Dehydration: Causes and Symptoms

Understand dehydration, its causes and symptoms and treatment. Train with BOISE RunWalk and attend clinics each Saturday morning on this topic and more.

What is dehydration?

Dehydration is the loss of water and salts essential for normal body function

The basic premise behind dehydration is an inadequate intake of fluids resulting in the body losing more fluid than it takes in. The fluid/salt balance needed to maintain healthy cells and tissues can be seriously disrupted with dehydration. 

heat_exhaustionDehydration can occur in as little at 30 minutes of exercise, especially in hot weather. The body relies on sweating to dissipate the heat generated from working muscles. Sweating also helps to maintain our core body temperature. Allowing our core body temperature to be maintained within a safe range is a key element in preventing heat related injuries which may initially be caused by dehydration. The amount of sweating necessary to sustain heat loss during vigorous exercise inevitably will lead to dehydration unless adequate fluids are ingested.

Dehydration will diminish an athlete’s performance and can lead to death if not corrected.  Dehydration is one of the most common factors for heat related sickness such as heat exhaustion and heat stroke. Both heat exhaustion and heat stroke cause numerous deaths each year. The most serious consequence of dehydration is impaired heat dissipation which can elevate the core body temperature to dangerously high levels resulting in heat exhaustion and potentially fatal heat stroke.

What causes dehydration?

Not drinking enough fluids, strenuous activity, excessive sweating, vomiting, diarrhea and heat exhaustion.

Strenuous activity such as running requires adequate fluid/electrolyte replacement or dehydration will occur. Excessive sweating due to climactic conditions and/or intensity of exercise can rapidly dehydrate individuals if corrective measures aren’t taken.

Vomiting, diarrhea, fever, diuretics, illness, a variety of medications such as an anti-inflammatory, low fitness levels, sleep deprivation, lack of heat acclimatization, staying in the sun too long, not drinking enough fluids, alcohol and caffeine can all be contributing factors to dehydration.

Who's at risk?

Adult and young athletes are at risk for dehydration, especially during the summer months, for any activity lasting longer than 30 minutes. Without proper hydration, the body can quickly lose water and other essential elements running the risk of kidney problems or even death. Children, due to their smaller stature are at an increased risk of developing dehydration. For all athletes, once dehydration starts, the deterioration can be quick.

Symptoms for Dehydration:

  • Headache
  • Dizziness
  • Confusion
  • Clumsiness
  • Excessive sweating
  • Thirst (dehydration occurs before you get thirsty)
  • Dark urine
  • Cramps
  • Reddened skin
  • Weak, irregular or rapid heart rate
  • Low blood pressure
  • General weakness
  • Rapid and shallow breathing
  • Heat exhaustion (increase in core body temperature and heart rate)
  • Heat stroke (very high core body temperature, reddened skin, initially normal to profuse sweating, death)

How to prevent dehydration?

 Drink before, during and after exercise, wear proper clothing, be acclimated to heat, avoid excess alcohol and certain medications (see your doctor).

The best preventative measure to ward off the possibility of becoming dehydrated is to stay hydrated. A good indicator of hydration is the output of large volumes of clear or pale-yellow urine. On a regular basis, darker-colored urine indicates a real possibility of chronic dehydration.

The fluid requirement for most runners is estimated to be 500 ml/hr. Drinking 8-20 ounces of fluids before exercise (about 2-3 hours), is highly recommended. Replacing lost fluids during exercise, especially when the duration is longer than 60 minutes, is critical. Consuming 6-8 ounces of fluid replacement every 20 minutes of exercise is a good rule of thumb to follow, especially during endurance events such as a marathon. Replacing fluids post exercise allows the body to hold onto the balance it seeks to achieve as well as aiding in muscle recovery, especially when the fluids contain high glycemic carbohydrates (sports drinks).  Choose a sport drink that contains at least 6-8% carbohydrate and electrolytes.

Proper clothing can influence your cooling efficiency. It is best to go with light colored clothes as they reflect light and in turn remain cooler than darker clothing. Choose materials that wick away the moisture, which allows for better air circulation, thus facilitating more rapid cooling. Cotton is not a good choice because it does not wick moisture away from the body.

Heat acclimatization takes about 10 days. Adapting to the heat helps our bodies make the necessary adjustments to promote better cooling.

Certain medications can cause dehydration by interfering with sweating, kidney function and diuretic effect. Checking with a physician is always best before taking medications while exercising. As a rule of thumb do not take any medications before checking with your physician.

Treating dehydration?

Cool yourself as soon as possible, re-hydrate and if necessary seek medical treatment.

Understanding the cause of dehydration is the first step towards treatment. If heat appears to be the cause of dehydration, rapid cooling is recommended. Cooling can be achieved by loosening an individuals clothing and moving out of the direct sun. Ice can be used if available and should be applied to the athlete’s groin, armpits and neck.

Rehydration or replacing lost fluids is essential in correcting dehydration. Fluids containing some salt (but not too much) are helpful. Salt and water work together to allow our bodies to achieve a healthy balance. The salt acts to draw water through permeable membranes, which aids in the distribution of fluids throughout the entire body. Too much salt however, can have a reverse affect, and pull too much water through the membranes and can lead to further dehydration. The more dehydrated a person is the less salt one should administer. Assess the level of dehydration, start by administering water and then add a sports drink, most of which contain safe amounts of sodium.

When dehydration is extreme, the body stops sweating. As a result, our core body temperature will increase to high levels, causing a heat stroke. Heat stroke is a serious medical emergency and requires medical attention. Individuals require intravenous saline solution to correct this degree of dehydration and it may take 48-72 hours under supervised medical care to correct heat stroke. Heat stroke is life threatening.

What is the best method for calculating the amount of fluid a person needs during exercise?

1. Baseline fluid requirements (recommended daily) for non-exercising individuals can be calculated by weighing the person in lbs and dividing that number by 2:

           140/2 = 7070 ounces fluid replacement necessary for daily maintenance

  • Wt (lbs) divided by 2 = minimum daily requirement of fluid for a non-exercising individual represented in ounces
  • Example: 140 lb male

2. Exercising individuals need to build on the minimum daily fluid requirement. The most accurate method for determining fluid replacement for an exercising adult is by weighing themselves before they run and weighing themselves again after they run. The change in weight represents the amount of fluid necessary to achieve an accurate fluid balance.

  • Example: 140 lb male

Pre-exercise weight: 140 lbs

Post-exercise weight: 139 lbs

  • 1lb of weight loss = 16 ounces of fluid to be replaced
  • This is in addition to the minimum daily requirement above.

It is also recommended that on hot days and additional 8-20 ounces of fluids be consumed prior to exercise (30-60 minutes before). An easy way to remember how much to drink is to drink often, aiming for at least 5-8 ounces of a sports drink every 10-20 minutes during exercise lasting longer than an hour or anytime the weather is warm. Optimal fluid replacement for most runners is 500ml/hr.


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