Marathon Nutrition: Hydration and Electrolytes

With the countdown to the NY Marathon it is important to consider how to prevent dehydration during the run and how to maintain electrolytes. Multiple studies have linked dehydration, measured as a loss of 2-3% body weight, with decreased performance. This is partly because dehydration increases muscle glycogen use, depleting your power stores, and decreases cardiac output, making your heart work harder. It can also lead to gastrointestinal distress. On the other hand, if you over hydrate you put yourself at risk of hyponatremia, or too low of sodium levels which can put you at risk for headache, dizziness, weakness or fainting. This is because you need to maintain a proper balance between water levels and electrolyte levels to sustain the perfect concentration of minerals for optimal performance. If you over hydrate, you increase the need to void during the race. That being said, you want to find the balance between becoming dehydrated and decreasing performance or drinking too much water and decreasing performance on the other extreme.

How do you know how much water to drink? Because race time environments, climates, and personal sweat levels vary, it is difficult to generalize recommendations for fluid intake. A good way to judge how much water you need to drink is to weigh yourself before and after an hour long run with similar climate to the climate that you will be racing in. Don’t drink water in between weigh ins. Then take the number of pounds that you lost and multiply it by 16 to get the number of ounces that you should take in per hour. Alternatively, you can use the following equation:

((# lbs lost x 16oz) + water consumed) / hours run = oz of water / hour

Once you have hydration under control, you need to think about how to maintain electrolyte levels, most importantly sodium. Sodium is necessary for your nerves to communicate to your muscles when and how to contract. It is also important in regulating amount of water retention which affects blood pressure. Symptoms of hyponatremia can be severe and in extreme cases fatal. Because you are sweating out electrolytes during the race you don’t want to wait until you get to the point of muscle cramping to replenish electrolytes. Some easy ways to replenish electrolytes are sports drinks, electrolyte tablets, or sodium packets. Just like anything that you will be consuming during race day, you should do “dress rehearsals” to determine which of these methods works best for you!

Coleman, E. Nutrition for the Marathon and Beyond: Optimize your performance by proper fueling. Marathon & Beyond. Sept/Oct 2012. 88-101. Jeukendrup, A. Nutrition for Endurance Sports: Marathon, Triathlon and Road Cycling. Journal of Sports Science. 2011.29.(S1).S91-S99. Stellingwerff, T. Contemporary Nutrition Approaches to Optimize Elite Marathon Performance. International Journal of Sports Physiology and Performance. 2013.8.573-578.

Wardenaar, F Nutrient Intake and GI Complaints of Ultramarathon Runners. International Journal of Sports Nutrition and Exercise. 2015.

Elizabeth Brewer, SPT & Kathleen Leninger, PT, DPT, NY Custom PT & Performance