Hydration and Fluid Replacement

Proper hydration is the most frequently overlooked performance aid in sports. Water is the most essential of all nutrients. It makes up approximately 60% of the human body and approximately 70% of our muscles. Water's main function is to remove the heat (calories) produced by working muscles — accomplished through perspiration; cooling takes place when sweat evaporates from the skin, and blood that was transferring oxygen to muscles is diverted to the skin to help eliminate heat.

If too much body water is lost through sweating, blood volume can decrease, compromising cardiac output and oxygen delivery to muscles. Dehydration can harm performance by reducing strength, endurance, and aerobic capacity, and without adequate rehydration, heat exhaustion and heat stroke can occur.

How much fluid? The foundation should be a balanced diet with adequate fluids, including milk and juices, plus high-water-content foods (oranges, melons, strawberries, tomatoes). The average person needs approximately eight 8-ounce glasses of water per day. Exercise increases fluid requirements by two to three quarts a day. Caffeine or alcohol further increases fluid requirements.

After exercise, the volume of fluid to replace depends on the weight lost during exercise: for every pound lost, 16 oz of fluid should be enough for rehydration. Thirst is not an adequate guide to fluid rehydration.

Water vs. sports drinks (recommendations): Both promote optimal cardiovascular function and temperature regulation. For exercise lasting less than one hour, water is a convenient, effective, inexpensive choice and remains the beverage of choice. The 1996 ACSM fluid-replacement stand (the edition this manual reflects) found that adding carbohydrate and/or electrolytes is worthwhile for events longer than one hour, with little performance difference from plain water in shorter exercise. That stand also found little physiological basis for sodium in the drink to speed intestinal water absorption when the previous meal supplied sodium; sodium's value is palatability, fluid retention and, in some heavy drinkers, protection against hyponatremia.

• Baseline: the Institute of Medicine's Adequate Intakes for total water (food plus drinks) are 3.7 L/day for men and 2.7 L/day for women, with about 80% from beverages (see Daily Fluid Needs and Hydration Targets).
• During exercise: the 2007 ACSM stand sets the goal as preventing a water deficit of more than 2% of body weight, with an individual plan built from pre/post weighing; drinks with electrolytes and carbohydrate can beat water alone in some situations (see Hydration Strategy During Exercise).
• Extra needs depend on the session: sweat rates run about 0.3–2.4 L/h (2016 AND/DC/ACSM joint position), so a flat "2–3 extra quarts a day" is only a rough figure for heavy training.
• Rehydration: about 1.5 L per kg lost for rapid, full recovery (2007), or 1.25–1.5 L per kg (125–150% of the deficit; 2016) — roughly 19–23 oz per pound, more than the 16 oz above, because urine losses continue. Include sodium.
• Thirst: the 2007 stand notes that if you wait for thirst during exercise you are already in deficit, so drink at intervals when you can; day to day, thirst and drinks with meals keep most people hydrated (Institute of Medicine 2005).

• Water ≈ 60% of the human body and ≈ 70% of muscle.
• Water's main role: remove heat from working muscles via perspiration.
• Dehydration reduces strength, endurance, and aerobic capacity; can cause heat exhaustion/stroke.
• Baseline need: ~eight 8-oz glasses of water/day.
• Exercise adds ~2–3 quarts/day; caffeine/alcohol increase needs further.
• Rehydrate ~16 oz of fluid per pound lost in exercise (historical guidance); 2007/2016 ACSM-backed figures are higher: 1.25–1.5 L/kg ≈ 19–23 oz/lb.
• Thirst is not an adequate rehydration guide.
• Water is best for exercise <1 hour; carbohydrate-electrolyte drinks help for exercise >1 hour (ACSM 1996 stand).
• Sodium in drinks aids palatability and fluid retention; the 1996 stand found little basis for it speeding intestinal water absorption.