Your Hydration Plan: Fluid Replacement Guidelines when Exercising

The successful maintenance of one’s hydration level is essential for optimal health and performance. During resting conditions in a cool climate this is not a particularly daunting task. However, avoiding dehydration while exercising and during exposure to high temperatures and humidity can present a serious challenge.

Our body loses water via urine, respiration, feces, and sweat. These losses are easily matched with regular consumption of fluids throughout the day. However, when we exercise, our loss of water through sweat is greatly increased. Sweat rates vary greatly between individuals and are greatest during high intensity exercise and exposure to hot and humid environments. Sweat rates between 0.5 to 3.0 quarts per hour are typical during these conditions. Because our body can only absorb approximately 1.2 quarts of fluid per hour (about 40 ounces), dehydration becomes an unavoidable consequence during these conditions. Dehydration can be easily assessed by dividing the amount of weight lost during exercise by body weight (before exercise). For example, if a 150 pound individual loses 4.5 pounds during a workout, they experience a 3% dehydration (4.5 lbs. / 150 lbs. = 0.03 or 3%).

Avoiding or minimizing dehydration should be a top priority for all people engaging in physical activity because of the important roles that water and electrolytes (both lost in sweat) play in maintaining health and allowing for peak performance. The maintenance of water balance (matching intake with output) is critical for normal physiological functions, such as temperature regulation, delivery of nutrients to our organs, removal of toxins from our organs, and disease prevention. In addition to playing a role in the maintenance of fluid balance by helping the body retain water, electrolytes are also needed for normal muscular contractions.

Dehydration will affect our ability to perform exercise by increasing heart rate, reducing oxygen delivery to the muscles, slowing metabolic processes, reducing muscle strength, and increasing feelings of fatigue. These effects worsen as the level of dehydration increases. Signs of dehydration, such as thirst, discomfort, and loss of appetite, can appear as soon as one loses about 2% of their body weight (for example, 3 lbs. of water loss in a 150 lbs. person). As dehydration worsens, the symptoms become more serious and can eventually become life threatening if not treated.

Physical Symptoms and Effects of Dehydration

Percent Dehydration* Symptoms
1% Few symptoms or signs of any thirst present; however, there is a marked reduction in aerobic performance (VO2 max)
2% Beginning to feel thirsty; loss of endurance capacity and appetite
3% Dry mouth; performance impaired, cramping, increased heart rate
4% Increased effort for exercise, impatience, apathy, vague discomfort
5% Difficulty concentrating, breathing, slowing of pace
6% - 7% Further impairment of temperature regulation, higher pulse and breathing, flushed skin, sleepiness, tingling, stumbling, headache
8% - 9% Dizziness, labored breathing, mental confusion, further weakness.
10% Muscle spasms, loss of balance, swelling of tongue
11% Heat Exhaustion, delirium, stroke, difficulty swallowing; death can occur
*Percent dehydration calculated by dividing weight lost during exercise by pre-exercise body weight.

Recently, the American College of Sports Medicine (ACSM) presented fluid replacement guidelines for optimum health and performance. These guidelines are intended to provide tools to create a customized fluid replacement program that prevents excessive dehydration and electrolyte imbalance.

Fluid replacement recommendations:

- Know your hydration needs. Considerable variability in water and electrolyte losses exists for each individual. In addition to body size, factors such as air temperature, humidity, exercise intensity, and clothing can all increase sweat rate considerably. Compare pre-exercise and post-exercise body weight to help determine your losses.
- Hydrate before you exercise. “Fluid should be consumed several hours before exercise to enable fluid absorption and allow urine output to return to normal levels. ACSM recommends consuming beverages with sodium and/or salted snacks to help stimulate thirst or retain fluids.” Drink more on hot and humid days.
- Minimize dehydration while you exercise. The amount and rate to consume differs depending on the individual and the conditions. There are additional performance and hydration benefits to consuming beverages containing electrolytes and carbohydrates compared to water, such as sports drinks, especially as exercise duration increases. Generally, 0.4 – 0.8 liters (approximately 12 – 24 ounces) should be consumed “ad libitum” (at your pleasure) every hour.
- Replace what you lost…and more! Rehydration should begin immediately after exercise and its goal is to replace what was lost during exercise. For optimal results, consume 150% of the volume lost to restore normal hydration status. This means for every pound lost during exercise consume approximately 24 ounces of fluid. The extra volume will help account for continued losses of water through sweat and urine. Water and sports drinks are optimal choices for rehydration beverages. Sodas, juices, milk products, and smoothies should be ‘watered down’ or avoided until adequate rehydration has occurred.

What should you drink?

The characteristics of your beverage play an important role in maintaining fluid and electrolyte balance. Here are some factors to consider for some common beverage choices.
Water – most quickly absorbed, but does not provide electrolytes or carbohydrates for energy, and it will reduce thirst and urine output. Water is optimal for shorter workouts (less than 1 hour) during mild environmental conditions.
Sports drink – provide electrolytes and carbohydrates for sustained energy. They are usually consumed in high volumes because they taste good, and will not reduce thirst and increase urine volume as much as when drinking water.
Fruit juice – provide too much carbohydrate, which slows down fluid absorption and may cause gastric upset. Dilute juices in a 1:1 ratio with water.
Soda – like juices, contain too much carbohydrate and the carbonation may lead to additional gastric discomfort. Sodas contain minimal amounts of electrolytes and should also be diluted for optimal rehydration results.

Your Hydration Plan

To avoid the detrimental consequences of dehydration, develop a personal hydration plan.
- Experiment with different types of drinks and different volumes of drinks prior to, during, and after exercise to determine what works best for you. Do not try a new beverage or plan on the day of an important event.
- Explore your local grocery or supplement store to see what drink options are available. Most sports drinks are labeled based on their intended use. For example, pre- and during exercise beverages (look for terms like ‘endurance’) include carbohydrates and electrolytes, and post-exercise beverages (look for terms like ‘recovery’) will usually contain carbohydrates, protein, and electrolytes.
- Weigh yourself before and after workout to better understand your hydration needs under differing exercise and environmental conditions.
- Aim to hydrate before exercise by consuming fluids within a few hours of your workout; minimize dehydration during exercise by consuming water or a sports beverage; and replace 150% of what you lost after exercise by consuming water or a sports drink.

Here is an example of a typical hydration plan I utilize with my athletes:
Pre-exercise
 - 60 – 120 minutes before workout or event, 16 ounces water
 - 15 – 30 minutes before workout or event 8 – 12 ounces of sports drink
During exercise
 - For short workouts or event (< 45 minutes), water as desired
 -For longer workout or event (> 45 minutes), alternate sports drink and water aiming for about 16 ounces per hour depending on exercise intensity and environmental conditions
 - Post-exercise
 - After short workout or event, immediate consumption of 8+ ounces of sports drink and water as needed
 - After longer workout or event, immediate consumption of 12+ ounces of sports drink followed by water and a drink (12+ ounces) containing both carbohydrates and protein over the next hour

References

1. ACSM Position Stand: Exercise and Fluid Replacement. Medicine and Science in Sports and Exercise. 39:2, 377-390, 2007.
2. American College of Sports Medicine releases new, revised hydration recommendations – ACSM.org
3. Wilmore, J., Costill, D. (2004). Physiology of Sport and Exercise (3rd Edition). Human Kinetics Publishing.