A new analysis involving 225 adults found no clear evidence that drinking fluids during prolonged exercise improves race times or overall performance, although hydration was associated with lower heart rates and core body temperatures.
By yourNEWS Media Newsroom
Drinking water during prolonged exercise may help regulate body temperature and reduce cardiovascular strain without necessarily improving athletic performance, according to a new scientific review that challenges the assumption that consuming more fluids automatically leads to greater endurance.
The systematic review and meta-analysis published Oct. 27 in Frontiers in Nutrition examined 24 randomized trials involving 225 adults who participated in exercise sessions lasting at least 60 minutes. Researchers found no clear improvement in time-trial performance or work completed within a fixed period when participants consumed fluids during exercise compared with drinking little or nothing.
The findings suggest that hydration may offer physiological benefits without consistently producing measurable improvements in athletic performance. Researchers emphasized that individual needs depend on workout duration, environmental conditions, sweat losses and personal fluid tolerance.
The review, conducted by Xiaojie Zhang, Ling Wang and Ziyu Liu, assessed several hydration strategies, including drinking during exercise versus consuming little or no fluid, following scheduled drinking plans versus drinking according to thirst, and consuming different quantities of fluids or sodium.
Of the 24 studies included, 21 contributed data to the quantitative analysis. The trials involved 170 men, 20 women and 35 participants whose sex was not reported.
Because the findings were drawn from separate experiments using different protocols, environmental conditions and performance measurements, the researchers cautioned against treating the pooled results as definitive recommendations for every athlete.
In four studies examining time trials or fixed-duration exercise, hydration produced an estimated performance difference of just negative 0.1%, with a 95% confidence interval ranging from a 3.6% decrease to a 3.5% improvement.
That range did not establish a meaningful advantage from drinking during exercise.
A separate analysis of three studies involving 21 participants found that hydration was associated with a 34.1% improvement in time to exhaustion. However, the researchers said the evidence was too uncertain to confirm a consistent benefit across different exercise settings.
Time-to-exhaustion tests measure how long someone can continue exercising before stopping, while time trials evaluate how quickly a person completes a set distance or how much work is performed within a particular period.
The distinction matters because endurance capacity measured under controlled laboratory conditions does not always translate into faster race times or better performance during ordinary workouts.
Although performance improvements remained uncertain, the researchers identified more consistent changes in physiological measurements.
Across nine studies involving 80 participants, people who consumed fluids during exercise had core body temperatures averaging 0.38 degrees Celsius lower than those who drank little or nothing.
In 12 studies involving 115 participants, hydration was associated with an average heart rate reduction of approximately 6.8 beats per minute.
These findings suggest that fluid intake may help the body manage heat and cardiovascular demands during prolonged physical activity.
However, the researchers identified substantial differences among the studies, and their statistical estimates indicated that the benefits might not occur consistently in every exercise setting.
The review classified the certainty of evidence for all critical and important outcomes as very low, reflecting small sample sizes, variation in study methods and limitations in the available data.
The findings therefore do not establish that drinking during exercise is unnecessary or that avoiding fluids improves performance.
Instead, they indicate that the relationship between hydration and athletic output is more complicated than simply consuming additional water.
The researchers also examined whether following a predetermined hydration schedule was superior to drinking when thirsty.
Three studies involving 27 participants found no clear performance difference between scheduled or individualized drinking strategies and self-regulated fluid consumption.
However, four studies involving 40 participants found that people following planned drinking schedules lost approximately 1.47 percentage points less body weight during exercise.
The researchers explained that this difference largely reflected the greater quantity of fluid consumed rather than independent evidence of improved athletic performance.
Changes in body weight during exercise are often used to estimate fluid losses through sweating, although those measurements can be affected by additional factors.
The review also examined whether larger fluid quantities or greater sodium intake improved endurance.
Three studies comparing higher and lower fluid replacement doses produced an estimated 2.7% performance difference, but the confidence interval was too wide to establish a reliable benefit.
Evidence concerning sodium supplementation was even more limited. Only two studies provided suitable comparisons of different sodium quantities while controlling for fluid and carbohydrate intake.
Those trials did not establish an optimal sodium dose for maintaining performance or preventing dehydration.
Sodium is an important electrolyte involved in fluid balance, nerve signaling and muscle function. It is lost through sweat, particularly during prolonged physical activity in hot environments.
However, athletes differ considerably in how much sodium they lose, making a single replacement strategy inappropriate for everyone.
The findings also need to be considered alongside decades of research examining dehydration, heat tolerance and exercise safety.
Physical activity increases metabolic heat production, and sweating helps regulate body temperature through evaporation.
When sweat losses are not adequately replaced, circulating blood volume can decline, placing additional demands on the cardiovascular system.
Earlier physiological experiments have demonstrated that dehydration can substantially reduce tolerance to exercise in hot conditions. One study cited in the scientific literature found that hypohydration reduced walking tolerance in a hot environment from 121 minutes to 55 minutes.
Other investigations have identified physiological changes associated with relatively small losses of body weight through sweating, although the severity of performance effects varies according to exercise intensity, temperature, humidity and individual conditioning.
The latest review does not contradict the established importance of maintaining hydration during strenuous activity. Rather, it questions whether consuming more fluids consistently produces better endurance results.
For shorter exercise sessions in moderate temperatures, many healthy adults can regulate fluid intake effectively by responding to thirst.
Longer training sessions, particularly those involving sustained sweating in hot or humid conditions, may require more deliberate fluid and electrolyte replacement.
People exercising in extreme heat, performing physically demanding outdoor work or completing multiple training sessions in a day may also need to pay closer attention to hydration before, during and after activity.
The researchers cautioned that replacing every ounce of sweat is not necessarily required to maintain performance and that excessive fluid consumption can create its own medical risks.
One serious complication is exercise-associated hyponatremia, a condition in which blood sodium concentrations become abnormally low, often because a person consumes more fluid than the kidneys can eliminate.
The condition has been documented among marathon runners, endurance athletes and participants in prolonged physical activities.
Symptoms can include nausea, headache, confusion and altered mental status. Severe cases may lead to seizures, pulmonary edema and potentially fatal neurological complications.
The risk underscores why drinking large amounts of water without regard to thirst, exercise duration or individual fluid needs can be dangerous.
Electrolyte-containing beverages may be appropriate during prolonged activity involving significant sweat losses, but sodium supplementation does not eliminate the dangers of drinking excessive fluid.
The review’s authors therefore emphasized an individualized approach rather than universal drinking targets.
Their findings also highlight the importance of distinguishing between physiological responses and athletic performance.
A lower heart rate or core temperature during exercise may indicate reduced cardiovascular or thermal strain, but those changes do not necessarily mean an athlete will run faster, cycle farther or complete more work.
Performance also depends on physical conditioning, pacing, carbohydrate availability, environmental conditions and other factors beyond hydration.
The researchers acknowledged several limitations, including the small number of participants in many trials, inconsistent exercise protocols, incomplete access to some original research materials and analytical decisions made after the review was registered.
They also noted that the available studies did not permit reliable conclusions about how hydration strategies compare across hot and temperate environments.
Those limitations prevent the findings from being interpreted as proof that drinking during endurance exercise offers no performance advantage under any circumstances.
The researchers concluded that average results favored hydration for some physiological measurements and exhaustion tests, while evidence concerning self-paced performance and optimal fluid or sodium intake remained inconclusive.
The findings support tailoring hydration practices to the demands of individual workouts, including duration, temperature, sweat rate and tolerance for consuming fluids while exercising.
For athletes and recreational exercisers, the central distinction is between drinking enough to maintain health and assuming that greater fluid consumption will automatically improve athletic results.
The new analysis indicates that hydration remains important for managing physiological strain and preventing dangerous dehydration, but consuming more water does not reliably guarantee a faster finish or better endurance performance.
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