Fluids in Sport

By Liz

Fluid in Sports

Why is fluid important during exercise?

Water is vital for the regulation of body temperature and muscle contractions. It is essential to keep hydrated during exercise to preserve body functions and support exercise performance. Dehydration increases body temperature, heart rate and fatigue leading to a higher perceived exertion. A decrease is seen in mental function, including decision making and concentration, and gastric emptying, resulting in stomach discomfort. Negative impacts have been detected when fluid deficits are as low as 2% (e.g. 1.4L0 in a 70kg athlete). 

Individual sweat rates and fluid losses vary widely due to genetics, body size, gender, exercise intensity, aerobic fitness and environmental conditions. Sweat rates increase with a rise in temperature, humidity and exercise intensity, therefore increasing the need for fluid intake. Sweat loss can occur with any exercise, even in water events and sports such as cycling where sweat is evaporated quickly by the air flow.

As a guide, you probably need more fluid if:

  • you sweat heavily
  • you have certain medical conditions, such as diabetes or heart disease
  • you have cystic fibrosis, which means you have a high concentration of sodium in your sweat 
  • you are using a medication that can act as a diuretic, causing your body to lose more fluid
  • you have a bigger body size
  • you are fit (because fitter people tend to sweat more and earlier in their exercise)
  • you are doing vigorous exercise
  • you are active in hot or humid conditions.


Dehydration and Performance

As dehydration increases, there is a reduction in physical and mental performance. There is an increase in heart rate and body temperature, and an increased perception of how hard the exercise feels, especially when exercising in the heat. Impaired skill level can also occur, along with mental fatigue that can impact concentration and decision making. Dehydration can also increase the risk of nausea, vomiting, diarrhoea and other gastro-intestinal problems during and after exercise.

Fluid and Hydration plus the factors influencing sweat rate 

Measuring fluid loss:

The most practical way to monitor your sweat loss is to measure changes in body mass, with corrections made for fluid/food intake, as well as urine losses before and after exercise. Record your weight in minimal clothing before you  begin your session and then again when you are finished. Be sure to be wearing the same clothes and towel dry any excess sweat on the body.

Each kilogram of weight loss is equal to ~1 litre of fluid deficit. Adding on the weight of any fluid or food consumed during the exercise session and subtracting urine output will provide an estimate of total fluid loss for the session. Divide that by the duration of exercise to provide a rate of sweat loss.

For example: an athlete weighed 55kg before exercise and 53.5kg at the end. During the 2 hour session he consumed 1L of fluid and lost 500ml in estimated urine losses.

Weight deficit (kg) = 55kg - 53.5kg = 1.5kg    Total sweat loss (L) = 1.5kg + 1kg - 500ml = 2kg    Sweat rate (L/h) = 2kg/2hr = 1L/hr    Fluid deficit (L) = 1.5kg - 1kg + 500ml = 0L 


To prevent dehydration, it is essential to ensure your body is adequately hydrated before commencing any exercise. Develop a fluid intake plan that allows you to replace most of your sweat losses during exercise. It is best to begin drinking early in the session and adopt a pattern of drinking small volumes regularly. If you can’t replace all of the necessary fluid, you should drink as much as is practical and comfortable. 

Drinks need to be cool, palatable and conveniently available or they will not be consumed. Sports drinks are recommended for high intensity ’stop-n-go’ activities and endurance sports of 60 minutes or greater. The tolerance to the type and amount of fluid we drink varies greatly with each person. To prevent discomfort in competition, trial your fluid plan during training to see what works best for you. Remember, when your body tells you its thirsty, you're already dehydrated!

During recovery you continue to lose fluids through ongoing sweat and urine losses. As a rough guide, for every 1kg of body weight lost, 1.5 litres of fluid needs to be replaced. So plan to replace 125% to 150% of this fluid within 2-6 hours after ceasing exercise.

Research has proven that consumption of sports drinks during exercise increases voluntary intake and improves performance compared to water. They are designed to provide the right balance of carbohydrate and fluid to ensure they are emptied quickly from the stomach preventing discomfort. The carbohydrates provide a top-up fuel for the brain and working muscles.

They also contain electrolytes such as sodium and potassium. Sodium is lost in our sweat and inadequate levels may cause muscle cramps.  Sodium increases fluid absorption and retention, and can also enhance fluid intake by driving the thirst mechanism. This is especially important in those with large or salty sweat losses.

Choose sports drinks that have 4% to 8% carbohydrate, 10 to 20 mmol/L sodium. Trial a variety of drinks in training before you decide what is best for you in competition.


Summary :Tips on hydration


Fluids before exercise:

  • Drink enough fluid daily to maintain weight and adequate pale yellow colour urine output.
  • The day before an event, drink extra water, 100 % juice and / or other nutrient-rich fluids such as non-fat or 1 percent milk.
  • 5-10 ml/kg BW in 2 to 4 hours before exercise to achieve urine that is pale yellow in colour
  • Including sodium in fluids/foods before exercise may help retain the fluid during exercise.
  • LIMIT beverages that contain alcohol.


Fluids after exercise:

  • Drink 1.5 L of fluid for every kilogram of weight lost during exercise.
  • Consume high carbohydrate foods and drinks within 2 hours after exercise to replenish glycogen.
  • In conjunction with drinks. Consume foods containing sodium (salt)(tomato or vegetable juice, pretzels, commercial soup, low fat cheese, salted nuts, vegemite) and foods containing potassium (vegetables, fruit, milk, legumes, or meat) to replace electrolytes. 


Can you drink too much and is it a problem?

In cool weather or when the exercise intensity is low, sweat losses may be small. Drinking more fluid than necessary has the potential to interfere with performance (and can be dangerous to health) in several ways. Over-hydration during exercise is called hyponatraemia (dilute levels of sodium in the bloodstream). Symptoms include headaches, disorientation and in severe cases, coma or death. It is important to note though that this is relatively rare and dehydration is a typically a more common issue for athletes though there is no evidence that dehydration has ever killed a marathoner or athlete (1) . But the outpouring that athletes must stay fully hydrated and drink before they become thirsty has spawned a new problem—overhydration. During the 2002 Boston Marathon a healthy, 28-year-old woman collapsed a few short kilometres from the finish line and died a day later. The cause of death was hyponatremia—too little sodium in her blood caused by drinking too much fluid before and during the marathon. According to the BMJ, 16 marathoners have died and more than 1,600 have become critically ill due to overhydration and hyponatremia.

Sports drinks don’t appear to prevent hyponatremia. A study of marathoners by Harvard-based researchers found that 13% had some degree of hyponatremia, and that it was just as likely to happen among those who guzzled sports drinks during the marathon as it was among those who stuck with water.







Information included in this document was obtained from:-

  • Sports Dietitians Australia and the Australian Institute of Sport
  • https://www.betterhealth.vic.gov.au
  • sportune.com.au
  • Dr Arthur Siegel, associate professor of medicine at Harvard Medical School and a medical advisor to the Boston Marathon, article in BMJ (British Medical Journal)





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