Categories: Health Nutrition Training

How much water should I drink? | Thermoregulation and hydration

February 16, 2017

Sports scientist, marathon runner and Ironman finisher Benjamin Garcia discusses the importance of hydration and how much you should drink daily.

In healthy individuals, total body water content makes up approximately 45–70% of total body mass, which if you are a 75kg male equates to between 34 and 53 liters. The water balance of an individual is controlled within a set range and this regulation of homeostasis plays a very important role in the vast majority of metabolic and physiological functions.

How much water should I drink?

How much water you need per day depends on your physiological makeup, your activity level and environmental conditions. However, as a baseline aim to consume 2 liters of water every day. To get a more detailed answer, read on.

Thermoregulation

When taking part in physical activity an individual’s metabolic rate increases. Coupled with the increase in metabolic rate the human body is only able to utilise approximately 25% of the energy to produce external movement. As a whole the human body is a remarkable and astounding piece of machinery, however this particular function is very inefficient. The remaining 75% of the energy is dissipated as heat.

During physical activity there are clearly high energy demands and as a result heat production is elevated. Just like water balance, the human body works very hard to maintain a constant internal environment and so when the body recognizes an increase in core temperature, it must react accordingly, which in this case is to dissipate some of this heat.

To protect against overheating the body relies on four physical processes to achieve heat loss:

  • Radiation
  • Conduction
  • Convection
  • Evaporation

Radiation, conduction and convection all contribute to overall heat loss, but it’s evaporation that provides the most protection against overheating. When the body senses a rise in core temperature, one of the responses is an increase in sweat rate. The increase in body water to the surface of the skin allows the heat to evaporate into the environment and thus, helps to the body to control the rise in body temperature.

Electrolytes and maintaining balance

The composition of body water is also an important factor in the regulation of physiological function, which include electrolytes. The major electrolytes are sodium, potassium, calcium, magnesium and chloride and are all present in different concentrations. These electrolytes play a crucial role in preserving the required chemical and electrical balance, safeguarding correct cellular function and as a result, facilitating electrical communication around the body. It is therefore important for their respective concentrations to be maintained by the body’s internal regulatory system.

As they are all found in sweat, the loss of these electrolytes will depend upon the composition of an individual’s sweat as well as their sweat rate. It is important to note that these two variables are manipulated by environmental conditions, the physiological makeup of an individual, the intensity that the individual is exercising, as well as natural variation over time.

Effects of dehydration

When physical activity levels increase, our body temperature increases and to help control this rise in temperature our bodies increases sweat rate. If fluid losses continue without the consumption of extra fluid a state of dehydration may occur.

Dehydration has been shown to cause an increase in heart rate, an increase in core body temperature and a significant decline in cardiac output, which is the amount of blood pumped by the heart around the body in one minute. This can obviously have a significant negative effect on performance.

If fluid losses continue without the consumption of extra fluid a state of dehydration may occur.

NATA (National Athletic Trainers’ Association) recommends the following: “Fluid replacement should approximate sweat and urine losses and at least maintain hydration at less than 2% body weight reduction.”

So what does all this mean for ensuring correct hydration and what should you be thinking about around training and competition to ensure you’re not dehydrated and firing on all cylinders?

To start with a very simple way to monitor how hydrated you are is urine colour. Use the below as a guide – a light colour means hydrated and dark means dehydrated highlighting that you may need to consume extra fluids.

Optimizing hydration pre-exercise

It’s important to start exercise in a hydrated state. Regular exercise, or neglecting to drink sufficiently throughout the day, may cause a fluid deficit and therefore could negatively affect the next training session or competition.

Aim to consume 2 liters of water every day, especially if your regularly taking part in physical activity which increases the need for extra fluids.

Aim to consume 2 liters of water every day, especially if your regularly taking part in physical activity which increases the need for extra fluids.

Drink little and often in the hours before exercise and avoid consuming large volumes in one go. Monitoring hydration status using the above pee chart will ensure you’re properly hydrated at the start of exercise.

Optimizing hydration during exercise

If a training session or competition lasts longer than 30–40 minutes, a number of factors including a decline in fluid volume and an increase in body temperature may contribute to fatigue and so taking on fluids during exercise is an important strategy to control these outcomes.

It is advised to listen to your body during exercise and drink to thirst. Depending on the type of training or sport you’re competing in, make every effort to have fluids available during exercise. Similarly to pre-exercise, aim to consume fluids little and often avoiding large quantities in one go.

Similarly to pre-exercise, aim to consume fluids little and often avoiding large quantities in one go.

When preparing fluids to consume during training or competition, consider the composition of fluid lost through sweat. Consuming fluids that contain electrolytes may prove to be beneficial. The only electrolyte to add to your drink consumed during exercise is sodium. Sodium promotes water and sugar uptake, which will help to preserve the drive to drink as well as fluid volume.

Optimizing hydration post-exercise

As part of your overall recovery strategy, rehydration is an important element after both training and competition. Key considerations for rehydration after exercise are the quantity of fluids and its composition. Aim to consume 150% of sweat lost after exercise to ensure effective rehydration. This can be calculated using a simple body weight measurement. For every kg of body weight lost during exercise that equates to 1 liter of fluid.

For example, if an athlete loses 1kg of body mass during exercise that athlete needs to consume 1.5L of fluid in the hours after exercise to rehydrate back to pre-exercise, resting levels.

  • Maintaining hydration should be an important consideration every day regardless of activity levels or environmental conditions. As a baseline aim to consume 2 liters of water every day. Remember that food contains water as well.
  • For training or competition ensure you start hydrated. Drink little and often in the hours before exercise and use urine colour as a guide.
  • During exercise listen to your body. Consume fluids little and often and avoid consuming large quantities in one go. Fluids containing sodium will promote water and sugar uptake and help to maintain the drive to drink.
  • Rehydration forms an important part of an athlete’s recovery strategy. Aim to consume 150% of sweat lost after exercise to ensure effective rehydration.