Hydration is a fundamental component of marathon running. Fluid balance directly influences performance, as a mere two percent loss of body weight from dehydration can lead to a significant drop in endurance. The body relies on sweating for thermoregulation, and a lack of fluid impairs this cooling mechanism, raising the risk of heat-related illness. Maintaining proper hydration prevents increased cardiovascular strain, regulates blood flow, and assists in avoiding common issues like muscle cramping and premature fatigue.
Hydration Loading: The Week Before
Achieving a “pre-hydrated” state begins days in advance. Runners should consistently increase fluid intake in the 48 to 72 hours leading up to the event to maximize fluid stores. Monitoring hydration status is easily done by checking urine color, which should consistently be a pale straw yellow. Baseline daily fluid intake (approximately 125 fluid ounces for men and 91 fluid ounces for women) is a starting point, adjusting upward for pre-race activities. Heavy or salty sweaters may benefit from a short-term hyperhydration strategy involving an electrolyte solution with a higher concentration of sodium, but limit or avoid diuretic beverages like excessive alcohol and caffeine.
Pre-Race Morning Intake
The few hours immediately before the start are important for topping off fluid levels without causing gastrointestinal distress or frequent bathroom stops. Experts recommend consuming fluid about two to three hours before the race begins to allow for absorption and excretion of any excess. A general guideline is to drink 16 to 20 fluid ounces of water or a sports drink during this window. This timing allows the body to process the fluid, ensuring light-colored urine and permitting a final restroom visit. Approximately 15 minutes before the start, consume 6 to 8 fluid ounces to prime the stomach and top off circulating fluid volume.
Strategy During the Race
Fluid management while running is highly individualized, dictated by a runner’s sweat rate, ambient temperature, and humidity level. While some guidelines suggest drinking “to thirst,” a structured approach is necessary for optimal marathon performance, aiming for approximately 400 to 800 milliliters (13.5 to 27 fluid ounces) of fluid per hour. This volume is often broken down into smaller, more frequent intakes, such as 6 to 8 fluid ounces every 15 to 20 minutes. The stomach can typically only empty about 24 to 28 fluid ounces of fluid per hour, so exceeding this range risks discomfort. Utilizing aid stations requires a tactical approach, such as slowing down briefly or pinching the cup to prevent spillage and allow for controlled intake.
Sweat Rate Calculation
Runners should determine their individual sweat rate during training by weighing themselves before and after a long run in similar weather conditions. The weight lost, plus any fluid consumed, indicates the volume of sweat loss targeted for replacement on race day. In hotter or more humid conditions, the frequency and volume of fluid intake must be increased to match the higher sweat losses.
Fluid Composition: Electrolytes and Carbohydrates
The composition of the fluid consumed is as important as the volume to support hydration and energy needs. Sweat contains water and electrolytes, particularly sodium, which must be replaced to maintain fluid balance and prevent hyponatremia. General recommendations suggest a sodium intake of about 450 to 700 milligrams per liter of fluid. Fluids used during the race should also contain carbohydrates, providing a readily available energy source.
Types of Sports Drinks
Sports drinks are categorized by their osmolality (the concentration of particles relative to blood). Isotonic drinks, which have a similar concentration, typically contain 6 to 8% carbohydrates and are a common choice for quick fluid and energy replacement. Hypotonic drinks have a lower concentration, allowing for faster water absorption but delivering fewer carbohydrates, which is beneficial in hot conditions where rapid rehydration is the priority. Runners must practice with their race-day sports drinks and gels during long training runs to ensure stomach tolerance under running stress.