The section focuses on the critical role of fluids, particularly water, in the human body, constituting approximately 60% of an average person’s body weight. The percentage can fluctuate based on various factors such as age, gender, body composition, and overall body size. Water is stored in different body locations, including fat, bone, muscle, and blood plasma. The section emphasizes the importance of euhydration, where the body’s water amount is adequate to meet physiological demands, as the daily goal for active individuals. It also highlights the dangers of extremes in fluid intake, namely hyperhydration (excess water) and hypohydration or dehydration (insufficient water).
The discussion introduces the concept of total body water, divided into intracellular fluid (ICF) and extracellular fluid (ECF). The ECF further includes interstitial fluid, intravascular water, and fluid in other locations. Despite compartmentalization, water easily moves between ICF and ECF. The composition of sodium, potassium, chloride, bicarbonate, and protein varies between ICF and ECF, playing a crucial role in fluid and electrolyte transport across cell membranes. The section emphasizes the body’s efficiency in maintaining homeostasis, adjusting water movement based on solute concentrations.
Terms such as osmolarity, isotonic, hypotonic, and hypertonic beverages are introduced, providing insights into fluid absorption and stomach emptying rates. The optimal absorption of fluids occurs when the solution is isotonic, having a solute concentration equal to that of blood.
The narrative then shifts to fluid balance during exercise, particularly addressing the increase in core body temperature, blood flow to the skin, and sweat loss as mechanisms to cool the body. Individual characteristics influence sweat rates, and well-trained athletes may have higher total body water values due to lean body mass and glycogen storage. The impact of dehydration, defined as a greater than 2% loss of body weight, on athletic performance is discussed. Factors contributing to dehydration, such as sweat losses, environmental conditions, and body surface area, are highlighted. Specific sports like football, hockey, and wrestling are noted to pose higher risks for dehydration based on various factors unrelated to exercise, such as equipment and weight class considerations. The section underscores the broader implications of normal hydration beyond athletic performance, emphasizing its crucial role in cardiovascular and thermoregulatory functions and the increased risk of heatstroke with dehydration, especially in hot, humid environments and at altitude.
The section delves into the issue of hyponatremia, a condition characterized by a low concentration of sodium in the blood, commonly observed in aerobic endurance athletes and first described in the 1980s. The exact mechanism of hyponatremia remains unclear, but contributing factors include the overconsumption of hypotonic fluids, excessive sodium loss through sweat, and extensive sweating combined with the ingestion of low-sodium fluids.
In events lasting less than 4 hours, symptomatic hyponatremia is often attributed to excessive fluid intake coupled with insufficient sodium consumption before, during, and sometimes after the event. The signs and symptoms of hyponatremia encompass disorientation, confusion, headache, nausea, vomiting, and muscle weakness. If left untreated, the condition can rapidly progress to severe outcomes such as seizures, brain swelling, coma, pulmonary edema, and cardiorespiratory arrest.
The text introduces the concept of hyponatremia, emphasizing its dangers arising from dangerously low blood sodium levels. It highlights that long bouts of exercise, combined with the consumption of only water, are a typical cause in athletes. Women, particularly in longer aerobic endurance events, may be at higher risk due to psychosocial and biological factors. The discussion notes that fluid intake recommendations for women have often been based on data from men, potentially leading to greater sodium dilution in the body.
Early signs and symptoms of hyponatremia may go unrecognized, appearing when blood sodium concentration reaches 130 mmol/L. As severity increases and blood sodium concentration drops below 125 mmol/L, more serious signs emerge, including altered mental status, seizures, respiratory distress, and unresponsiveness. Severe cases of hyponatremia can lead to coma and death.
The section emphasizes the ease with which athletes can become hyponatremic and dehydrated by solely choosing water or food and drinks with minimal sodium. For individuals with high sweat rates and sodium concentration in sweat, standard commercial sports drinks may not provide sufficient sodium to prevent hyponatremia. A general recommendation is to opt for sports drinks containing a minimum of 20 mEq sodium (460 mg) per liter of fluid.
While there’s no concrete recommendation regarding electrolyte intake before exercise, athletes often consume salty foods and drinks beforehand to prevent hyponatremia. Daily salt intake, especially for those prone to salty sweating, is encouraged. In some cases, the use of salt tablets during exercise may be warranted, provided they are consumed with enough fluid to maintain fluid and electrolyte balance. The advice for active individuals is to limit fluid intake to minimize dehydration and consume sodium-rich foods and beverages during exercise lasting longer than 2 hours to prevent excessive drinking and mitigate the risk of developing hyponatremia.
The section addresses the measurement of hydration status in athletes, emphasizing the importance of assessing hydration during training to ensure optimal hydration both during exercise and at rest. The ideal hydration testing method should be sensitive and accurate enough to detect total body water changes of 2% to 3% body weight, and it should be practical in terms of time, cost, and technical requirements in field settings.
Urine specific gravity (USG) is identified as a quantifiable field test and the preferred method for assessing hydration status before exercise in athletes. Athletes can also assess their own hydration status by considering urine quantity, color, and changes in body weight. However, relying on a single measure is discouraged due to each having its limitations.
Urine color, influenced by urochrome levels, can vary based on urine concentration. A urine color chart, depicting a linear relationship between urine color, specific gravity, and osmolarity, is introduced. The importance of considering dietary compounds that may affect urine color, such as B-complex vitamins, beta carotene, betacyanins, and certain artificial food colors and medications, is highlighted. Athletes are advised to be mindful of these factors when using urine color as a gauge of hydration status.
A well-hydrated athlete’s urine should be pale, akin to diluted lemonade, while darker, concentrated urine indicates dehydration. Orange or brown urine requires immediate medical attention. Body weight fluctuations of plus or minus 1% for well-hydrated individuals in energy balance are considered normal, although factors like bowel movements, eating habits, and hormonal fluctuations in women can influence morning body weight changes.
Weighing before and after exercise sessions provides a useful estimate of fluid losses, aiding in determining whether athletes meet their fluid needs during training. The section also acknowledges potential changes in hydration status during different points in a female’s menstrual cycle.
Moving on to the connection between hydration and performance, the text underscores the significance of maintaining fluid and electrolyte balance during aerobic endurance exercise due to the increased likelihood of dehydration, overheating, and electrolyte imbalance. It notes that attention to fluid balance during strength and power exercise is often less than in aerobic endurance sports, attributed to the shorter duration of many strength and power events and the ready availability of fluids.
The discussion concludes by emphasizing the crucial role of fluid and electrolyte balance in aerobic endurance exercise, citing research that shows a 2% loss of body weight can impair exercise performance in both hot and temperate environments. However, it acknowledges variations among athletes, with some being better regulators of heat and requiring different fluid strategies, as demonstrated in a study of Ironman triathletes where a 3% reduction in body mass during competition had no adverse effects on thermoregulation or body temperature.
The section discusses hydration practices before, during, and after exercise, addressing the importance of starting exercise in a hydrated state with normal electrolyte levels. It emphasizes the significance of hydration practices during the day, including fluid and high water content food consumption, to maintain optimal hydration. For individuals who have lost significant fluid without replenishing adequately, an aggressive preexercise hydration protocol is recommended.
Before exercise, athletes are advised to consume approximately 5 to 7 ml of fluid per kilogram of body weight at least 4 hours prior. An additional 3 to 5 ml/kg of fluid is recommended 2 hours before exercise, especially if the individual is not urinating or if the urine is dark. The inclusion of sodium-rich foods is suggested to stimulate thirst and retain fluids. However, hyperhydrating with water before an event is discouraged due to the risk of increased urination and potential dilution of sodium levels, leading to hyponatremia.
During exercise, the goal is to prevent excessive dehydration and electrolyte imbalance. Athletes are recommended to consume 3 to 8 ounces of a 6% to 8% carbohydrate–electrolyte beverage every 10 to 20 minutes during exercise lasting longer than 60 to 90 minutes. This helps in hydration and promotes better performance. Carbohydrate consumption during exercise is highlighted for maintaining blood glucose levels and reducing fatigue. The ideal sport drink typically contains sodium (20-50 mEq/L fluid), potassium (2-5 mEq/L fluid), and a carbohydrate concentration of 6% to 8%.
After exercise, the aim is to fully replenish fluid and electrolyte deficits. Athletes are advised to consume 150% of the lost weight within 6 hours after exercise for normal hydration. This translates practically to ingesting 20 to 24 ounces of fluids for every pound of body weight lost during training. While plain water is effective, consideration of a sport drink or water with electrolyte-containing foods is recommended for electrolyte replacement. Alcoholic and caffeinated beverages, while contributing to daily hydration, are best avoided in the first few hours after activity for rapid rehydration.
The section then shifts focus to strength and power performance, noting that while numerous studies provide specific recommendations for fluid consumption in aerobic endurance athletes, fewer have explored the effects of dehydration on strength and power. Mixed results are attributed to methodological differences. Studies have shown that hypohydration may decrease strength by approximately 2%, power by 3%, and high-intensity endurance by 10%. However, the impact of dehydration on strength and muscular endurance varies among individuals, and the multifaceted nature of many sports may make dehydration likely to affect performance in different ways.
Despite some studies showing no effect of dehydration on strength and muscular endurance, a review indicates that hypohydration is likely to decrease strength, power, and endurance. The potential consequences of dehydration on athletic performance include subpar performance, increased risk of heat illness, and a heightened risk of developing rhabdomyolysis, a serious injury to skeletal muscle.
This section explores the impact of dehydration on strength training, emphasizing the scarcity of research on this topic and the absence of a clear consensus on whether dehydration decreases muscle performance. Studies suggest that inducing a hypohydrated state may alter the endocrine and metabolic internal environments, affecting the anabolic response post-resistance exercise. Additionally, research on muscle damage markers indicates that more dehydrated participants performed less total work, although the differences were not statistically significant.
Despite some data suggesting potential benefits of hydration for athletes engaging in resistance training, a scientifically based hydration protocol has not been established. Therefore, the emphasis is on encouraging resistance-trained athletes to prioritize good hydration techniques throughout the day to maintain optimal hydration levels before, during, and after training.
The section then shifts focus to age-related fluid needs, particularly in children and the elderly. Children are highlighted as a group with increased risk due to factors such as higher heat generation per unit body mass, greater core body temperature increase when dehydrated, and common voluntary dehydration during exercise. Sweating thresholds, sodium and chloride losses in sweat, and individual fluid prescriptions are discussed in the context of children’s fluid needs during exercise. Practical recommendations include drinking until the child does not feel thirsty plus an additional 4 ounces for children and 8 ounces for adolescents. Taste is emphasized as a critical factor in beverages for children.
For elderly individuals, age-related physiological changes, such as decreased renal functioning, altered blood flow responses, reduced sweat rates, and changes in thirst sensation, are discussed. These changes can predispose older adults to fluid imbalances and dehydration during exercise. Elderly persons are also at risk for hypernatremia, elevated blood sodium concentrations, primarily due to decreased thirst. Rehydration periods are recommended to be longer for older adults, and fluid replacement guidelines for younger athletes can be adapted for masters athletes. The section emphasizes the need for sport nutritionists to be aware of dehydration issues in these populations and educate parents and coaches accordingly.