Hydration levels are a concern for athletes and John Q. Public. In my office we use the Total Health System to accurately measure hydration levels. Dehydration has cognitive consequences, negatively affecting response time, coordination, tracking, short-term memory, attention, and mental focus (1). Feelings of fatigue take over faster and more recent research has even shown that dehydration increased brain activity related to painful stimuli (6).
Performance wise, dehydration can decrease strength by 2%, power by 3%, and high-intensity endurance by about 10% (1). Some of the reasons dehydration affects performance, especially for endurance activities combined with heat, include reduced plasma blood volume (leading to reduced stroke volume, increased heart rate), a decrease in blood flow to the skin (reducing the sweating response and heat dissipation), and an increase in core temperature (4,5).
So how much should an athlete consume to keep hydrated? The following chart lists recommendations from the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine’s Fluid Replacement Guidelines (1):
If exercising less than an hour, water will generally meet hydration needs. For longer duration exercise, especially in the heat, a sports beverage will help replace the fluids and electrolytes lost.