Comprehensive Study Guide on Hydration in Sports

Overview of Hydration in Sports from Universidad de Tarapacá

These study notes are based on the presentation by Nta. Daniel Chávez López of the Universidad de Tarapacá (Universidad del Estado) regarding hydration in the context of sports and physical activity. The material provides an encyclopedic look at the biological, physiological, and practical aspects of fluid management for athletes.

Body Fluid Distribution and Composition

Total body mass is divided into solids and fluids, with significant differences based on biological sex. In females, body mass is typically composed of $45\%$ solids and $55\%$ fluids. In males, body mass is typically composed of $40\%$ solids and $60\%$ fluids. The total fluid volume is further divided into two main compartments: the intracellular fluid (LIC) and the extracellular fluid (LEC). Approximately $2/3$ of total body fluids are found within the cells (intracellular), while $1/3$ is found outside the cells (extracellular). The extracellular fluid is further partitioned between the interstitial fluid ($80\%$) and the blood plasma ($20\%$), which flows through the capillaries. The barrier between the intracellular and extracellular compartments is the cell membrane, while the capillary wall separates the plasma from the interstitial fluid.

To illustrate this distribution in a practical context, consider a male weighing $80\,kg$. His Total Body Water (Agua Corporal Total) would be approximately $60\%$ of his body weight, calculated as $80\,kg \times 60\% = 48\,LT$. This total volume is segmented as follows: the Intracellular Fluid (LIC) accounts for approximately $40\%$ of body weight ($80\,kg \times 40\% = 32\,LT$), and the Extracellular Fluid (LEC) accounts for approximately $20\%$ of body weight ($80\,kg \times 20\% = 16\,LT$). Within the LEC, the Interstitial Fluid (representing $3/4$ of the LEC) totals $12\,LT$, and the Plasma (representing $1/4$ of the LEC) totals $4\,LT$.

Daily Water Balance

Maintaining hydration requires a balance between water intake and water losses. Daily water intake is derived from two primary sources: the ingestion of liquids and foods, and endogenous metabolic production, which contributes between $250$ and $350\,ml/day$. Daily water losses occur through four main pathways: insensible losses (skin and diffusion), which range from $450$ to $1400\,ml/day$; fecal losses, ranging from $100$ to $200\,ml/day$; urinary losses, ranging from $1000$ to $1500\,ml/day$; and respiratory losses, ranging from $250$ to $350\,ml/day$. The total estimated daily balance of intake and loss typically falls between $1300$ and $3450\,ml/day$.

International Recommendations for Daily Fluid Intake

Various international health organizations provide guidelines for daily water intake (measured in liters per day). The World Health Organization (WHO, 2003) recommends $2.5\,LT$ for men and $2.0\,LT$ for women. The Institute of Medicine (IOM, 2004) suggests higher values of $3.7\,LT$ for men and $2.7\,LT$ for women. The National Health and Medical Research Council (NHMRC, 2006) recommends $3.4\,LT$ for men and $2.8\,LT$ for women. The European Food Safety Authority (EFSA, 2010) offers a tiered recommendation: for men, $2.5\,LT$ if sedentary and $4.5\,LT$ if active; for women, $2.2\,LT$ if sedentary and $4.5\,LT$ if active.

Evolution of Hydration Recommendations for Athletes

Expert guidelines for athlete hydration have evolved significantly over the last few decades. In 1996, the American College of Sports Medicine (ACSM) recommended $10$ to $12\,ml/kg/h$ and advised drinking as much as possible to avoid any weight loss during exercise. By 2000, the National Athletic Trainers' Association (NATA) defined deshydration categories based on body weight loss: minimal deshydration is a loss of $1$ to $3\%$, moderate is $3$ to $5\%$, and severe is greater than $5\%$.

In 2006, the International Marathon Medical Directors Association (IMMDA) updated recommendations to $6$ to $8\,ml$ of liquid per kilogram of body weight per hour of exercise (roughly $400$ to $500\,ml/h$ or $150$ to $200\,ml$ every $20$ minutes), cautioning against taking more fluid than necessary to compensate for the deficit. The 2007 ACSM position stand emphasized pre-hydration ($5$ to $7\,ml/kg$ four hours before exercise), the consumption of sodium ($20$ to $50\,mEq/L$), and specific protocols for heat and humidity (approximately $0.5\,LT$ plus salts in the hour prior, divided into four $200\,ml$ doses every $15$ minutes). By 2015, organizations like the Ultra Sport Science Foundation and experts like Hoffman et al. shifted focus toward drinking according to thirst to prevent exercise-induced hyponatremia and overhydration, while still managing dehydration risks between $4\%$ and $6\%$.

Impact of Dehydration on Athletic Performance

Dehydration exceeding $2\%$ of body weight is a critical threshold that diminishes performance, particularly in aerobic exercise. As the percentage of weight loss increases due to lack of fluid intake, the performance decline becomes more pronounced. Scientific evidence supports several key points:

1. Prolonged exercise fatigue results from both dehydration and the depletion of substrates. Studies on soldiers and athletes show that dehydration above $2\%$ of body mass impairs cognitive performance (Grandjean, 2007; Lieberman, 2012; Masento et al., 2014).

2. Research by Armstrong, Costill, and Fink (1985) demonstrated that a $1.5$ to $2\%$ loss in body mass reduced performance in races of $1500\,m$, $5000\,m$, and $10,000\,m$ by decreasing speed, with adverse effects being more severe in longer distances.

3. Coyle (2004) identified that dehydration reduces endurance through interrelated mechanisms: increased cardiovascular strain (due to hyperthermia and reduced blood volume) and the direct effects of hyperthermia on muscle metabolism and neurological function.

Thermoregulation and Environmental Factors

Thermoregulation is the regulation of body temperature, which involves a balance between heat gain and heat loss. Heat gain stems from the basal metabolic rate, the thermal effect of food, muscular activity, and the environment. Environmental heat influence is determined by three main factors: physical activity, ambient temperature, and relative humidity. Heat is lost or transferred via radiation, convection, evaporation, and conduction. The core body temperature is typically maintained around $37^{\circ} C$.

An "Heat-Tension Index" (Indice Tensión - Calor) maps relative humidity against ambient temperature to determine safety zones. For example, at $30^{\circ} C$ and $60\%$ humidity, the perceived temperature is $32^{\circ} C$, falling into an "endurable" zone. However, as humidity or temperature rises, conditions move through "undesirable" and "dangerous," eventually reaching a "health risk" zone. High humidity significantly impairs the body's ability to lose heat through evaporation (sweating).

Physiological Consequences and Heat Illnesses

Dehydration triggers several physiological changes: an increase in core body temperature, elevated cardiovascular tension (characterized by lower blood volume, lower stroke volume, and decreased blood flow to muscles), altered metabolic and central nervous system (CNS) function, and an increased rate of glycogen utilization. These factors can lead to a spectrum of heat-related illnesses:

• Heat Cramps: Characterized by involuntary muscle spasms.
• Heat Exhaustion: The most common heat illness, caused by ineffective cardiovascular adjustments. Symptoms include decreased central blood volume, low blood pressure, headache, nausea, dizziness, and general weakness.
• Heat Stroke: A grave condition requiring immediate medical attention. It involves a very high core temperature, the cessation of sweating, hot and dry skin, and potential organ failure or death.

Exercise-Associated Hyponatremia (HAE)

Hyponatremia occurs when blood sodium ($[Na^+]$) levels fall below the normal range ($136$ to $142\,mEq/L$). In athletes, it is usually triggered by activity lasting more than $4$ to $5$ hours combined with the excessive intake of plain water (overhydration). The severity of symptoms corresponds to sodium concentration:

• Mild Hyponatremia ($130$ to $135\,mEq/L$): Gastrointestinal inflammation and moderate nausea.
• Moderate Hyponatremia ($125$ to $130\,mEq/L$): Headache, vomiting, swelling, unusual fatigue, and confusion.
• Grave Hyponatremia (below $120\,mEq/L$): Seizures, respiratory collapse, coma, brain damage, and death.

To prevent hyponatremia, athletes should follow a hydration plan suited to their predictable losses, use sports drinks for sessions longer than $3$ hours (the glucose-sodium transport mechanism aids intestinal water uptake), slightly increase salt in meals before long events in high heat, and be educated on "warning symptoms" to stop exercise and seek help.

Monitoring Hydration Status

Several biological markers can be used to assess hydration, each with varying practical utility and validity. Total body water has low practical utility but is valid for acute and chronic changes (cutoff: $< 2\%$ loss). Plasma osmolarity has medium practical utility (cutoff: $< 290\,mOsmol$). Both Urine Specific Gravity ($< 1.020\,Usg$) and Urine Osmolarity ($< 700\,mOsmol$) have high practical utility and are excellent for monitoring chronic status. Body weight is highly practical and valid for both acute and chronic monitoring (cutoff: $< 1\%$ change).

A subjective but effective tool is the Urine Color Chart. Values from $1$ to $3$ indicate an optimal hydration pattern. Values from $4$ to $7$ indicate dehydration. A value of $8$ suggests a severe issue or the presence of blood in the urine, necessitating a medical consultation.

Calculating Sweat Rate

The sweat rate (Tasa de sudoración) helps personalize hydration plans. It is calculated using the following formula:

$\text{Sweat Rate (ml/min)} = \frac{\text{Weight Lost (g) + Liquid Ingested (ml) - Urine (ml)}}{\text{Minutes of Activity (min)}}$

The Role and Composition of Sports Beverages

Beverages are categorized by their tonicity relative to human blood: hypotonic (generally before exercise), isotonic (during exercise), and hypertonic (after exercise). A standard hydration index for various drinks shows that oral rehydration salts, skim milk, and whole milk are highly effective at retaining fluids, often more so than plain water, while coffee and beer have lower retention indices.

Sports drinks containing Carbohydrates (CHO) can maintain body temperature as effectively as water while improving performance in prolonged events. However, solutions with more than $15$ to $20\%$ CHO can significantly delay gastric emptying and cause gastrointestinal distress. Solutions between $5$ and $8\%$ CHO empty from the stomach as effectively as water. According to consensus, a hydrating beverage for athletes should provide:

• Carbohydrates: $50$ to $80\,g/l$ ($5$ to $8\%$ concentration).
• Energy: $80$ to $350\,kcal/l$.
• Composition: More than one type of carbohydrate (not just glucose).
• Osmolarity: Between $200$ and $400\,mOsmol/l$.
• Sodium: Between $20$ and $60\,mmol/l$ ($460$ to $1380\,mg/l$).

Market examples include Gatorade ($5.83\%\,CHO$, $444.4\,mg/l\,Na$), Powerade ($5.83\%\,CHO$, $416\,mg/l\,Na$), and SIS GO ($7.2\%\,CHO$, $1000\,mg/l\,Na$).

Practical Fluid Replacement Protocols

Fluid consumption should be structured into three phases:

• Pre-exercise: Drink $5$ to $7\,ml/kg$ during the $4$ hours before exercise. In hot/humid environments, consume $500\,ml$ in the preceding hour. Salty foods help stimulate thirst and retention.
• During exercise: Drink $6$ to $8\,ml$ of liquid per kg of body weight per hour of exercise. The ideal liquid temperature is between $15$ and $21^{\circ} C$. Avoid excessive dehydration but do not overhydrate.
• Post-exercise: Rehydration should begin immediately, even without thirst. It is recommended to drink at least $150\%$ of the weight lost within the first $6$ hours (e.g., if $1\,kg$ is lost, drink $1.5\,LT$). If losses exceed $7\%$ and involve vomiting or inability to drink, intravenous (IV) replacement may be necessary; otherwise, oral rehydration is sufficient.

Individualization and Adaptability

Fluid consumption can be trained and adapted through progressive exposure. Studies (e.g., Le Bellego et al.) show that athletes can increase their voluntary consumption when given specific drinking programs over several weeks. Hydration needs are highly individual, influenced by age, sex, body fat percentage, acclimatization, training level, and relative skill. The future of sports nutrition lies in the personalization of these strategies, adapted to the specific individual and their unique performance objectives.