Athletes need to consume fluids before, during and after exercise because dehydration of more than 2% body weight can cause exertional heat illness such as heat cramps, heat exhaustion or heatstroke.
Water
Approximately 73 percent of our lean body mass is water. Without food, the body can maintain life for weeks; without water, only days. But while the importance of water cannot be overstated, the number one protection against the harmful effects of dehydration and cramping is consumption of the right combination of fluids with electrolytes.
Since water by itself contains no electrolytes, carbohydrates or flavor benefits, athletes typically "voluntarily dehydrate" by not consuming enough of it. To avoid this voluntary dehydration, athletes must be convinced to participate in their own hydration based on personal preferences in fluid choices and sweat rates.
Additionally, beverages that are carbonated and contain caffeine or alcohol can lead to further dehydration and should be avoided. Caffeine or alcohol can lead to increased urine output and reduced fluid retention while carbonated beverages may reduce fluid intake due to stomach fullness.
Electrolytes
The inclusion of sodium chloride (table salt) in fluid-replacement beverages is highly recommended in physical activities exceeding 4 hours in duration or during the initial days of hot weather. Adding modest amounts of salt (0.3 to 0.7g/L) to a replacement beverage can offset sodium losses in sweat and reduce muscle cramping associated with exercise. More importantly, sodium enhances the palatability of the ingested fluid and can maximize the amount of fluid consumed because of improved taste.
Electrolytes (sodium, potassium, calcium, and magnesium) are dissolved in the body as electrically charged particles called ions that modulate body fluid exchange within the various fluid compartments of the body. Perhaps the most important role of sodium and potassium is that of establishing the proper electrical gradients across cell membranes. This electrical difference inside and outside the cell is required for the transmission of nerve impulses and for the stimulation and contraction of muscle. These minerals are also important in controlling the acid-base qualities of body fluids, especially blood.
Carbohydrates
Athletes can also benefit from including carbohydrates in their rehydration program, especially in activities lasting more than one hour. Athletes should consume carbohydrates at the rate of 1L per hour of exercise to maintain optimal carbohydrate metabolism.
Cramping
While the effects of dehydration on exercise performance can be subtle, the onset of muscle cramping is very apparent. Cramps are painful, involuntary contractions of skeletal muscle during or immediately after exercise. The exact etiology of skeletal muscle cramping is still unknown. Fluid and electrolyte imbalances are one possible cause. Another possible cause involves abnormal spinal control of motor neuron activity, particularly when a muscle contracts in a shortened position. Important risk factors include muscle fatigue and poor stretching habits.
