Runners’ Hydration Levels

heat_0I was so thirsty during a weekend half marathon a couple days ago. It was hot, but I normally don’t get THAT thirsty. I drank so much during the race, I got waterlogged and it was tough to finish. I clearly wasn’t hydrated properly.

This caused me to pause and start researching hydration. What is the best hydration technique for an endurance runner in a marathon?

In the article “Fluid Replacement During Marathon Running,” by Tim Noakes, M.D., published in the “Clinical Journal of Sports Medicine,” it states, “During endurance exercise, about 75 percent of the energy produced from metabolism is in the form of heat, which cannot accumulate. The remaining 25 percent of energy available can be used for movement. As running pace increases, the rate of heat production increases.”

The greater the body mass, the greater the heat production. Thus, to keep oneself cool, you need to drink fluids to ensure proper sweat availability for evaporation and circulatory flow.

But how much is enough? Surprisingly, the article didn’t give specifics. Rather, it just states for runners to drink ad libitum, meaning, drink what you need, but no more than 400 to 800 mL per hour. Drinking too much water can lead to hyponatremia, which is when electrolyte balance falls too low. Too much body water can dilute the serum sodium.

I conducted more research and found helpful information from the International Marathon Medical Director’s Association (IMMDA). Being a visual learner, this chart helped me to discern proper fluid intake levels:

Fluid Intake for Marathoners During a Race*

Finish Time
Race Pace
Fluid Intake
Fluid Intake Total
< 4 hours 10-12 oz / 20minutes 3.5-4.0 liters
< 8 minutes/mile 30-36 oz / hour
1000-1250 ml/hour
4-5 hours 8 oz / 20 minutes 3.0-3.5 liters
9-10 minutes/mile 24 oz / hour
750 ml / hour
> 5 hours 4-6 oz / 20 minutes 2.5- 3.0 liters
> 10 minutes/mile 18 oz / hour
500-600 ml/hour


For anyone running in an upcoming race, this hydration chart is helping me to gauge what I need to do to not feel so water logged and hopefully finish strong.