The question of drinking water while running often stems from the fear of developing a side stitch or the uncomfortable feeling of a sloshing stomach. While improper drinking can cause discomfort, mid-run hydration is not inherently bad. Drinking during a run is necessary for health and performance, but the timing, volume, and fluid type are critical factors. A successful hydration strategy requires balancing the body’s need for fluid with its limited capacity to absorb it during physical activity.
The Physiological Need for Mid-Run Hydration
Hydration directly regulates the body’s cardiovascular and thermoregulatory functions during exercise. When running, the body continuously loses fluid through sweat, and this loss must be addressed to maintain performance. Sweat rates vary significantly among runners, ranging from roughly 0.2 to over 2.5 liters per hour depending on intensity, genetics, and environmental conditions.
Even a mild fluid deficit can compromise the body’s efficiency. Losing just 1 to 2% of body weight due to fluid loss negatively influences endurance performance. This level of dehydration increases physiological strain, causing the heart rate to rise by approximately 3 to 5 beats per minute and the core body temperature to increase by 0.12°C to 0.25°C for every 1% of body mass lost.
The rise in core temperature occurs because dehydration reduces the overall plasma volume, restricting blood flow to the skin for cooling. This forces the heart to work harder to deliver oxygen and cool the body, accelerating fatigue. Consuming fluid mid-run is necessary to mitigate the steady decline in function that accompanies sweat loss.
Why Improper Drinking Causes Gastrointestinal Distress
The most common complaints related to mid-run drinking are abdominal cramps, nausea, and the sensation of fluid sloshing, which are symptoms of gastrointestinal (GI) distress. These issues result from either the mechanical effects of running or a physiological mismatch in fluid ingestion. During intense running, the body prioritizes blood flow to the working muscles and the skin, reducing circulation to the digestive system.
This reduction in blood flow, known as splanchnic hypoperfusion, impairs the gut’s ability to function and absorb nutrients. When a runner rapidly ingests a large volume of fluid, the stomach becomes distended, which triggers discomfort and can slow the rate at which the fluid passes into the small intestine. This gastric distension is a primary cause of side stitches and the sloshing sensation.
Furthermore, consuming fluids highly concentrated in carbohydrates, known as hypertonic solutions, can exacerbate GI issues. A hypertonic drink draws water from the body into the stomach and intestines to dilute its concentration, which can delay gastric emptying and lead to bloating, cramping, and diarrhea. The key to avoiding distress is managing the volume and concentration of the fluid consumed.
The Risk of Water Intoxication
While dehydration is the more common concern, a severe, though less frequent, danger is exercise-associated hyponatremia (EAH), or water intoxication. Hyponatremia occurs when the concentration of sodium in the blood drops below 135 millimoles per liter. This is caused by drinking an excessive amount of fluid, usually plain water or other sodium-poor beverages, over a long period.
The excess fluid dilutes the blood’s sodium, causing water to shift into the body’s cells, including those in the brain, leading to dangerous swelling. Symptoms often begin with non-specific signs like nausea, headache, and fatigue, but can progress to confusion, seizures, and coma in severe cases. EAH is most often seen in ultra-endurance athletes or slower marathon runners who drink far more than they lose.
A 2002 Boston Marathon study found that 13% of finishers had some degree of hyponatremia. In these cases, athletes often gain weight during the event because they retain more water than they excrete. For long-duration efforts, proper sodium replacement is necessary to prevent this potentially life-threatening condition.
Practical Strategies for Safe Running Hydration
The safest and most effective hydration strategy involves small, frequent sips rather than guzzling large volumes at once. The body’s ability to absorb fluid during exercise is limited, often maxing out in the range of 400 to 800 milliliters per hour. Aiming for small intakes, such as 4 to 8 ounces every 15 to 20 minutes, helps match absorption capacity and minimizes gastric sloshing.
For runs lasting less than 60 minutes, plain water is sufficient to meet fluid needs. However, for efforts extending beyond 60 to 90 minutes, especially in warm conditions, incorporating electrolytes becomes beneficial. Sports drinks or electrolyte mixes help replace the sodium lost through sweat, which can range from 200 to over 2,000 milligrams per liter of sweat.
A modern and safe approach is to use thirst as a primary guide, drinking when the body signals a need for fluid. This strategy helps athletes avoid the dangerous overhydration that leads to hyponatremia. Runners can also practice pre-hydrating by consuming 16 to 20 ounces of fluid in the two to three hours before starting a run.