What is Dehydration?

Dehydration is the loss of body water or fluid from within the cells (intracellular), surrounding tissues (extracellular), liver and skeletal muscles that is not replaced adequately. A well trained, acclimated athlete will typically lose 1 liter of fluid per hour due to: carbohydrate and fat utilization; water released from stored glycogen used during exercise as well as that which is lost through metabolism. 

As carbohydrate stores are used, water loss occurs. This loss equates to approximately 3 kg of body weight. For a 176lb (80 kg) athlete, this loss represents a 3-4% loss in body weight.  According to the British Journal of Sports Medicine, this reduction in body weight post-competition is shown to be within normal levels and does not represent a significant health risk to the well-trained athlete. Keep in mind additional weight loss occurs due to respiration and perspiration.

Acute, pre-exercise body weight loss at or above 3% can greatly affect endurance performance. Dehydration impairs the body’s ability to sustain exercise intensity for prolonged periods of time. Dehydration also increases heart rate and body temperatures, reduces the heart’s output and stroke volume (amount of blood pumped throughout the body per stroke), and reduces blood flow to the skin.

Preventing Dehydration:

Endurance athletes should strive to start exercise well but not overly hydrated. During exercise lasting more than 1 hour, it is recommended to structure fluid intake so as to maintain exercise-induced body weight loss to 2-3%. Knowing how much to drink is an art and varies from individual to individual. Monitoring your body weight pre and post-exercise will help you to know what is the right amount to for you to drink.

Electrolyte and Carbohydrate Content of Fluid:

Electrolytes are chemical substances that are needed for muscle contraction. Sodium (Na+) and Potassium (K+) and Magnesium (Mg+) are the primary electrolytes for determining intracellular and extracellular water volumes. Most sports drinks contain 20mmol•L-1 (or 360mg) of sodium. Even if fluid is ingested at sufficiently high rates to replace all water lost, that fluid must also contain sodium in the same concentrations found in sweat. During exercise, your body loses between 900 to 1400 milligrams of sodium per liter of sweat. Again, trial and error during training runs can help you to determine the amount of sodium that will work for you.

It’s important to keep in mind, that during prolonged exercise, the balance of sodium and potassium in the body determines the water balance in the body not the other way around. Too much water and not enough electrolytes can cause hyponatremia with potentially fatal effects. Supplemental sodium capsules can help keep your electrolyte levels balanced. Research has shown that drinking isotonic CHO-electrolyte drinks that contain 4-8% CHO (carbohydrate), are rapidly absorbed (as quickly as or quicker than water), provide fuel and can delay dehydration.

How to Eat During an Ultra-Distance Event

Fueling chartDuring energy-demanding endurance events, carbohydrates will be the ultimate source of energy for our body and our brain. Each gram of carbohydrate provides 4 calories. How many calories you need will depend on your size, your mountain bike fitness level and how hard you’re working. The harder the effort, the higher the percentage of carbohydrates-to-fats used (see graph) and the more difficult it is to process the food and drink we consume.

For a 176lb (80 kg) athlete, caloric expenditure can range from 600-1200 calories per hour. The average athlete can store approximately 1600-1800 calories from carbohydrates either in the form of blood glucose or muscle and liver glycogen. The average athlete can generally absorb 250-300 calories per hour or approximately .7-1 gram of carbohydrates/kg of body weight per hour. It is highly recommended to eat frequently to help resist fatigue from glycogen depletion.

The best source of energy for ultra-endurance biking or running is carbohydrates. There are 2 main types: simple and complex, each has a different chemical structure and is converted to energy differently.

Simple carbohydrates, also known as simple sugars, are made up of short chains of sugars and are very easy to digest; Glucose, Fructose and Sucrose. Maltodextrin, a complex carbohydrate, provides quick energy but is absorbed more slowly than simple sugars. Even though simple carbohydrates provide fast energy for immediate use, they should be used intermittently with complex carbohydrates so as to keep blood sugar levels even and decrease the chance of gastric distress.

Complex carbohydrates, or complex sugars, are longer chains of sugars, are more difficult to digest and take longer to be used for energy. Such foods include: breads, grains, potatoes, and fruit. Both types of carbohydrates when combined with small amounts of fat or protein will keep you fueled for a successful event.