Generally, the guidelines are based upon every hour of exercise and the duration, intensity and so on. However, it's not as simple as weighing oneself to determine sweat rates because losses change with conditions, effort, fitness and durations of exercise.
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Changes to body mass also affect total weight loss. About a two to three percent loss in body mass is expected during endurance events due to the use of energy from stored carbohydrate, fat and proteins-not because of dehydration.
It's no surprise to most that dehydration affects an individual's capacity to exercise, but every individual processes nutrients differently and training is the time and place to determine needs (within recommended ranges of intake).
The body's ability to perform decreases when body fluids are out of balance by even by the smallest of amounts.
However, there are individual differences and many elite athletes actually finish and win events with body mass losses of two to three percent. Their body weight (mass) is lower, but body water actually may increase due to releases of water from stores in muscle glycogen. Therefore, fitter athletes may have an advantage in fluid replacement strategies.
To maintain and enhance training, performance and promote recovery the athlete should consider a plan of fluid intake before, during and after exercise. It's important to take in enough fluids, but not more than can be emptied from the stomach.
More: Cutting-Edge Hydration Strategies
The general recommendation is no less than 400mL (14 ounces) and no more than 800mL or 28 ounces for each hour. Elite athletes will be on the lower end and less experienced on the higher end.
Fluids and Nutrition Needs for Events and Training
Daily Water Ingestion: 0.5 of body weight in ounces, generally 6 to 9 glasses each day
Pre-Event (1 hour prior): 12 to 16 ounces of sports drink (mixed at 40 grams CHO)
Pre-Event (15 to 30 minutes prior): 8 to 12 ounces of cool water
Pre-Training: For working out, ingest 30 minutes prior, 300 calories (40 grams of CHO and optionally 25 grams of protein. For example, CytoSport "Pre-Performance" is an excellent pre-training (CHO/protein) combination and pre-event product
DuringEvent/Training: Ingest nothing until 15 minutes into event/training. Then drink 4 to 6 ounces (two to three sips) each 15 minutes (minumum 14 and maximum 28 ounces per hour, if very hot.
After Event/Training: 16 to 245 ounces during the next 45 to 60 minutes.
For all athletes, fluid replenishment should be a primary task before, during exercise, following exercise and especially, within the context of the density (how close are the workouts), duration (length of time) and intensity of workouts in addition to environmental conditions.
Fluid management and maintaining a balance between sweat losses and reestablishing fluid balance can make for much improved performance and of course, the quality of workouts and for enhancing recovery.
More: The Truth About Sports Drink After Exercise
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