Relentless Forward Progress_ A Guide to Running Ultramarathons - Bryon Powell [44]
The most effective way to measure sweat rate is to conduct a sweat test on yourself. To do this, weigh yourself completely naked before going out for a run of 60 to 90 minutes in length. After weighing yourself, do not eat, drink, or use the bathroom until you’ve completed your run, at which time reweigh yourself completely naked. After your post-run weigh-in, calculate the difference between your starting and finishing weight. Each pound you lost during the sweat test represents 16 ounces of sweat. Divide the total number of ounces lost to sweating during the test by the number of minutes you ran. Multiply this number by 60 to determine your sweat rate in number of ounces per hour.
Keep in mind that sweat rate varies markedly depending on the exact environmental conditions and level of exertion. Therefore, you may want to conduct multiple sweat tests to determine your sweat rate under a variety of conditions. In particular, if you are determining your hydration rate with a particular goal race in mind, conduct the sweat test under conditions that mimic likely race conditions as closely as possible.
As you gain experience, you’ll likely be able to estimate how much fluid you should be consuming in various situations. No matter how much you are sweating, it’s difficult for the body to process much more than 24 to 28 ounces of water per hour.
Electrolytes and Hydration
During long bouts of exercise, be sure to take in electrolytes. In fact, it’s crucial to maintain a proper balance between water and electrolyte intake—this balance plays an important role in muscle, digestive, and other bodily systems.* Electrolytes include water-soluble ions such as sodium, potassium, calcium, magnesium, and chloride. Each of these electrolytes is shed from the human body in sweat. While modern diets provide sufficient amounts of these minerals for less active individuals, sweat loss from prolonged exercise easily results in short-term electrolyte deficiencies when insufficient amounts of electrolytes are consumed during exercise.
* The most dangerous hydration–electrolyte imbalance, dilutional hyponatremia, is caused by overhydrating while consuming insufficient electrolytes. See the Water–Electrolyte Balance table for more information regarding the symptoms and treatment of dilutional hyponatremia.
Replacing sodium is important once you start replacing fluids during runs, and it becomes crucial as the length of the run approaches ultra distances. In warm weather it’s typical to consume 400mg of sodium** per hour, while 800mg or more may be prudent in hot weather. Potassium loss is small enough that the consumption of a sports drink or electrolyte tablet to replace sodium provides the necessary potassium. I will leave the debate as to which other electrolytes are necessary during exercise up to the various sports drink and electrolyte manufacturers. The highly variable inclusion or exclusion of electrolytes other than sodium and potassium suggests a lack of scientific consensus on the subject.
** The given amounts are expressed for sodium. This equates to 1 to 2 grams of sodium chloride, aka table salt.
The three most common ways to replenish electrolytes while running are through sports drinks, food, and dietary supplements. Each of these means of electrolyte replacement varies significantly in terms of the types and quantity of electrolytes. With that in mind, it’s easiest to focus on one or two primary sources of electrolytes during a particular run. Often these sources are a sports drink and electrolyte tablets. However, even if you focus on these two sources, don’t turn a blind eye if you consume significant amounts of sodium from other sources such as broth, snack foods, or salted potatoes commonly found at ultras.
Electrolyte-containing sports drinks are the most familiar way for an athlete to replenish electrolytes on the go. Most often, these sports drinks are available in pre-mixed