Optimal hydration improves health and well-being. Hydration is an important consideration if you lead an active lifestyle, whether that be competing in sport, exercising for enjoyment and health or working in a physically demanding occupation.
During exercise, a range of physiological changes occur in the body to maintain the supply of energy substrates and oxygen, and to allow the removal of waste products, carbon dioxide and heat.
Drinking when you feel thirsty may be an adequate approach to staying hydrated, but some people may need to establish a personal drinking plan to establish their needs and monitor their hydration status.
Physical activity can lead to fluid losses of 1-3 litres an hour and fluid intake should be adjusted accordingly. Levels of hydration have a significant impact on performance and a person’s experience of activity (perceived effort). Dehydration can adversely influence decision-making, cognitive performance and reaction times.
To encourage activity and improve performance it is necessary to avoid dehydration. The loss of small amounts (5-8%) of total body water causes thirst, dizziness, lethargy and fatigue. Losses of more than 10% of total body water causes confusion, drowsiness and overheating. Severe dehydration can cause reduced blood volume, increased heart rate, reduced skin and muscle blood flow, impaired thermoregulation, headache and nausea.
Exercise performance is impaired when an individual is dehydrated by as little as 2% of body weight. The capacity to perform high-intensity exercise, which results in exhaustion within a few minutes, may also be impaired by prior dehydration.
The main reasons dehydration has an adverse effect on exercise performance are:
- reduction in total blood and cardiac output – a reduced maximal cardiac output is the most likely mechanism by which dehydration decreases a person’s VO2max
- decreased blood flow to the skin to aid thermoregulation
- decreased sweat rate
- increased core temperature (above 40 degrees Celsius) – the body temperature will rise if it cannot dissipate al of the heat it generates during exercise, which could result in a range of clinical conditions – from heat cramps to heat stroke
Overhydration is possible if an athlete trying to avoid dehydration drinks too much water (more than can be lost through sweat). Excess fluids may dilute the sodium concentration in the blood which is also a potential health hazard. It is important to consider the amount of fluids consumed before, during and after exercise and to consider the composition of your drinks.
Before exercise – individuals should begin exercise adequately hydrated: Drink 400-600ml of fluids in the two hours before beginning exercise
During exercise – drink sufficient fluid to prevent dehydration from exceeding 2% bodyweight: drink 150-300 ml every 15-20 minutes (volume depending on sweating rate) – by monitoring body weight before and after exercise you can tell if you drank too little, too much, or just the right amount. Each kilogram of weight loss represents about one litre of sweat loss. Also, if you are urinating less often than usual or your urine is a darker colour than usual, you may be dehydrated.
Water is generally a sufficient fluid replacement for exercise lasting 40-60 minutes and electrolytes will be replaced during consumption of a healthy, balanced diet. For longer lasting exercise a drink containing carbohydrates and electrolytes may better maintain performance. A drink containing a low concentration of glucose (<5%) will empty from the stomach more quickly and better facilitate rehydration.
Since every person will have different fluid needs, ask the following questions to establish a drinking plan that works for sport you practice and for each of the different situations that you may practice in (eg: warmer temperatures)
- How did you feel during your training session or event?
- How was your performance?
- How much weight did you lose during this session? – your weight loss should usually not be more than about 2% of you total body weight
- Did your fluid intake make you feel uncomfortable?
- Did you spend time drinking when you didn’t need to?
- Did you have to stop to pass urine?
There are a range of strategies for supporting people active in sport or physical activity to stay adequately hydrated, and an individual should find a strategy that suits their circumstances, including their sport and personal characteristics.
Information from the European Hydration Institute
Enjoy Good Food for Good Health