Athletes and others who begin exercise with a less than normal volume of body water are likely to experience adverse effects on cardiovascular function, temperature regulation, and exercise performance. Increasing body water stores above normal, well-hydrated levels by drinking fluids shortly before exercise is also likely to improve cardiovascular function and temperature regulation when it is impossible to ingest sufficient fluids during exercise. Under most circumstances, it is wise to drink at least 500 ml (16 oz) of fluid before sleeping on the night before exercise and at least another 500 ml first thing in the morning to help ensure a state of euhydration. Next, one should drink another 500-1000 ml about 1 h before and an additional 250-500 ml about 20 min before exercise to "top off` fluid stores. The type of fluid consumed before exercise is important. Fluids containing carbohydrate (to supply energy) and small amounts of sodium chloride (to help maintain thirst and reduce urine formation) are likely to have more beneficial effects than plain water. The addition of glycerol to a pre-exercise hydration beverage usually is ineffective and can produce headaches and nausea.
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