The Rules About Fluids

How Much Fluid Should I Drink During Endurance Events?
It is IMMDA’s desire to educate and ensure that participants consume proper amounts of fluids during endurance events to remain healthy and perform well. Too much or too little may bring about health concerns and/or poor performance.

What should you drink?
If your event or workout is longer than 30 minutes you should drink a sports drink instead of water. The added carbohydrates and electrolytes speed absorption of fluids and have the added benefit of energy fuel and electrolytes. There is actually decreased benefit to watering down or diluting sports drinks or alternating sports drinks with water.

How much should you drink?
Drinking too much or too little can be of risk to health and performance. Hyponatremia (low blood salt level due to abnormal fluid retention from over-drinking) and dehydration (due to net fluid losses from under-drinking) are conditions easily averted by understanding your individual body needs. Just as you have a unique face and fingerprint, your body’s need for fluid is individual as well. Body weight, gender, climate and sweat rate are just a few variables that individualize your needs. It is normal to lose a small amount of bodyweight during a half or full marathon: bodyweight will re-equilibrate over the next 24 hours through the consumption of sodium and fluids with meals. A weight loss of more than 2% or any weight gain are warning signs that justify immediate medical consultation and indicate that you are drinking improperly.

Try to drink to thirst.
This advice seems way too simple to be true; however, physiologically the new scientific evidence says that thirst will actually protect athletes from the hazards of both over- and under-drinking by providing real time feedback on internal fluid balance. If you are not thirsty, try to refrain from drinking. Do not feel compelled to drink at every fluid station nor follow the cues of other runners – their fluid needs are probably very different from your own. If you are “over-thinking” and feel you cannot rely on this new way of thinking, experiment in your training with one of these other ways, realizing each has its own cautions as well.

Approximation of Fluid Replacement
There are individual variations: “one size does not fit all.” IMMDA members endorse thirst as the best scientifically supported method for you to use. These alternate methods may not take into account changes in ambient conditions, running speed and terrain, which can change your level of thirst.

Runners and walkers who are interested in the endurance “experience” rather than pursuing a “personal best” performance, must resist the tendency to over-drink. Runners/walkers planning to spend 4-6 hours on the course are at risk for developing fluid-overload hyponatremia and usually do not need to ingest more than one cup (3-6 oz: 3 oz if you weigh approximately 100 lbs and 6 oz if you weigh approximately 200 lbs) of fluid per mile. Athletes should avoid weight gain during an event.

Some participants may find that adjusting their intake to pace or time is easier for them (as shown below) but remembering thirst is the best method:
Adjust the rate of fluid intake to race pace: slower race pace = slower drinking rate; maximum intake of 500 ml/hr (4-6 oz every 20 min) for runners with greater than 5 hour marathon finishing times (10-11 min/mile pace). Weight monitoring is also important: if you gain weight during your workout or event, you are drinking too much.


For a more highly motivated runner/walker who desires a numeric “range,” a fluid calculator can provide an estimate of body fluid losses as a generalized strategy for fluid replacement. Participants concerned about peak performance are advised to understand their individualized fluid needs through use of this fluid calculator but ALWAYS defer to physiologic cues to increase fluid intake (thirst, concentrated dark urine, weight loss) or decrease fluid consumption (diluted or clear urination, bloating, weight gain) while participating. It is also important to recognize that if you use this method in one climate and then travel to a different climate for your event, the humidity will change your sweat rate and therefore your fluid needs.

Fluid calculator to calculate sweat rate:

  1. Weigh nude before the run.
  2. Run/walk at race pace for one hour. (One hour is recommended to get a reliable representation of sweat rate expected in an endurance event.)
  3. Track fluid intake during the run or walk; measure in ounces.
  4. Record nude weight after the run/walk. Subtract from starting weight. Convert the difference in body weight to ounces.
  5. To determine hourly sweat rate, add to this value the volume of fluid consumed (in Step 3).
  6. To determine how much to drink every 15 minutes, divide the hourly sweat rate by 4. This becomes the guideline for fluid intake every 15 minutes of a run.
  7. Note the environmental conditions on this day and repeat the measurements on another day when the environmental conditions are different. This will give you an idea of how different conditions affect your sweat rate.

Good luck in your training. Experimenting with your fluids can be a fun exercise. Remember to keep in mind that the consumption of beverages and foods containing sodium or carbohydrate should be guided by the goal to minimize loss of body weight and prevent weight gain.

Tips for Race Day

  • Check your urine 1/2 hour before the race, if clear to yellow (like light lemonade) you are well prehydrated. If dark and concentrated (like iced tea) drink more fluids.
  • During the race, drink when you are thirsty and no more
  • DO NOT take any product with ephedra in it. Ephedra increases your risk of “heat illness.” It should not be used while training or on race day.
  • Stay away from dehydrating agents such as cold medicines, anti-diarrhea products, sinus meds and caffeine, which all can lead to dehydration; you may take them again a few hours after finishing the race. No more than 200mg caffeine race morning, which is the equivalent of 2 cups of coffee. If no contraindication, take ½ baby aspirin (81 mg) the morning of the race. It will help blood flow within your heart.

Too Much Fluid Can Be Harmful
Most athletes understand the importance of drinking fluids, but some don’t understand that drinking too much can be harmful as well. Over-hydrating can lead to a dangerous condition known as hyponatremia (low blood sodium). Runners or walkers out on the course for long periods, losing lots of sodium in sweat, are at risk. Overzealous drinkers who drink lots of water in the days prior to the race and then stop at every fluid station along the course also may risk hyponatremia. This condition can lead to nausea, fatigue, vomiting, weakness, sleepiness, changes in sensorium and in the most severe instances, seizures, coma and death.

To Avoid Hyponatremia Follow These Easy Guidelines:

  • Follow the fluid recommendations.
  • Include pretzels or a salted bagel in your pre-race meal.
  • Favor a sports drink that has some sodium in it overwater, which has none.
  • After the race, drink a sports drink that has sodium in it, eat some pretzels, a salted bagel or any other salty food.
  • In the days before the race, add salt to your foods(provided that you don’t have high blood pressure or
  • your doctor has restricted your salt intake).
  • Eat salted pretzels during the last half of the race.
  • Carry two small salt packets with you, eat one just before starting and the second during the last half of the race.
  • Stop taking non-steroidal anti-inflammatories 24 hours before your race and do not start again until a minimum of six hours after finishing the race (see list at right).

During Training: Weigh In Daily
Step out of bed every morning and onto the scale. If you’re anywhere from 1% to 3% lighter than yesterday, re-hydrate by drinking 8 ounces of fluid for each pound lost before training again. Over 4% lighter, re-hydrate and back off that day’s training intensity and see a doctor.

And After Workouts?
Weigh yourself right before and after workouts. For every pound you lost, drink a pint of electrolyte replacement fluid that has carbohydrate and protein in it. Carbohydrate replaces your glycogen (stored glucose) and the protein aids in muscle recovery. A good recovery drink has a ratio of 3-4:1 (carbohydrate : protein). Chocolate milk has been shown to be a great recovery drink; try it during training.