With the onset of long, hot summer days, hydration becomes increasingly important. Sweat losses during prolonged exercise can easily reach 1.5 liters an hour and when you add in a hot, humid climate, that number can exceed 2.5 liters per hour. This can easily turn into dehydration and poor performance if you don’t have a strategic hydration plan.
Fluid loss of as little as 1 percent of total body weight can lead to an elevation in core temperature during exercise. Fluid loss of 2 percent can impair endurance performance. Loss of 3 to 5 percent of body weight results in cardiovascular strain and impaired ability to dissipate heat. At this level of dehydration, athletes can experience a variety of symptoms including impaired mental concentration, slower response time, muscle spasms, and heat exhaustion. At loss of 7 percent body weight, collapse is likely, and fluid loss of greater than 10 percent is likely to result in death.
Why is dehydration so harmful for athletes?
As we lose water through sweat and respiration during exercise, plasma volume (the amount of water in the blood) decreases. As plasma volume declines, blood flow to the skin (which is one way the body regulates body temperature) is reduced. At the same time, cardiac output is impaired and stroke volume (the quantity of blood pumped from the heart with each beat) is reduced. Heart rate increases to compensate but cannot offset the deficit in stroke volume and exercise performance begins to suffer.
Cognitive function, reaction time, and concentration are also negatively impacted by dehydration. General malaise, weakness, and headaches are also common—a combination that will interfere with mental reserves and harm athletic drive and motivation.
Sports medicine experts recommend that every athlete training in the heat develop a hydration plan. The first step is to start a body weight log to record body weight before and after training. Change in body weight correlates with loss of water with each pound (0.45 kg) lost equaling 1 pint (0.5 L) of fluid. Sweaty clothes should be removed for accuracy because sweat trapped in clothing will increase weight recordings.
The body weight log can identify both acute hydration needs after single training sessions as well as chronic dehydration, such as the progressive loss of 5 to 10 pounds over the course of a week. Another tool is to calculate an athlete’s Sweat Rate. This is a calculation based on body weight and fluids in/out over the exercise time period. It requires you to weigh yourself before and after exercise, and account for all fluid intake and urine output during training:
|Body Weight Before Exercise
|Body Weight After Exercise
|Change in Body Weight (B-C)
|0.7 kg = 700 g
|Drink Volume During Exercise
|Sweat Loss (D+E-F)
|Sweat Rate Per Minute (G/H)
|Sweat Rate Per Hour (I X 60)
This calculation tells us that you should consume a bit over half a liter per hour to keep up with sweat rate and maintain a good hydration status.
The ACSM provides additional fluid recommendation guidelines, with fluid replacement ideally occurring as part of the workout nutrition plan and taking place in three general time frames:
- About 2 hours prior to training, consume about half a liter (17 oz.).
- At 15 to 20 minutes before exercise, consume 250 to 500 ml.
- Drink 115 to 185 ml every 15 minutes of exercise.
- Consume larger volumes (8 to 16 oz.) for faster stomach emptying.
- Use a solution of electrolytes and 6 percent or less carbohydrates for faster hydration.
- Drink 500 ml in the 2 hours after exercise
- In the 24-hours post-exercise, continue drinking, consuming 16 ounces for every pound lost, until body weight has reached baseline and urine is nearly clear.
What about electrolytes?
Electrolytes are charged minerals that allow for proper hydration within our cells, our bloodstream, and in the extracellular space that allows nutrients to migrate into cells from the bloodstream. The electrolytes we are primarily concerned with are sodium, chloride (which typically follows sodium), potassium (which opposes sodium), and magnesium. Sweat rates among athletes vary widely as do sodium concentrations in sweat (0.46 to 2.3 g/L of sodium). Additionally, sodium intake is diet-dependent with people who eat primarily processed and packaged foods, as the average Westerner does, getting a relatively high sodium intake. On the other hand, people who eat diets of whole foods, often need to add sodium to their diets. Heavy sweating may increase these needs.
Potassium requirements are rarely met in the standard American diet. The average American potassium intake is 3,300 mg whereas the National Research Council recommends 4,700 mg of potassium a day, so it takes careful planning to reach the adequate level. Magnesium levels are also rarely sufficient in the diet, and magnesium is easily depleted during exercise, with high levels of magnesium present in sweat.
There is not one size fits all recommendation for electrolytes, however, most people who sweat heavily will benefit from including electrolytes in their workout fluid plan. Additionally, combining electrolytes with glucose can decrease urine output, which helps the body to gain water quickly during dehydration. It also increases palatability to encourage athletes to drink. A drink containing some salt and around 6 percent carbohydrates appears to be best for rehydrating during and after intense exercise. Recovery drinks can be purchased as pre-made drinks (Gatorade, Powerade) or as powders that can be added to water. They can also be made at home by mixing one-part fruit juice with four parts water and some salt.