Optimal Hydration

Establishing a Hydration Plan for Marathons


How important is hydration to marathon runner safety and performance?

Staying properly hydrated is important during marathon racing for both safety and performance. Hydration status in marathon runners depends on the balance between sweat losses and fluid replacement. Dehydration occurs when fluid losses are not adequately replaced.  Sweat rates are increased by:

  • Warm, humid weather conditions
  • Male Gender (Men sweat 30% more than women of the same weight)
  • Fitness Level (More Fit = More Sweat)
  • Acclimatization (More training in the heat = More sweat)
  • Running Pace (Faster pace = More Sweat)

Increased sweat rates result in body water loss and accelerate the onset of dehydration, increasing the risk of early fatigue and heat-related illnesses in runners.  Keeping the body properly hydrated improves marathon safety and performance by maintaining blood volume and cardiovascular function.  Dehydration decreases blood volume, increases heart rate, and impedes heat loss, all of which may cause marathoners to slow their pace or become ill. 

It is also possible for runners to drink too much fluid, which can result in a potentially fatal condition called exercise-associated hyponatremia.  Hyponatremia often occurs in slower marathon runners who have the greatest opportunity to drink more fluid than they lose in sweat.  

Balancing fluid intake with sweat losses to avoid both dehydration and hyponatremia and finish the race with a 2-5% weight loss is the fluid replacement goal for a safe a marathon experience. Faster runners (< 3 hrs) should be more concerned with avoiding dehydration than developing hyponatremia. It’s important to determine your individual fluid needs because there is no single recommendation that applies to everyone, and drinking to thirst is adequate for slower runners.



What is exercise associated hyponatremia and why is it dangerous?

Exercise-associated hyponatremia (EAH) is defined as a blood sodium concentration less than 135 mmol/liter (135-145 is considered normal).  The sodium in a sports drink may delay the onset of hyponatremia, but drinking too many sports drinks can still result in hyponatremia.  This usually occurs in slower runners who are on the course for more than 4 hours; most commonly from drinking too much water or sports drink before, during, or after the marathon race.  Drinking too much fluid dilutes the blood sodium concentration if the kidneys do not properly clear the extra body water and becomes dangerous if it causes the brain to swell. Significant brain swelling can lead to death. Some other factors associated with EAH are gaining weight during the race, not losing any weight during the race, warm and cold temperatures, NSAID use, and small stature.



What are signs or symptoms of exercise-associated hyponatremia?

Symptoms begin to appear when serum sodium levels fall below 135 mmol/L and water begins to flow into the body cells.  Early symptoms of brain swelling in hyponatremia include weight gain, puffiness (e.g., swollen fingers, tight-fitting watch or rings), nausea, progressively worsening headache, vomiting, and a sense of “just not feeling right.”  More serious symptoms include confusion, irritability, agitation, or seizures, which left untreated can progress to serious brain swelling, lung congestion, coma, and death.

It can take time for hyponatremia symptoms to appear, so keep an eye out for these symptoms even a few hours after the race.  During or after exercise, it is critical not to drink large amounts of fluids if you are not sweating heavily and/or if you have not lost weight (see symptoms above) until you are urinating normally.



What are the signs of dehydration?

Signs and symptoms of dehydration include headache, thirst, dizziness, nausea, weakness, abnormal chills, thick saliva (i.e., difficulty spitting), irritability, and fatigue.



What is the best way to avoid dehydration?

The best way to avoid dehydration is to drink enough fluid to minimize loss (2-5%) of body weight during the race, but avoid over-drinking and weight gain.  In fast runners, thirst often lags behind and may be a late indication of dehydration and faster runners need to know how much fluid to replace each hour of running.  However, in slower runners thirst indicates that you are becoming dehydrated and should begin to replace your sweat losses. 



How do I know how much to drink?

The amount of sweat you lose should be a bit more than the amount of fluid you drink.  You can measure your hourly sweat rate so that you can know how much to drink per hour.  Follow these easy steps:

The amount of sweat you lose should be a bit more than the amount of fluid you drink.  You can measure your hourly sweat rate so that you can know how much to drink per hour.  Follow these easy steps:

1. Weighing yourself nude or in as minimal running clothes. (THIS is PRE-WEIGHT)
2. Run for 1 hour in the conditions and at the pace you expect to race.  Do not drink during this run.
3.  At the end of the run, strip down, towel off, and reweigh yourself. (THIS is POST WEIGHT)
[PRE-WEIGHT minus POST WEIGHT = maximum amount of fluid replacement per hour during the race]

SWEAT LOST IN 1 HOUR in ounces is your SWEAT RATE. No more than that amount should be replaced in each hour of your race. You should also weigh yourself periodically before and after training runs because your sweat rate will change based on your fitness and the time of year in which you are training. Most runners will lose 2-3 pounds of body mass during the course of a marathon and losses beyond that are most likely due to loss of body water.  During training and racing, drink at regular 15 to 20 minute intervals to minimize loss of body weight to approximately 2-5% of your starting body weight by the end of the race.  For example, if you begin the run weighing 140 lb and end the run weighing 137 lb, you have kept your weight nearly neutral for body water loss.  If your weight drops significantly below 137 lb this may begin to impair your performance.  If your weight is above 140 lb, it is a clear indication that you drank too much.  Drinking too much of any fluid (overhydrating) can lead to exercise associated hyponatremia.  If you are running at a slow pace (longer than 4 hours for the marathon) and you do not know your sweat rate, you should drink each time you notice feeling thirsty to stay safe from both hyponatremia and dehydration. 



How can I avoid both dehydration and hyponatremia?

Develop your own hydration program using these tips:

  • You’re unique, so don’t copy other runners.  Some runners need less fluid than you, while others will need more.  Learn your individual hydration needs. Fluid needs vary widely and slower runners need to be very cautious with their fluid intake while faster runners may need to drink more to replace higher volume sweat losses.
  • Try to match fluid intake to just below weight loss.  For example, if you lost 2 lbs (32 oz) during a 2 hour run, you should try to drink close to 24-30 oz but not more during that long run.
  • During a marathon, you should lose 2-3 pounds.  If you do not lose weight, you are 7 times more likely to get hyponatremia.
  • Do not overdrink.  Weight gain during a run is a sure sign of overdrinking.
  • If you are feeling the effects of hot weather, slow your pace. Drinking more fluid will not make you less hot or cool you down.  The first step, slow down or stop and rest.
  • If you are a slow runner, determine the fluid intake that keeps your weight balanced with a slight 2-5% loss during a long run or drink when you are thirsty.  The rate of sweat and weight loss for the same distance varies according to weather conditions and running speed. 
  • Keep your urine a pale yellow color like lemonade, neither dark like apple juice (dehydration) nor clear like water (overhydration)
  • Recognize the warning signs of dehydration like feeling faint or light headed with standing, rapid heart rate, sunken eyes, dry mouth, feeling very thirsty, or dull headache.  Try some fluids to see if you improve.
  • Recognize the warning signs of hyponatremia like water sloshing in your stomach, severe and worsening headache, or feeling puffy or bloated in the hands and feet, nausea, upset stomach, or wheezy breathing. Stop drinking until you begin to urinate and the symptoms resolve.
  • If you are not feeling well during or after the race and simple changes do not make you feel better, seek immediate medical attention.
Bill Roberts, M.D.

Founder and Past-President, IIRM (formerly ARRMS)
Past-President, American College of Sports Medicine
Medical Director, Medtronic Twin Cities Marathon

Pierre d’Hemecourt, M.D.

Immediate Past-President, IIRM
Co-Medical Director, Boston Marathon

George Chiampas, D.O.

Past-President, IIRM
Medical Director, Bank of America Chicago Marathon

John Cianca, M.D.

Past-President, IIRM
Medical Director, Chevron Houston Marathon

Joseph Chorley, M.D.

Associate Medical Director, Chevron Houston Marathon


The International Institute for Race Medicine (IIRM) is committed to ensuring that every event and medical personnel have accurate medical information and guidelines to promote athlete safety.



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