What is water and what does it do?

Water is a combination of hydrogen and oxygen. It is the basis for the fluids of the body. Water makes up more than two-thirds of the weight of the human body. Without water, humans would die in a few days. All the cells and organs need water to function.

Water serves as a lubricant. It makes up saliva and the fluids surrounding the joints. Water regulates the body temperature through perspiration. It also helps prevent and relieve constipation by moving food through the intestines.

Hypohydration (total body water below normal) impairs the body’s ability to regulate heat resulting in increased body temperature and an elevated heart rate.  Perceived exertion is increased causing people to feel more fatigued than usual at a given work rate.  Mental function is reduced which can have negative implications for motor control, decision making and concentration.  Gastric emptying is slowed, resulting in stomach discomfort.  All these effects lead to impairment in exercise performance.  Most types of exercise are adversely affected by hypohydration, especially when they are undertaken in hot conditions, and negative effects have been detected when fluid deficits are as low as 2% (i.e. a deficit of 1.4 litres for a 70 kg person). Dehydration of greater than 2% loss of body weight increases the risk of nausea, vomiting, diarrhoea, and other gastro-intestinal problems during exercise. The good news is that by drinking regularly during exercise, you can prevent declines in concentration and skill level, improve perceived exertion, prevent excessive elevations in heart rate and body temperature and improve performance.

Food Sources

You get some of the water in your body through the foods you eat. Some of the water is made during the process of metabolism. But drinking water is your main, and best source, of water.

You also get water through liquid foods and beverages, such as soup, milk, and juices. Alcoholic beverages and beverages containing caffeine (such as coffee, tea, and colas) are not the best choices because they have a diuretic effect; they cause the body to release water.


It is challenging to identify the exact amount of water you should drink as each person has individual needs, but experts usually recommend drinking six to eight 8-ounce glasses of water daily.

Fluid requirements vary remarkably between people and between exercise situations.  Fluid losses are affected by:

  • Genetics – some people innately sweat more than others
  • Body size – e.g., larger athletes tend to sweat more than smaller athletes
  • Fitness – fitter people sweat earlier in exercise and in larger volumes
  • Environment – sweat losses are higher in hot, humid conditions
  • Exercise intensity – sweat losses increase as exercise intensity increases

How much do athletes drink?

Typically athletes replace 30-70% of sweat losses during exercise.  Fluid replacement is an issue for all sports including those such as swimming and water polo conducted in wet environments, and sports conducted in air conditioned stadiums. Drinks need to be cool, palatable and conveniently available or they will not be consumed. Research shows that fluid intake is enhanced when beverages are cool (~15 °C), flavoured and contain sodium (salt).  The sensation of fluid in the mouth sends nerve signals to the brain that switch off the drive to drink.  When fluids such as water, juice and cordial are consumed (low in sodium), the desire to drink is often switched off before the athlete has consumed sufficient fluid to match sweat losses.

Immediately after exercise, monitor your weight change to estimate your final fluid deficit. During recovery, you will continue to lose fluids through sweating and urine losses, so plan to replace 125-150% of this fluid deficit over the next 2-6 hours. For example, if you lost 1 kg (1000mL), you will need to drink 1250-1500mL to fully re-hydrate. Drink fluids with your recovery snacks and the following meal to achieve this goal.

Side Effects

Too little water

If you do not drink enough water each day, the body fluids will be out of balance, causing dehydration. When dehydration is severe, it can be life-threatening. Dehydration reduces the rate of fluid absorption from the intestines, making it more difficult to reverse the fluid deficit. You may end up feeling bloated and sick if you delay fluid replacement. It is impossible to ‘train’ or ‘toughen’ your body to handle dehydration.

Too much water

Consuming fluid in excess of requirements may cause some gastrointestinal discomfort.  In extreme cases, a condition called hyponatraemia can occur.  Hyponatraemia (low blood sodium levels) causes symptoms similar to dehydration and is potentially life threatening.  It is not common but can occur in prolonged endurance events (> 2 hours) when large volumes of low sodium drinks (such as water) are consumed and sweat losses are small.  Those most at risk are small females who have long race times (in for example marathons).  This group of athletes tends to have small sweat losses and plenty of time to consume large amounts of fluid during the event.