Hydration is the act of supplying the body with adequate levels of water. Water must be replaced continually because water evaporates during respiration (breathing) and sweating and through urination. Approximately 60%–70% of the body is composed of water (70% represents water content in lean body mass), and up to 70% of the brain is made up of water, so it is important that water balance is safely maintained within the body with proper hydration.
Adequate hydration before, during, and after exercise is essential to the health and performance of athletes and nonathletes alike. Exercise usually causes higher levels of evaporative sweating because the body releases fluid to help reduce the rise in body temperature created during aerobic activity. An increase in sweating leads to a greater loss of water and puts the body at risk for dehydration. Dehydration can affect athletic performance and can jeopardize health in crucial ways. If the loss of water from dehydration is significant, an athlete's performance can suffer dramatically, and the health of the athlete might be compromised. However, hyperhydration (overdrinking) can also create health concerns. Being knowledgeable about the best way to rehydrate and which liquids are best to consume during exercise is key to better performance, better health, and proper recovery.
Body water is typically lost through increased sweating. Sweating occurs to cool the body down because as the muscles work faster and harder during exercise, they continually generate more heat, increasing core body temperature. As water is lost through sweating, it is being removed both from inside and outside cells, which alters electrolyte levels and increases heart rate. This effect begins to limit the ability of the body to transfer heat from the muscles to the skin where it can then be released into the environment to help cool the body back to its core temperature. If this does not occur properly, a person can have heat injury.
When dehydration occurs during exercise, nearly all of the body systems are affected. When one system is affected by dehydration, others follow. For example, the cardiovascular system begins to work harder (heart rate increases), affecting the thermoregulatory (temperature control) system that, in turn, affects the muscular system. This effect makes continual hydration very important to bring those systems back into balance and provide the energy the body needs for repair and recovery during and after exercise.
Dehydration of greater than 2% of body weight negatively affects endurance performance. However, losses of up to 7% can be tolerated in strength and power sports without affecting performance. Drinking fluids and eating healthy food rehydrate the body. Certain fluids assist with hydration more than others. For example, isotonic drinks, often called sports drinks, absorb the fastest, yet drinking some fluids, such as strong alcoholic beverages, can actually lead to dehydration because the drinks act as diuretics.
The American College of Sports Medicine (ACSM) recommends that athletes preparing to exercise maintain adequate nutrition and fluid balance. Mostly, the ACSM reminds athletes to drink enough fluids before and during exercise to avoid losing any more than 2% of their body weight. Although needs vary based on activity level, weight, and sweating rates, nonathletes need to drink a minimum of 1–2 L of fluids a day. Athletes might require up to 10 L a day and be adequately nourished and hydrated before beginning a vigorous activity. Athletes also should consume fluids throughout exercise, beginning early and replenishing consistently to compete with the loss of fluids through sweating. The best fluids to drink are cooler than the room or outdoor temperature (between 59°F and 72°F [15–22°C]) since this assists the body in regulating core temperature, which rises from exertion, and because cooler liquids taste and feel better to most people, which inspires them to continue hydrating.
During exercise lasting longer than three or four hours or in conditions of high heat or humidity, flavored sports drinks are better than water alone, as long as the drinks contain electrolytes, sodium, and any flavor that encourages drinking. (By the time a person feels thirsty, dehydration has already begun.) Electrolytes are essential because they help the function of nerves and muscles. Electrolytes are ionized particles—sodium, calcium, potassium, and chloride—that support the transfer of nutrients into cells and help with elimination of wastes from cells. It is important to offset electrolyte loss during times of excessive sweating to help the body maintain a homeostatic state and to assist the body in muscle repair and cellular nutrient exchange.
Sodium is added to sports drinks for two reasons: It helps to generate greater thirst, thus ensuring that the urge to hydrate continues, but, more important, fluids with the right sodium content mimic blood salinity and cause them to be more quickly absorbed into the blood than water alone, which has no salt content. For athletes or individuals preparing to exercise for less than one hour, hydrating with water might be best, but for high-endurance exercise lasting several hours, good-quality sports drinks with sodium, electrolytes, carbohydrates, and some protein content usually are recommended. Carbohydrates are added to sports drinks because for intense workouts and endurance training, muscle fatigue can set in. The body metabolizes carbohydrates into glucose that fuels muscles during intensive workouts. This process allows the body to continue working and may even help in the recovery process after exercise.
When temperature rises suddenly or when athletes perform in climates warmer than climates they are accustomed to, hydration becomes a greater concern because more fluid escapes through sweat. Body temperature is also likely to rise more dramatically, which affects performance in important ways. A process known as acclimatization, in which athletes gradually prepare their body for the new conditions, is recommended. When temperatures are warmer and where humidity percentages are higher, sweat rate generally increases for the same duration of exercise. This reaction requires greater fluid intake before, during, and after exercise. It is strongly advised that hydration practices be well monitored, especially during the first week of exercise in a new climate. Athletes should drink fluids every 20 to 30 minutes in smaller amounts throughout exercise.
Certain athletes, such as wrestlers, boxers, rowers, those competing in judo, and body builders, typically do not hydrate before or during training because they train hard to fit into a lower weight class before competition. This strategy is assumed to improve their ability to compete; however, it is especially critical for these athletes to hydrate immediately after the activity ends. Stressing muscles in a dehydrated state weakens them and puts stress on the entire body. It is strongly advised that these athletes not use diuretics, exercise in saunas, or wear rubber suits to advance dehydration and fit into a lower weight class. These measures can result in dangerous levels of dehydration, which can cause injury to athletes.
The recovery process after exercise is just as important for individuals seeking to maintain fitness, achieve healthy immunity, control weight, and fight off disease as it is for competitive athletes. Research indicates that building gradually into an exercise routine is best, rather than stressing muscles and endurance immediately and too consistently. Muscles need time to repair and recover. Working out two days in a row and then allowing a day of rest is advised.
It is advisable to incorporate an appropriate warm-up and cool-down routine into any workout. Warm-up prepares the muscles for more strenuous activity and can help prevent injury such as muscle pulls and tears. Lactic acid builds up during exercise and slows muscle movement. Gentle stretching after exercise also is advised. Doing so helps with the release of lactic acid buildup to reduce the possibility of muscle stiffness after exercise or competition and will help to increase flexibility.
Carbohydrates are a necessary part of the recovery phase because the body breaks them down into glucose that is metabolized into fuel for every cell. A body that is properly fueled and has rested after exercise is more prepared for the next round of exercise or competition. Researchers state that facing exercise without proper hydration or fuel for muscles and cells is counterproductive to fitness and competition goals and can lead to health concerns.
Weighing before and after exercise and hard training is important. If an athlete has a 1% or more loss in body weight, it can affect health and performance. Too much water lost in body weight can lead to a medical emergency.
When dehydration becomes severe, or when external conditions cause the body temperature to rise too high, a person can have heat injury or exhaustion and complications. If the body's temperature rises to 101°F–104°F (38°C–40°C), muscle weakness and fatigue set in. At 104°F–105°F (40°C–40.5°C), a person becomes confused and begins to experience extreme muscle weakness and imbalance. Above 105°F (40.5°C), the body ceases to sweat and loss of consciousness sets in.
There has been some research on hyperhydration, which some athletes attempt before exercise. Hyperhydration, however, causes side effects such as headaches and gastrointestinal distress that can greatly inhibit athletic performance.
There is danger when an individual fails to recover after activity. Exercise physiologists discuss a condition known as overreaching, in which individuals exercise frequently at maximum levels or overtrain long term; they can have symptoms indicating their bodies no longer allow them to exercise or perform at the levels they are attempting. Overreaching begins as severe muscle soreness and stiffness, becoming severe muscle fatigue. Individuals who overreach can experience mood changes such as tension, depression, anger, and confusion. Resting heart rate becomes elevated. Levels of important hormones and metabolites often are out of balance. When overreaching becomes more severe, it can be referred to as overtraining syndrome. Individuals with the syndrome need to cease exercising for many weeks or sometimes months.
There is no set amount of water to drink every day that applies to each individual, but avoiding dehydration is important for everyone. Hydration is responsible for maintaining water balance for cells and organs in the body and for continuing function of muscle, thermoregulatory, and cardiovascular systems, among others. Dehydration caused by increased sweating from exercise can compromise performance and even pose serious health risks to athletes and nonathletes. Some symptoms of dehydration are extreme thirst, fatigue, confusion, irritability, nausea, vomiting, gastrointestinal disturbance, cessation of sweating, and even loss of consciousness.
Electrolyte replacement is critical during and after exercise because of the important role electrolytes play in body chemistry. Electrolytes play a key function in muscle contraction and the delivery of nerve impulses that control muscles. Electrolytes are critical to delivering nutrients to cells and in eliminating waste material from cells. Many experts recommend drinking enough fluids during and after exercise but argue against using intravenous (IV) fluids after workouts or competitions.
Hydration and electrolyte replacement are important parts of overall recovery in exercise. The muscles need time after training to repair and strengthen. Incorporating steps such as warming up and cooling down can help to prevent injury and accelerate the recovery period after exercise. Overexercising or training too hard for too long without rest can have damaging results to athletes.
See also Dehydration ; Endurance training ; Fatigue ; Hydration .
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American College of Sports Medicine, 401 W. Michigan St., Indianapolis, IN, 46202-3233, (317) 637-9200, Fax: (317) 637-7817, (888) 463-6332, http://www.acsm.org .
National Association for Health and Fitness, 10 Kings Mill Court, Albany, NY, 12205, (518) 456-1058, firstname.lastname@example.org, http://www.physicalfitness.org .
Julie Jordan Avritt
Revised by Teresa G. Odle, BA, ELS