Nutrition: Hydration

By Marjorie Hagerman

It is essential to take onboard enough fluid to maintain an adequate level of body hydration. During training, heat is generated as a by-product of energy production to fuel the muscles, and this heat must be dissipated in order to stop the body's core temperature from rising to a dangerously high level. The body can rid itself of heat by:

• Dilating the blood vessels of the skin, which in turn increases the flow of blood to the skin and releases the heat to the environment by radiation and convection.
• Secretion of sweat onto the surface of the skin requiring heat calories to evaporate the moisture, causing a cooling reaction.

In hot weather especially, it is the cooling by evaporation process that allows exercise to continue, but only if these sweat losses are replaced. When training in hot weather, sweat losses from the body can be in excess of two litres per hour, and these need to be replaced during and following training. Some practical guidelines to help maintain optimal fluid balance during training:

• Cool fluids (5 to 10°C) are more quickly absorbed from the stomach and small intestine.
  If a sweet drink is preferred, the carbohydrate content should be present in no greater than an 8% solution, so as not to delay fluid emptying from the stomach or absorption of fluid from the intestinal tract into the blood.
• Drink 400 to 600ml 2 to 3 hours before exercise.
• During exercise, rehydrate by drinking 200 to 300 ml of cold fluid every ten to 20 minutes of activity. It's important not to wait until you feel thirsty to replace fluids. Thirst usually doesn't develop until 1 to 2% of body weight is lost through dehydration, and performance can be adversely affected at a 2% loss. The neurophysiologic stimulus for thirst is inadequate during and following exercise.
• Following exercise, it is recommended that 800ml of fluid should be consumed for every pound of weight lost through sweating. Rehydrate within two hours of exercise.
• In general, use of mineral supplements such as salt tablets to replace electrolytes lost in sweat is not necessary for rowers engaging in usual training regimes. Adding a little extra salt to daily meals and including high-potassium foods, such as citrus fruits and bananas, should easily replace the small amount of electrolytes lost.
• Loss of valuable electrolytes in sweat depends on such factors as gender, body size, heat adaptation, fitness, and environmental conditions. Losses of sodium, calcium, potassium, chloride, and other important ions will vary among individuals. Commercial drinks will provide adequate replacement of electrolytes and, at the same time, replenish carbohydrate stores. More importantly, however, drinking an isotonic sports drink and water will restore depleted body fluids for both the muscle and its transport systems and also ensure successful thermoregulatory function.