Exercise and Hydration: How much water is needed for proper hydration during exercise?

People perform physical activity throughout a range of environmental conditions such as, temperature, humidity, sun exposure and wind conditions. Depending upon the metabolic rate,

environmental conditions and clothing worn, exercise can induce significant elevations in body (core and skin) temperatures. Body temperature elevations elicit heat loss responses of increased skin blood flow and increased sweat secretion .Sweat evaporation provides the primary avenue of heat loss during vigorous exercise in warm hot weather; therefore sweat losses can be substantial.

Hydration or water balance during exercise depends on the difference between water gain and water loss. Water gain occurs from consumption of fluids during exercise while water loss during exercise predominantly occurs as sweat loss, urine, respiration.

Body weight measurement; provide a simple and effective tool to assess hydration. The before-exercise body weight measures are used with the post exercise body weight corrected for urine losses and drink volume. When possible, nude weights should be used to avoid corrections for sweat trapped in the clothing.

It is assumed that 1mL of sweat loss represents a 1-g loss in bodyweight (i.e., specific gravity of sweat is 1.0g ml/1). Fewer than 2% decrease in post exercise body weight in comparison to pre-exercise weight is widely acceptable. Anything above 2% difference indicates dehydration.

Dehydration increases physiologic strain and perceived effort to perform the same exercise task, and this is accentuated in warm-hot weather. The National Athletic Trainers' Association recommends the following hydration guidelines for exercise:

1. Two to three hours pre-exercise: 500 ml to 600 ml fluid of water or sports drink.
2. Ten to 20 minutes pre-exercise: 200 ml to 300ml of water or sports drink.
3. During exercise: Fluid replacement should approximate sweat and urine losses and at least maintain hydration at less than 2% body weight reduction. This generally requires 200 ml to 300 ml of water or sports drink every 10 to 20 minutes. Include carbohydrates in the beverage if the exercise is intense or lasts more then 45-50 minutes. Water alone will suffice, and save calories, if the exercise is moderate or less than 45-50 minutes.
4. Post-exercise: Ingest fluid equal to 150% of the weight loss (difference between pre and post exercise weight), ideally within two hours, and no more than four to six hours after the event. Carbohydrate in the drink will improve the rate of intestinal absorption of the fluid as well as replenish glycogen stores in the muscles and liver.

The goal of prehydrating is to start of physical activity euhydrated and with normal body electrolyte status. Prehydrating with beverages should be initiated at least several hours before exercise to enable fluid absorption and allow urine output to return to normal levels. The goal of drinking during exercise is to prevent excessive (92%body weight loss from water deficit) dehydration and excessive changes in electrolyte balance from compromising performance and health. Because there is considerable variability in sweating rates and composition between individuals, individualized fluid replacement programs are recommended.

Note: This content is based on ACSM Position stand on Exercise and Fluid Replacement, 2007. The content of this blog should not be considered as a medical advice. The content is developed keeping in mind, the reader as a middle aged, other wise healthy individual. If you have a diagnosed medical condition, ensure to consult your health care provider.