Fluid and electrolyte replenishment prior to exertion

Adequate hydration ensures optimal physiological responses and performance. Athletes who enter competition dehydrated are at a disadvantage. For example, in a study by Armstrong et al., athletes ran 5,000 m (approximately 19 min) and 10,000 m (approximately 40 min) under conditions of normal hydration and dehydration. When dehydrated by approximately 2% of body weight (using a diuretic before exercise), running speed decreased significantly (by 6-7%) in both cases. In hot climates, dehydration impairs performance even more.

To ensure adequate hydration, ACSM recommends that athletes consume a nutritionally balanced diet and adequate fluids within 24 hours prior to the event, particularly during the pre-exercise meal, to stimulate the necessary hydration prior to training or competition.

If people live in a hot climate, free fluid intake is often insufficient to meet their fluid needs. This was verified by a study conducted with Puerto Rican soccer players. The athletes were monitored during 2 weeks of training. When they were allowed to drink as much as they wanted throughout the day (average consumption was 2.7 liters per day), the total amount of water in their bodies by the end of the first week was 1.1 liters less than with mandatory fluid intake of 4.6 liters per day. In other words, free fluid intake did not replenish fluid losses and forced the athletes to begin training or competitions already dehydrated.

It is recommended to consume approximately 500 ml (approximately 17 ounces) of fluid approximately 2 hours before exercise to ensure adequate hydration and allow time for the body to eliminate excess water consumed.

Indeed, subjects who consumed fluid 1 hour before exercise had lower core temperatures and heart rates than when they did not consume fluid.

Monitoring urine color and volume is an important practical means for physically active individuals to assess their hydration status. Dark urine color and relatively low volume indicate dehydration and are a signal to consume more fluid before exercise. Monitoring urine volume is a common recommendation for mining workers who are constantly exposed to high temperatures and humidity.

It has been suggested that glycerol solution taken before exercise may provide cardiovascular and thermoregulatory benefits in the heat. Hyperhydration caused by glycerol consumption is accompanied by a weight gain proportional to the amount of water retained (usually 0.5–1.0 kg). Fluid retention occurs because glycerol molecules, after absorption and distribution into body fluids (excluding the aqueous humor and cerebrospinal compartments), cause a temporary increase in osmotic pressure, causing a temporary decrease in urine formation. As glycerol molecules are removed from body fluids in the following hours, the osmolality of the plasma decreases, urine formation increases, and excess water is excreted.

There are a number of reasons why it is unwise to recommend glycerol-induced hydration to athletes.

• Athletes incur metabolic costs for excess body weight.
• There is no compelling evidence that glycerol-induced hyperhydration is physiologically beneficial.
• Side effects of glycerin absorption range from mild symptoms of bloating and dizziness to more severe symptoms of headache, dizziness, and nausea.

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