How to manage dehydration due to vigorous exercise and sport

Our bodies contain about 60% fluid, with almost half of this residing in the muscle cells. During vigorous or prolonged exercise large quantities of water and electrolytes can be lost through sweat, as the body promotes heat loss. Smaller quantities of water can also be lost through breathing (respiration).

Individual sweat rates and fluid losses vary widely. The rate of sweating also varies depending on such factors as the type and intensity of exercise, air temperature, humidity and body exposure to sunlight. Sweat rates of 0.3 to 2 litres per hour are common in active sports people both during training and competition.

A change in body weight due to water loss of 1% is a marker of early dehydration. At 2% water loss, athletes may begin to experience a decline in physical and mental performance.

And it does not take long to get dehydrated!

For example, a 70 kg athlete undergoing intensive training, with a sweat rate of 1.5 Litre per hour, will hit the 2% dehydration level within the first hour of exercise.

It is critical that athletes maintain a healthy state of hydration or “euhydration” through proper fluid uptake prior to, during and after intense exercise.

Hydralyte is designed to promote rapid and effective rehydration.

Signs of dehydration

The early signs of dehydration (1-2% body mass loss) are:

• Sticky or dry mouth
• Increased thirst
• Decreased urine output and dark yellow urine

As dehydration worsens (2-4% body mass loss) signs include:

•  Feelings of lower energy (lethargy)
•  Poorer concentration
• Skin can lose its elasticity

Dehydration greater than about 4% can have serious health implications such as cardiovascular strain and heat stroke.

Dehydration and Performance

Many clinical studies have been carried out to explore the effects of dehydration on exercise performance. At 2% water loss or more the following impacts have been observed:

• Dehydration can reduce exercise performance (speed and power output)
• Dehydration can reduce the time to exhaustion
• Dehydration increases the perception of exercise difficulty
• Dehydration can reduce mental performance (therefore alertness, concentration, visual motor skills and decision-making)
• Dehydration and sodium loss can lead to muscle cramps
• Dehydration is a risk factor for heat exhaustion and heat stroke, both serious conditions

What is tonicity?

Tonicity describes the concentration of solute molecules in a solution (glucose, sodium, potassium etc) and refers in this context to blood plasma. The numeric measurement of tonicity is osmolality.

The osmolality of blood plasma in healthy individuals is about 280 – 295 mOsm/kg. A solution with the same osmolality as plasma is termed “isotonic”.

A solution with lower osmolality is called hypotonic and one with greater osmolality than blood plasma is called hypertonic. 

Why is Hydralyte hypotonic?

It has been found that solutions slightly hypotonic to blood, with osmolality of about 200 – 260 mOsm/kg, provide more rapid and effective rehydration and replacement of electrolytes. Solutions that are hypertonic, including many sports drinks, are inferior and may interfere with the rehydration process.   

Hydralyte is hypotonic and has osmolality in the range 220 – 245 mOsm/kg.

Fluid Replacement During Exercise

The goal of hydration during exercise is to replace fluid and electrolytes lost through sweat and prevent the body from losing more than 2% body weight from such losses. 

This is best achieved by first estimating the rate of sweat loss for a particular individual and exercise (in Litres per hour) and then following a hydration plan.

Sweat loss rates are highly variable and for serious athletes it is recommended to assess their own using the “change in body mass” method.   

Evaluation of Hydration Status

The simple way to assess hydration status is to look for the common signs of dehydration – increased thirst, dry mouth and concentrated or dark yellow urine. When a person is well-hydrated their urine is generally clear and light yellow in colour. 

More sophisticated techniques that provide a numerical value include measurement of urine specific gravity, urine colour charts, and urine osmolality (Ref 1,2).

The most reliable way of evaluating dehydration resulting from vigorous or prolonged exercise is by measuring change in body mass (refer below).
 

Typical Rates of Sweat Loss

Low intensity exercise –  e.g. walking and gentle cycling: 0.2 – 0.4 L/h

Medium to high intensity exercise: 0.4 – 0.6 L/h or more

Change in Body Mass Method to Determine Sweat Rate 

The following procedure may be used by an individual athlete. (It is advised that any calculations are checked with a healthcare professional or registered sports coach) 

1. Immediately prior to the start of the exercise episode (sports or training), measure the pre-exercise body mass (A in kg) on accurate scales. Athlete should remove all clothing except for underware. 
2. Immediately after the exercise, towel off the excess sweat, and measure, in underwear, the post-exercise body mass (B). Calculate the total body mass change (C = A - B  kg). 
3. Record the drink volume consumed during the exercise episode, between weighings, (D) expressed in kg (Assume 1L = 1 kg and 1 ml = 1g)
4. Record the urine volume passed during exercise, between weighings (E) expressed in kg (Assume 1L = 1 kg and 1 ml = 1g). 
5. Calculate the sweat loss (F) based on F = C + D - E (in kg) and express as "Litres"
6. Calculate the average sweat rate (G) as F (Litres) devided by the exercise duration in hours.
7. Check the calculations with a healthcare professional or sports coach. 

Example: An athlete trains for 2 hours, with Pre- and Post- body mass measurements of 70 kg and 68 kg. During the episode he drinks 1000 ml water and passes 500 ml urine. Determine his average sweat rate for that exercise under the specific climate conditions.

C = A - B = (70 - 68 kg) = 2 kg

F = C + D - E = (2 + 1 - 0.5 kg) = 2.5 kg = 2.5 Litres

G, the average sweat rate =  2.5 L / 2 hours = 1.25 L/hour

The athlete is losing fluid from perspiration at a rate of 1.25 L/hour 

       

Suggested Dosage

Exercise episodes of 1h duration or less:
Take 200  ml Hydralyte solution within 2h prior to exercise.
Consume additional fresh water, during exercise, according to thirst and comfort.

Exercise episodes of 1 – 3h:
Take 200 – 400 ml Hydralyte solution within 2h prior to exercise
Consume Hydralyte up to the rate required to replace sweat losses, drinking while thirsty or displaying the signs of dehydration.
In addition drink fresh water freely according to thirst and comfort. 

Exercise episodes greater than 3h:
For athletes and sportsman undertaking regular and prolonged exercise of greater than 3h, it is recommended to seek advice of a healthcare professional to determine an hydration plan.

In such cases, athletes may choose to estimate their sweat loss rate based on the “change in body mass method”

Hydration Pharmaceuticals can provide guidance to sports clubs, gymnasiums, personal trainers and other groups on how to evaluate hydration status and estimate fluid loss.

Warnings

Hydralyte contains approximately 0.8 mg/ml potassium. If you have kidney disease or are taking heart or blood pressure medicines – consult your doctor before using Hydralyte.
Hydralyte contains sodium. Refer to packaging for content.

 References

1.  Shirreffs SM and Maughan RJ. 1998. Urine osmolality and conductivity as indices of hydration status in athletes in the heat. Medicine and Science in Sports and Exercise 30 (11): 1598-1602.
2. Shirreffs SM. 2003. Markers of hydration status. European Journal of Clinical Nutrtion 57 Suppl 2: S6-S9