One of the most important fueling techniques that is often overlooked is the importance of adequate hydration. In addition to the daily body losses from breathing and normal gastrointestinal, kidney, and cooling functions of the body, even more losses occur during exercise in order to dissipate the heat generated as a byproduct of muscular work. Hot/humid environments exacerbate losses further to keep the body in safe temperature ranges. There is a lot of complexity and individual responses as it relates to fluid losses during exercise. However, generally, a weight loss of just 1% of body weight (1.5 pounds for someone weighing 150 pounds) can negatively affect the body’s ability to cope with the stress of exercise, which in turn increases risk of injury. A weight loss of 2% or more of body weight (3 pounds in someone weighing 150 pounds, 4 pounds in someone weighing 200 pounds), negatively affects cognitive function and aerobic performance, especially in hot weather. When 3-5% of body weight is lost, performance declines for anaerobic/high-intensity activities, as well as aerobic performance in cooler temperatures. Severe losses are considered those of 6-10% of body weight and have more significant effects on exercise tolerance, decreases in cardiac output, sweat production, and skin and muscle blood flow (Thomas et al., 2016).
Prior to discussing race day hydration, it’s important to ensure that daily hydration needs are being met. Many athletes show up to events already in a dehydrated state, so the first step is to prevent this. Monitoring urine color and volume can be a practical way of monitoring daily hydration – urine should be light in color (lemonade) and produce a good volume. Some use a general recommendation of 1 ounce of fluid per pound of body weight, but there are caveats to this recommendation, as some have different sweat rates, body composition, and activity levels that affect the daily needs. Fluids are also found in the foods that we eat, contributing to overall fluid balance. Therefore, monitoring of daily urine can be a practical tool.
The table below outlines basic recommendations for fluid intake for before, during and after activity. It’s important to point out that there can be individual variability in these recommendations, so working with an experienced sports dietitian is warranted if you have experienced challenges with your hydration.
Basic Fluid Requirements
Here are some tips to help you stay hydrated on a daily basis, and provides general guidelines if you do not know your sweat rate. Click on graphic to enlarge.
Personalizing Your Hydration: Sweat Rate
The experts at the American College of Sports Medicine have concluded that since fluid and electrolyte needs are widely variable based on genetics and environmental conditions (hot vs. cold, humid vs. dry, etc.), runners (and all athletes) should know their sweat rate and aim to replace the total amount lost during the exercise bout. Once your sweat rate is determined, it can be used in planning regular drinking intervals during workouts and race days (Sawka et al., 2007). Research is inconclusive if thirst is an accurate indicator of hydration status during exercise, but it appears to be more accurate in short duration (less than an hour), in cooler conditions, and for lower intensity exercise (Kenefick, R.W. 2018).
How to Calculate Sweat Rate
(Pre-Exercise Weight – Post-Exercise Weight) + Fluid Intake During Activity = Sweat Rate
During 1 hour workout:
Pre-Weight = 165
Post-Weight = 164
Fluid Intake During Workout = 16 ounces
(165-164) = 1 pound or 16 ounces lost
+ 16 ounces of fluid consumed during workout
= 32 ounces (2 pounds) of sweat lost per hour
This athlete should drink 8 ounces of fluid every 15 minutes to closely match sweat rate.
It is recommended to calculate sweat rate in the environment that closely mimics race day, and several different times throughout the training cycle.
In addition to consuming adequate fluid, it’s important to remember to include electrolytes, especially in hot and humid environments and/or if one is a heavy/salty sweater. Sodium is the primary electrolyte lost in sweat, in addition to others in much smaller quantities. Heat injury and muscle cramps are consequences of dehydration and inadequate sodium/electrolytes. Those most susceptible include who sweat profusely, are “salty” sweaters (skin and clothing caked with a white salt residue after exercise), and those who eat little salt in their diets. Loss of sodium and other electrolytes is another factor that is highly individualized, so an area in which we recommend you work with an experienced sports dietitian. General recommendations for replacing sodium/electrolytes in these situations include increasing sodium by adding salt to sports drinks (1/2 teaspoon salt to 32 ounces sports drink), choose an “endurance” formula sports drink with 200 mg sodium per 8 ounces, or use sports specific salt products. Also include high-sodium foods in their daily diets, such as salted pretzels, tomato juice, soup, and cheddar cheese. It’s important to know that muscle cramps can be caused by multiple factors, but if you are prone to cramping, evaluating your hydration and electrolyte practices is a good first step.
Too much hydration can be unsafe. Hyponatremia (low blood sodium) can result from drinking more fluids than you lose in urine and sweat during exercise. Those at highest risk for this life-threatening condition are endurance athletes who are small, female, slower, lose a lot of sweat, and “salty” sweaters, and those with high fluid intake. Extremely hot environmental conditions also increase the risk of hyponatremia. Symptoms of hyponatremia include:
- Bloating, swollen hands and feet
- Nausea & vomiting
- Confusion, disorientation
- Seizures, coma, death
To prevent hyponatremia, those who train for more than an hour at a time, need to be careful not to drink too much water before or during exercise, and to consume sport drinks instead of water during and after events. Several days prior to an endurance event, athletes should consume sport drinks or salty foods and water. Better yet, personalize your hydration.
With all the choices in sports drinks these days, it can often be confusing on where to begin. Here are some tips to get you started:
- Choose water when engaging in light activity in moderate conditions, for example: less than one hour of exercise in a cool environment.
- Choose a sports drink for moderate to heavy activity, endurance sports (long run), and stop-and-go sports (interval workouts), especially in warm, humid weather because they help to replace carbohydrate used during activity and electrolytes lost in sweat.
- Sports drinks with 14-19 grams of carbohydrate per 8 ounces provide adequate carbohydrate without stomach discomfort. Higher levels of carbohydrate slow absorption in the stomach and can cause pain.
- Refrain from diluting sports drinks. They are made to deliver a balanced mix of carbohydrates and electrolytes. This helps with absorption and recovery.
- Choose a sports drink with a mixture of different sources of carbohydrate, such as a blend of sucrose, glucose, and fructose. Drinks that contain mostly fructose and/or high-fructose corn syrup can cause gastrointestinal upset.
- Do not drink carbonated sports drinks. Carbonated drinks tend to make you feel full faster, therefore you drink less. Also, carbonated sodas do not contain sufficient electrolytes to fully replace those lost from sweat.
- Experiment with different flavors and brands during practice, never during competition.
Tips for Staying Hydrated
- Practice drinking during training so you know which brands and flavors of fluids you can tolerate during competition.
- Prevent dehydration by knowing the warning signs: unusual fatigue, lightheadedness, headache, dark urine (similar to the color of apple juice), dry mouth.
- Monitor urine color to determine hydration level. A small volume of urine that has a strong odor and is dark in color indicates dehydration. Keep your urine the color of lemonade or straw to assure adequate hydration. You may be drinking too much water if your urine is clear.
- Start activity with a belly full of liquid and keep your stomach comfortably full while you exercise.
- Keep in mind your fluid needs before, during, and after exercise.
- Take gulps instead of sips.
Remember that just a small amount of weight lost during exercise can negatively affect the body’s ability to cope with the stress of exercise. No one wants their workout to feel harder than it should, so, cheers!