Runner's World translated clinical hydration data into a practical marathon guide. Learn how to measure sweat rate, calculate fluid needs, and adjust sodium.
On September 16, 2026, Runner's World UK published a detailed marathon hydration guide for endurance athletes. The article translates clinical fluid and sodium recommendations into practical race day strategies. Hydration researcher Lindsay Baker, PhD, director of the Gatorade Sports Science Institute, contributed to the guidelines. Sports dietitians Meghann Featherstun and Starla Garcia also provided their professional insights.
The primary conclusion of the publication is that marathon hydration must be individualized rather than based on fixed volume targets. The experts translated fluid and sodium recommendations from the American College of Sports Medicine (ACSM) and the Institute of Medicine (IOM) into personalized ranges. For baseline daily intake, the article cites approximately 3.7 litres of total water from beverages and water-containing foods for men. It cites 2.7 litres of total water for women.
These baseline figures specifically apply to adults aged 19 to 50. However, Runner's World describes those figures strictly as a baseline rather than a universal marathon target. The guide emphasizes that daily fluid requirements increase based on exercise, sweating, weather, and other personal factors. Rather than attempting to follow one universal rule, athletes should calculate their ranges using body weight and sweat rate.
The guide outlines a practical method for determining individual fluid needs during a training cycle. Runners can estimate their personal sweat rate by weighing themselves immediately before and after a training run. The article treats each 500-gram reduction in body mass as approximately 500 millilitres of sweat loss. Sweat rate varies with weather, heat acclimation, speed, body weight and genetics.
Therefore, athletes should record their fluid consumption alongside their exact workout conditions. For pre-exercise hydration, the guide cites an ACSM recommendation of 2 to 4 millilitres per 500 grams of body weight. Athletes should consume this fluid during the two to four hours before exercise. For a 70-kilogram athlete, Runner's World translates that body weight range into approximately 300 to 600 millilitres of fluid.
Baker recommends checking your urine approximately two hours before exercise to gauge baseline readiness. If urine is dark or production is low, she advises an additional 3 to 5 millilitres per kilogram of body weight. Pale yellow urine serves as a practical sign that the runner has reached a reasonable pre-race hydration state. On the day before a marathon, Baker advises keeping hydration broadly similar to a normal day.
Runners should avoid aggressively drinking abnormally large volumes of water right before race morning. Instead, they should combine their ordinary baseline needs with fluid lost through sweating. This includes accounting for fluid lost from a pre-race shakeout run or extensive walking at a race expo. On race morning, Featherstun recommends approximately 500 millilitres of a preferred sports drink with the pre-race meal.
This meal should ideally occur two hours before the start. For a later start, the article gives an example of 500 millilitres four hours before the race. The athlete would follow this with another 500 millilitres two hours before the race for a total of 1 litre. The guide advises sipping rather than chugging and stopping roughly two hours before the start.
Featherstun describes the two-hour pre-start period as particularly important for overall preparation. She advises runners to avoid trying to correct their hydration immediately before the starting gun fires. Stopping fluid intake two hours early reduces the likelihood of needing a bathroom stop early in the race.
During the marathon, the stated practical objective is to limit dehydration to approximately 2 to 3 percent of body weight. The article explicitly cautions that runners should not assume they must replace 100 percent of their sweat loss. Runner's World states that people can tolerate only approximately 700 millilitres of fluid per hour while running. The guide warns runners not to treat that maximum tolerance figure as a required intake target.
Avoiding severe fluid depletion is critical for maintaining optimal cardiovascular performance. According to Garcia, even a 1 percent reduction in body weight makes the heart beat three to five times faster per minute. For a 70-kilogram runner, that 1 percent reduction equals exactly 700 grams of lost mass. A 2 to 3 percent body weight loss can significantly impair overall athletic performance.
Garcia notes that inadequate hydration can increase fatigue, reduce cognitive function, and potentially contribute to muscle cramping. Featherstun provides a general practical suggestion of taking at least a few sips at each aid station. This minimum baseline equates to approximately 100 to 200 millilitres per hour. However, the article says that runners can vary widely in their actual fluid requirements on race day.
Some athletes may need only occasional sips during cooler events. Others may tolerate approximately 500 to 600 millilitres per hour depending on their sweat rate and course conditions. For heavy sweaters, the guide discusses approximately 400 to 800 millilitres of fluid per hour. It pairs this volume with 450 to 700 milligrams of sodium per litre based on its IOM-based discussion.
For runners not relying on course drinks, Garcia recommends products containing at least 500 to 700 milligrams of sodium per serving. Runners may combine servings or use water alongside salt tablets, but they must test this combination heavily during training. Practicing the strategy prevents gastrointestinal distress or ending up with an excessively sweet drink.
For ambitious endurance athletes over 35, applying these specific fluid and sodium ranges is a foundational strategy for performance. As we age, assuming our hydration needs remain identical to our younger days is a critical mistake. We must adjust our fueling and hydration to protect our aging cardiovascular systems from excessive strain. The guide correctly focuses on pre-race and in-race fluid management strategies.
However, it does not provide a separate post-race protocol or a clinical discussion of exercise-associated hyponatremia. Therefore, mid-life athletes taking blood pressure medication or managing cardiovascular conditions should seek individualized medical advice before increasing sodium intake. The Runner's World article does not provide condition-specific clinical protocols for hypertension, kidney disease, heart disease or diabetes.
Hitting my forties brought a harsh reality check. The track workouts were not getting slower, but the days after them felt significantly heavier. Instead of forcing my old Tuesday and Thursday intensity schedule, I looked at the data on Masters athletes and muscle protein synthesis. I pushed my second hard session to Friday, allowing an extra forty eight hours of low intensity recovery.
My total weekly volume stayed the same, but the quality of my intervals skyrocketed. Respecting the recovery curve after forty applies to hydration management as much as it applies to weekly mileage. If we severely dehydrate our bodies during a long effort, the resulting cardiovascular strain lengthens our necessary recovery window. We encourage athletes to read our fueling and hydration approaches for detailed guidance on balancing these physical stressors.
Testing your intake ranges in training ensures you do not shock your digestive system on race day. Garcia recommends testing electrolyte drinks containing 300 to 500 milligrams of sodium mixed with water during the training cycle. This practice helps runners assess stomach tolerance and establish flavor preferences well before race week arrives. The article also advises checking which sports drink the race supplies and testing it thoroughly before race day.
Runners must decide in advance whether to use course fluids or carry the same products used in training. Hydration planning must also integrate seamlessly with carbohydrate intake. Carbohydrates are described as the primary fuel for exercising muscles during endurance events. Baker stresses the importance of consuming energy gels during races to reduce the risk of a late race collapse.
Runners who develop nausea can adopt evidence-based carbohydrate targets using higher-carbohydrate drinks as an alternative delivery method. Whether using solid fuels or liquids, endurance athletes must practice their personal hydration strategy heavily to avoid mid-race complications. Ultimately, Reendure translates complex sports science into practical training guidance for older athletes who want to stay competitive. The Runner's World guide validates our approach to measuring, testing, and adjusting protocols based on actual biological feedback.
We rely on clear, structured routines that maximize performance without ignoring the realities of an aging body.
A personalized hydration and sodium strategy limits cardiovascular strain, prevents late stage performance drops, and ultimately protects your long term athletic longevity.
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