapt to increased fluid intake, so practice drinking during training.
  • Experiment with different beverages and choose one which you like the taste of, and what seems to suit you
  • Experiment with different drink containers during training to see which suits you best
  • For those who suffer from heat cramps, a fluid which contains sodium may be of benefit. The Red8 hydration drink now contains more sodium than previously

Can we drink too much fluid?

We know that some individuals who consume a lot of fluid during competition lasting over 6 hours, can suffer from hyponatremia. This is where the body has become overloaded with water and so blood sodium levels have become dangerously low.

Hyponatremia has been seen in ironman athletes, marathon runners who take over 5 hours to complete the race (backmarkers) and in the military.

Symptomatic hyponatremia is characterised by expanded total body water and weight gain over a race. Some of the symptoms of moderate hyponatremia include, headaches, confusion, restlessness, disorientation, and slurred speech. Severe symptoms include seizures, agitation, stupor, vomiting, coma, pulmonary edema and death. Symptoms of hyponatremia are caused by edema of the vital organs, brain and lungs.

In some ironman events, it has been recommended that fluid ingestion should not exceed 500-1000ml per hour.