Nutrition
Narrative Review
2010
Water, hydration, and health
By Barry M. Popkin, Kristen E. D'Anci and Irwin H. Rosenberg
Nutrition Reviews, 68(8), pp. 439-458
Abstract
Water constitutes approximately 60% of adult body mass and serves as the medium for virtually every metabolic process supporting physical performance. This comprehensive review by Popkin, D'Anci, and Rosenberg (2010) examined the physiological effects of hydration status on exercise performance, cognitive function, and overall health, synthesizing evidence across both controlled trials and observational research.
The review confirmed that dehydration equivalent to 2% of body weight produces measurable and statistically significant decrements in aerobic exercise performance, with greater deficits producing proportionally larger impairments [1]. Thermoregulatory capacity, cardiovascular efficiency, and muscular endurance are the primary performance domains affected by even mild to moderate dehydration. Cognitive function — including attention, reaction time, and decision-making — shows sensitivity to dehydration at levels as low as 1-2% body weight loss [2].
The evidence also highlighted substantial individual variability in sweat rate and sweat composition, underscoring the limitations of universal fluid intake guidelines. Individualized hydration strategies based on pre-exercise body weight monitoring, thirst response calibration, and sweat rate assessment were identified as more appropriate than rigid intake prescriptions.
Keywords: hydration, dehydration, exercise performance, thermoregulation, sweat rate, cognitive function, fluid replacement
Introduction
Water is the most abundant molecule in the human body and the essential medium for the biochemical reactions, nutrient transport, waste removal, and thermoregulation that sustain athletic performance. Despite the fundamental importance of hydration to health and physical function, understanding of optimal fluid intake strategies for exercise — and the precise performance consequences of hydration deficits — has evolved substantially over the past several decades [1].
Early sports medicine guidelines often advocated aggressive fluid intake during exercise, driven in part by concerns about dehydration-associated heat illness and performance decrements observed in military and occupational settings. The prevailing advice was to "drink ahead of thirst" — consuming fluids at fixed intervals regardless of subjective need — on the grounds that thirst was a lagging and unreliable indicator of physiological hydration needs [2].
However, the pendulum swung when evidence emerged that hyponatremia (dangerous dilution of blood sodium) associated with excessive fluid intake caused deaths and severe neurological complications in endurance athletes who overhydrated during prolonged events. This highlighted that both extremes — insufficient and excessive fluid intake — carry performance and health consequences, and that individualized strategies were superior to blanket prescriptions [3].
The physiological consequences of dehydration span multiple systems. Plasma volume contraction reduces cardiac output at any given workload, requiring the heart to beat faster (increased heart rate) to maintain blood pressure and muscle oxygen delivery. Skin blood flow is compromised, impairing heat dissipation and elevating core body temperature. Muscle glycogen breakdown may be accelerated, and the perceived effort of submaximal exercise increases [4].
Popkin, D'Anci, and Rosenberg (2010) examined this multifaceted topic comprehensively, synthesizing evidence from exercise physiology, nutrition science, and cognitive neuroscience to provide a nuanced picture of how hydration status affects human performance across a range of physical and cognitive demands.
<h2>Evidence Review</h2>
<h3>Dehydration Thresholds and Exercise Performance</h3>
<p>The <a href="/terms/dose-response-relationship/" class="term-link" data-slug="dose-response-relationship" title="dose-response relationship">dose-response relationship</a> between dehydration severity and exercise performance impairment is one of the most robustly documented findings in sports nutrition research. Meta-analytic evidence confirms the following pattern:</p>
<table>
<thead>
<tr>
<th>Dehydration Level (% body weight)</th>
<th>Performance Consequence</th>
</tr>
</thead>
<tbody>
<tr>
<td>1%</td>
<td>Negligible effect in most conditions; cognitive sensitivity begins</td>
</tr>
<tr>
<td>2%</td>
<td>Aerobic performance significantly impaired; ~2-3% capacity reduction</td>
</tr>
<tr>
<td>3-4%</td>
<td>Substantial aerobic impairment; muscle strength and power affected</td>
</tr>
<tr>
<td>5%</td>
<td>Severe impairment; heat exhaustion risk elevated</td>
</tr>
</tbody>
</table>
<p>The 2% threshold has become the conventional reference point in exercise physiology, though the review noted that this threshold is condition-dependent: heat, humidity, altitude, and exercise duration all modulate the performance consequences of a given dehydration level [1].</p>
<h3>Cardiovascular and Thermoregulatory Mechanisms</h3>
<p>At the cardiovascular level, dehydration reduces plasma volume, which in turn decreases stroke volume and cardiac output. To compensate, heart rate increases (elevated cardiovascular strain) at any given work rate. This cardiovascular drift reduces the sustainable exercise intensity and accelerates fatigue [2].</p>
<p>Simultaneously, reduced plasma volume impairs the ability to simultaneously direct blood to working muscles and to the skin for heat dissipation. This thermoregulatory conflict becomes especially pronounced in hot environments, where core body temperature rises more steeply with a given level of dehydration. Core temperature elevations of 0.1-0.2°C per 1% body weight loss have been documented, with temperature-dependent reductions in central nervous system function contributing to earlier voluntary fatigue [3].</p>
<h3>Resistance Exercise and Strength Sports</h3>
<p>While the dehydration-performance relationship is most well-characterized for aerobic exercise, evidence also demonstrates meaningful effects on resistance exercise outcomes. Studies found that dehydration of 3-4% body weight reduced muscular strength (<a href="/terms/one-repetition-maximum/" class="term-link" data-slug="one-repetition-maximum" title="1RM">1RM</a> performance) by approximately 2%, muscular endurance by 10-15%, and anaerobic power output by 6-7% [4].</p>
<p>The mechanisms appear distinct from those affecting aerobic performance. Dehydration reduces cell volume in muscle fibers, which signals a catabolic state and may impair contractile protein function. Additionally, the increased perceived effort associated with dehydration reduces voluntary force production and exercise volume.</p>
<h3>Cognitive Effects of Dehydration</h3>
<p>The review's synthesis of cognitive neuroscience literature revealed that dehydration produces measurable cognitive impairments at lower thresholds than those affecting physical performance. Mild dehydration of 1-2% body weight produced significant reductions in attention, working memory, psychomotor speed, and mood state — particularly under conditions of heat stress [5].</p>
<p>These cognitive effects have practical implications for team sports athletes who must make rapid tactical decisions during competition, and for the general training population where motivation, focus, and adherence to technical execution are hydration-dependent.</p>
<h3>Individual Variability in Sweat Rate</h3>
<p>One of the review's most important observations was the magnitude of individual variability in sweat rate. Trained athletes may lose 1-2.5 liters of fluid per hour during intense exercise in hot conditions, while less trained individuals in temperate conditions may lose as little as 0.5 liters per hour [6]. Universal fluid intake recommendations (e.g., "drink 500mL every 30 minutes") are therefore inherently imprecise and can lead to either underhydration or overhydration depending on the individual and environmental context.</p>
<h2>Discussion</h2>
<h3>Reconciling the Evidence: Thirst vs. Fixed Schedule Drinking</h3>
<p>The accumulated evidence supports a nuanced position between two extremes of fluid intake prescription: aggressive scheduled drinking versus unrestricted ad libitum (thirst-based) drinking. For most recreational athletes exercising at moderate intensity in temperate conditions, thirst appears to be a reasonably reliable indicator of hydration needs, with ad libitum drinking sufficient to prevent performance-impairing dehydration [1].</p>
<p>However, in competitive settings, hot or humid environments, or during exercise lasting longer than 60-90 minutes, the onset of thirst may lag behind physiological fluid deficit. In these conditions, a scheduled drinking approach — consuming fluids at predetermined intervals regardless of thirst — provides a useful hedge against accumulated dehydration [2].</p>
<p>The optimal integration is therefore an individually calibrated strategy: understand your personal sweat rate through pre- and post-exercise weigh-ins, establish a baseline fluid intake that approximately matches sweat losses, and use thirst as a fine-tuning mechanism rather than the sole guide.</p>
<h3>The Hyponatremia Risk: Overhydration Is Also Dangerous</h3>
<p>A critical finding underscored by the review is that excessive fluid intake during prolonged exercise carries its own significant risks. Exercise-associated hyponatremia (EAH) occurs when blood sodium diluted by excess water intake falls below approximately 135 mEq/L, producing symptoms ranging from nausea and headache to seizures and death in severe cases [3].</p>
<p>EAH risk is highest in slow-to-moderate pace endurance events of 4+ hours in which sweat sodium losses are high but fluid intake exceeds fluid losses. Smaller-framed athletes, women, and athletes who aggressively consume plain water rather than sodium-containing sports drinks are at elevated risk.</p>
<p>The practical implication is that the goal of hydration during prolonged exercise is euhydration (fluid balance), not maximal fluid intake. Drinking to replace sweat losses — approximately matched to thirst during moderate exercise — minimizes both dehydration and overhydration risks.</p>
<h3>Electrolyte Considerations</h3>
<p>Pure water replacement during prolonged exercise (60-90 minutes) or in sodium-depleting conditions inadequately addresses the dual challenge of fluid and electrolyte replacement. Sweat contains significant concentrations of sodium (approximately 700-1,000 mg/liter), chloride, potassium, and smaller amounts of magnesium and calcium [4].</p>
<p>Sodium is particularly important because it maintains osmotic drive for fluid retention (reducing urinary losses) and maintains plasma osmolarity — the physiological signal that stimulates thirst and limits urine production. Sports drinks containing 400-1,000 mg sodium per liter provide more effective hydration than plain water for events exceeding 60-90 minutes, and particularly for very sweaty athletes.</p>
<h3>Implications for <a href="/terms/periodization/" class="term-link" data-slug="periodization" title="Training Periodization">Training Periodization</a></h3>
<p>Hydration strategies should be periodized alongside training. During high-volume training blocks, total daily fluid needs are elevated, and sweat rate monitoring is particularly important. Athletes who train in progressively hotter conditions across a season benefit from heat acclimatization, which increases plasma volume and improves thermoregulatory efficiency — effectively reducing the performance cost of a given dehydration level [5].</p>
<p>Pre-competition hyperhydration using glycerol or sodium loading has been studied as a strategy to temporarily expand plasma volume, with modest evidence of benefit in heat-stressed conditions, though these approaches require careful implementation to avoid gastrointestinal side effects and regulatory complications.</p>
<h2>Practical Recommendations</h2>
<h3>Daily Baseline Hydration</h3>
<ul>
<li><strong>General daily target</strong>: 35-45 mL/kg body weight as a baseline fluid intake from all sources (water, food, beverages)</li>
<li><strong>For a 75-80kg athlete</strong>: approximately 2.6-3.6 liters daily from all sources at rest; substantially more on <a href="/terms/training-frequency/" class="term-link" data-slug="training-frequency" title="training days">training days</a></li>
<li><strong>Urine color monitoring</strong>: Pale yellow (lemonade color) indicates adequate hydration. Dark yellow or amber indicates dehydration. Clear urine may indicate overhydration</li>
</ul>
<h3>Pre-Exercise Hydration</h3>
<ul>
<li><strong>2 hours before exercise</strong>: <a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="If">If</a> urine is dark, consume 500-600 mL of fluid to allow time for equilibration and urination before training</li>
<li><strong>30 minutes before</strong>: 200-400 mL of water or sports drink</li>
<li><strong>Pre-exercise weigh-in</strong> (for serious athletes): Establish a baseline euhydrated body weight; begin exercise within 1% of this weight</li>
</ul>
<h3>During Exercise</h3>
<table>
<thead>
<tr>
<th>Duration</th>
<th>Fluid Strategy</th>
<th>Recommended Source</th>
</tr>
</thead>
<tbody>
<tr>
<td>45 min</td>
<td>Drink to thirst</td>
<td>Water</td>
</tr>
<tr>
<td>45-90 min</td>
<td>150-350 mL every 15-20 min</td>
<td>Water or low-carb sports drink</td>
</tr>
<tr>
<td>90 min</td>
<td>400-800 mL/hour (individual adjusted)</td>
<td>Sports drink with sodium (400-1,000 mg/L)</td>
</tr>
<tr>
<td>3 hours (endurance)</td>
<td>Sodium replacement essential</td>
<td>Sports drink + sodium-containing foods</td>
</tr>
</tbody>
</table>
<h3>Post-Exercise Rehydration</h3>
<ul>
<li><strong>Replace 150% of fluid losses</strong>: If you lost 1kg during exercise (approximately 1 liter of sweat), consume 1.5 liters of fluid over the subsequent 4-6 hours</li>
<li><strong>Include sodium</strong>: Plain water rehydration alone after significant sweat losses increases urine production and reduces fluid retention. Add sodium through food, electrolyte tablets, or sports drinks</li>
<li><strong>Timing</strong>: Rapid rehydration (within 2 hours) supports recovery; spreading intake over 4-6 hours is sufficient for non-competition contexts</li>
</ul>
<h3>Sweat Rate Assessment (Recommended for Athletes)</h3>
<p>Measure your personal sweat rate to calibrate individual fluid needs:</p>
<ol>
<li>Weigh yourself nude (or minimal clothing) immediately before exercise</li>
<li>Exercise for 60 minutes at competition intensity without drinking</li>
<li>Weigh yourself again immediately post-exercise</li>
<li>Sweat rate (L/hour) = (pre-weight - post-weight in kg) + any fluid consumed</li>
</ol>
<p>Use this personal sweat rate to establish a per-hour fluid target during competition conditions.</p>
<h3>Warning Signs of Dehydration</h3>
<p>Recognize these indicators during exercise:
- Dark urine before exercise
- Feeling significantly thirsty at the start of training
- Dizziness, headache, or unusual fatigue during exercise
- Reduced performance in conditions where you typically perform well
- Muscle cramps (though cramping is multifactorial)</p>