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The Importance of Fluid Replacement During Exercise


Your body is so efficient at many processes, but needs water to do most of them. One of the greatest ways we lose fluid from our bodies is through sweat. Our bodies try to cool themselves by dissipating heat during exercise in warm or hot weather in the form of sweat. This causes changes in the amount of water and electrolytes a person has and can affect athletic performance and health.

To maintain optimal fluid balance, you need to replenish the fluid losses. The rate of fluid loss during exercise is affected by duration, intensity, temperature, wind, humidity and how much or types of clothing you are wearing. Between different sports the difference in losses can be significant, as well as the differences between people with varying fitness levels. Due to this fact, there are not specific guidelines for fluid replacement.

As your muscles become more active, metabolic heat is transferred from the muscles to the blood and then to the body's core. This causes physiological adjustments that transfer heat from the core to the skin to be released and cool the body. When the weather is colder, or less humid, the amount of sweat your body loses is small in comparison to when there is a higher heat stress. If sweat is not able to evaporate from the body and drips, your body is signaling that a higher sweat rate is needed to achieve the necessary evaporative cooling. Increased air motion (wind) can assist the evaporation and minimize the amount of sweat being dripped.

When you live in a higher temperature area your body acclimatizes to the weather and you are able to achieve higher and more sustained sweating. If the area is humid, causing wet skin, or if your body is dehydrated, your sweating rate is curbed. How much electrolyte loss occurs is dependent on the concentration of electrolytes in the sweat and how much fluid your body is expelling. If you are dehydrated, your body can cause the concentration of sodium and chloride loss to increase, but your body is not better able to reabsorb these electrolytes. As you acclimate to temperatures, your body is better able to reabsorb chloride and sodium, and generally reduces the sodium concentrations in sweat.

If you are an athlete training or heavily exercising, you may want to monitor your body's weight changes during exercise to calculate how much fluid you are losing. From there, you can determine how much fluid you need to replenish your body. Weigh yourself naked early in the morning and after urination to determine your baseline, and then after a specific time of exercise. Subtract your body weight after your workout from your pre-workout weight, and subtract any urinary loss. If you drank beverages during your workout, this also needs to be added in.

Physiologic stress increases when you are dehydrated. You measure physiologic stress by core temperature, heart rate and perceived exertion. The more water you lose, the more physiologic strain is experienced by your body. This impairs both mental and aerobic performance. Your aerobic performance decreases when you're dehydrated because of the increase in your cardiovascular strain, core temperature, need for glycogen, and changes in your metabolic functions. This affects your ability to concentrate, do skilled tasks or strategically plan. You become at risk of heat exhaustion and/or heat stroke, skeletal muscle cramps, or in some long endurance events, hyponatremia.

Fluid Replacement

Prior to exercise, hydrate so that you start physical activities with normal hydration and electrolyte levels. Normal hydration is achieved when there has been adequate time since last exercise session and enough beverages consumed. Consuming liquids hours before you start exercising ensures that your urine output and body functions have returned to normal.

During exercise, monitor how you feel. If you are exercising at a high intensity for long periods of time, monitor weight changes. Consuming drinks that contain electrolytes and carbohydrates can be beneficial for some activities, dependent on the intensity and duration. Sodium in these beverages works to stimulate your thirst, carbohydrates for energy and general electrolyte replenishment.

After exercise, make sure to replace fluid and electrolyte losses by consuming regular meals and beverages over the next 24 hours. If you plan to exercise again sooner than that and feel significantly dehydrated, more focus will be needed to achieve rehydration. Drink 1.5 liters of fluid for every kilogram lost or 24 ounces for each pound. This rate of 150% of sweat losses is required because of the additional urine output that will occur from increased intake.

Some people find it hard to drink enough water to meet their needs. Beverages that are flavored, cooled and containing sodium may enhance the voluntary intake. Another trick is to fill up liter bottles (like soda pop bottles) with your daily fluid needs every morning and put them in your refrigerator, so you can visually see how much you need to consume and easily track. Dehydration can have serious side effects, so always work hard to make sure you are consuming enough fluids.


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Emily DeLacey MS, RD is a Registered Dietitian and currently working in Jamaica as a HIV/ AIDS Prevention Specialist. She attended Central Washington University for her Bachelor's Degree in Science and Dietetics and continued on after her internship to Kent State University for her Master's Degree in Science and Nutrition, with a focus on public health and advocacy. She served as a U.S. Peace Corps Volunteer in Malawi 12'-14' working as a Community Health Advisor in a rural village, immersing in the joys of life without electricity or running water. She has been to 20+ countries and 47 of the 50 states in the US. Traveling, adventuring and experiencing new cultures has made her a passionate advocate for the equality of nutrition and wellness for all people.

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