Why, exactly, do we as ultramarathoners need to drink so much fluid? What is so magical about being hydrated? If you think about it, the result of the human body running and running and running boils down to one thing – heat production. As we exercise, we produce heat, especially as the intensity increases. If we run slower, we don’t produce “cool,” we just produce less heat. So before we get so overheated that we suffer heat illness, our body attempts to regulate our temperature through several methods: evaporation, convection, radiation and conduction. Since evaporation is our most efficient “refrigeration system” and accounts for 80% or more of our heat loss during exercise, that is where this article will focus.

The biggest factor in ultrarunning is that evaporation requires water and causes water loss from our bodies. Ah…hydration again! We need to drink to replace the liquids lost in sweat. And we need to sweat to allow the evaporation mechanism to keep our bodies cool and dissipate our accumulated heat. And we need to stay cool to maximize our performance and allow our bodies to run as fast as our training and genetics allow.

Much has changed regarding hydration in the past few years. Tim Noakes has written about the research in his book, Waterlogged, which led to the new dictum, Drink to your thirst. And Loyola University’s Jonathan Dugas further explains the misunderstanding regarding the idea of fluid balance and volume replacement. Fluid balance is the key to maintaining performance. Being fluid balanced means you keep that fluid concentration the same inside and outside the cells. “Drinking even half of what you sweat out during a race will accomplish this goal, a wonderful revelation for those who have suffered bloated runs for far too long.” Volume replacement may end up causing hyponatremia, which is potentially fatal. So we need to replace water lost in sweat, but not too much…

Sweating is a response influenced by our training and fitness level. We sweat sooner and more as we increase our miles and conditioning. Several other factors can also affect sweat rate – intensity of effort, temperature, the clothing we wear and the duration of the exercise. In an ultra race, the temperature and the length of the race are the same for everyone. We can control our clothing by choosing lightweight, light colored, breathable fabrics. Garments saturated with sweat (even if they are quick-drying) provide the most effective evaporative cooling; sweat dripping off you or your clothes is ineffective at cooling. So that leaves intensity as the modifiable factor. Average sweat rates at 65% intensity are about a liter per hour. As intensity increases, so does body temperature and the need for regular, and maybe more, fluid ingestion to replace losses and continue cooling our bodies.

Cardiovascular Drift

In addition to an increase in body temperature with increased intensity, there is an increase in metabolic cost known as “cardiovascular drift” with longer durations of exercise. This is the slow rise in heart rate and oxygen consumption with the same work effort. This means the intensity of our effort is increasing even though we are not running any faster. Glucose ingestion helps prevent cardiovascular drift, but in a long-distance ultra, this phenomenon will happen eventually.

Sweat Rate

It really helps to know your personal sweat rate…don’t put this off any longer! To do this, weigh yourself naked before a one hour run. It is easiest to do this if you do not drink or eat anything. It is ideal to do this test in conditions that will mimic those in your race. When you’re finished, towel off, pee if you have to, and weigh yourself again nude. The amount of weight you lost equals approximately the amount of fluid you lost in that hour, with one pound equaling 16 ounces. Once you have the amount of liquid per hour that you lost, do the same thing again on one of your long runs, and drink your “sweat rate” in liquids per hour. After a few hours of running, especially if the temperatures are warming up, you may find that you are still short. Which in itself is normal, but you may need to drink more water or add more salt, or both, as the day and you get hotter, if you become thirsty. If you have gained weight after many hours of running, this is a red flag for hyponatremia. You may be slowing down or decreasing the intensity and actually need less liquid per hour. Remember, as you become more trained, your sweat rate may change. You do not want to become hyponatremic – remember fluid balance versus volume replacement.

How Much Salt

What is the role of salt, or sodium, when ultrarunning? Sodium helps with water levels – inside your cells, outside you cells and in your blood. Too much water intake dilutes the sodium outside the cells, which then become leaky, allowing water across the cell membranes resulting in too much water inside the cells. The cells then swell, even brain cells, and the outcome is hyponatremia. Too little water and the blood becomes thick resulting in a slowing down of oxygen and nutrient delivery or dehydration.

Since sodium helps stabilize our water levels, learning your sweat rate can also help you with the amount of sodium you need to replace when running under different conditions. Some of the factors that affect this are:

How Much Salt Is In Your Sweat

Individuals have different amounts of sodium in their sweat that can range from a low of 220 mg. to over 1,000 mg. per pound of sweat. Average is about 500 mg. sodium per pound.

How Much You Sweat

Sweat rates normally range from 16 to 64 ounces per hour or one to four pounds. (Remember one pound equals 16 ounces.)

How Much You Exercise In The Heat

Training helps with acclimatization. The same ultrarunner can lose upwards of 1,000 mg. of sodium per pound of sweat if they are not used to the heat. However if acclimated that same runner’s loss can drop to 300 mg.!

How Long You Are Running

The longer you sweat, the more electrolytes you may lose. Pay attention to drinking sports drinks with sodium (some of the newer endurance drinks have twice the amount of sodium per the same amount of carbohydrate), eating salty snacks and taking electrolyte tablets as the miles accumulate.

While some ultrarunners can tolerate running while dehydrated better than others, your performance will be best when you are replacing most of the fluids you lose. Studies show that body weight loss is normal (up to 2% or more) and usually not detrimental to your time. But accumulated and excessive dehydration can spell disaster in the form of DNF’s or slower-than-usual results. Be sure to start your training run or race hydrated, and then DRINK TO YOUR THIRST. This is the best current advice from the experts, but even they say this mechanism is not a failsafe. It is possible to push “past dehydration” (for awhile anyway) and it is also possible for the brain itself to overheat, causing the usual protective system to fail and heat illnesses to occur. Be proactive by acclimating and experimenting with different fueling and hydration strategies to see which one works for you.


About Author

Sunny Blende, M.S. is a Sports Nutritionist who writes and counsels individuals and teams on fueling for enhanced performance and making healthy food choices. Currently she writes the nutrition column for UltraRunning magazine and runs ultras herself. She has presented at the National RRCA Convention, the National Rowing Convention, Nike San Francisco Marathon Expo, and the Runners World San Francisco Marathon and worked as an assistant with the Los Angeles Marathon Association. An avid master competitor herself, she trains and competes in the San Francisco Bay Area and beyond. Sunny received her Bachelor of Science degree from University of Southern California and her Masters in Human Nutrition degree from University of New Haven. (Photo at left, by Luis Escobar).


  1. Nice article Sunny. I’ve read Dr. Noakes’s “Waterlogged” and I encourage others to also. I don’t think we need to measure sweat rate, because thirst rules the day. Even the “reluctant” drinkers and race leaders who experience the most water loss in race fields seem to do quite well.
    Sodium losses in sweat vary depending on day to day dietary sodium intake, heat adaptation, and quite possibly sodium stores that are not yet understood. Thanks to you, and Dr. Noakes’s advice, runners have been able to back off on excessive drinking, but the harder thing for runners to back off on is sodium “replacement.” If a 150 lb runner is expectated to lose 5 lb that means the runner has less vascular volume, which in turn concentrates the vascular sodium. Typically, thirst and effort will keep that runner at 145 lb or whatever weight training adaptations so dictate, for the remainder of the race. Introducing sodium “replacement” during the race, is likely to raise the serum sodium concentration even further, wouldn’t it? And how will the runner adapt to these higher blood sodium levels- the runner will lose that sodium through sweat and urine. The concern here is that sodium will need to travel out of the body, and it needs to do that in water. I’m not aware of a clnical trial that clears this up, but it seems to me that sodium supplementation would lead to loss of precious water, and quite possible side-effects of higher sodium levels?

  2. I understood Waterlogged to explicitly state that there is no connection between hydration and body cooling. From the book, I would take your statement that >As intensity increases, so does body temperature and the need for
    regular, and maybe more, fluid ingestion to replace losses and continue
    cooling our bodies.< to be incorrect and more of the same old misinformation. Fluid ingestion does not cool the body. Our bodies are not like car engines, and water/Gatorade is not engine coolant. The winning runners in marathons and ultras have lost the most weight and sweat, and tend to take in the least fluid, and are the most dehydrated. The only thing that cools a runner is to slow down.

    • Thought: What about the heat capacity of water though? Notice the difference in the temperature of water vs soil (or any material) when exposed to an increase in heat energy. Water has a high heat capacity and therefore will absorb more energy before raising in temperature. Think Ocean vs. sand. I would think that a well hydrated body (cells, tissues, blood, etc) would be able to buffer temperature swings much better than a dehydrated one?

      • It seems to make sense, but I don’t think that water works that way (that simply anyway) in the body. I don’t know. I do know the research shows that test subjects and controls’ rectal temps increased at the same rates as they hammered exercise and to the same levels regardless of the fluids they ingested.

        My (anecdotal!) evidence was a half marathon I ran after reading the book, taking no fluid other than a small bottle of double-strength accelerade, from which I sipped, to get the sugar (which apparently does enhance performance). I tend to sweat a ton, up to 50 ounces/hour. In a 1:45 half marathon with probably 10 ounces of fluid, I actually ran negative splits, finishing at a 7:00 pace uphill the last mile. Of course, that was but one experiment, with one subject — me.