How Much Water Should You Drink During Training?

If your answer is “as much as possible” or “until I’m not thirsty,” you might be surprised to learn that hydration is far more nuanced than simply drinking when you feel like it. Many endurance athletes fall into the trap of either overhydrating or underhydrating, both of which can negatively impact performance and, in extreme cases, pose serious health risks.

The Science of Hydration

Hydration plays a critical role in maintaining endurance performance. Water is essential for thermoregulation, cardiovascular function, and muscular efficiency. However, there’s a fine balance between drinking too much and too little.

• Dehydration occurs when fluid loss exceeds fluid intake, leading to a decline in blood volume, increased heart rate, and decreased ability to regulate body temperature. Even a 2% loss in body weight due to dehydration can significantly impair performance.

• Overhydration (Hyponatremia) happens when an athlete consumes excessive amounts of water without replacing electrolytes, diluting blood sodium levels. This condition can lead to confusion, nausea, muscle weakness, and in severe cases, even death.

To optimize hydration and prevent these risks, athletes should approach fluid intake strategically rather than arbitrarily.

Why “Drink When Thirsty” Isn’t Always Reliable

Many athletes rely on thirst to guide their hydration, but this isn’t always an accurate measure. By the time you feel thirsty, you may already be slightly dehydrated. Additionally, factors such as adrenaline, focus, and race-day excitement can suppress thirst cues, leading athletes to underestimate their hydration needs.

Conversely, some athletes overhydrate based on the assumption that more water equals better performance. This is a dangerous misconception, as excessive water intake can dilute sodium levels, causing hyponatremia. The key is finding a middle ground—hydrating enough to maintain optimal function without overloading the system.

How to Determine Your Hydration Needs

1. Calculate Your Sweat Rate

Sweat rate varies significantly between athletes based on genetics, environment, and intensity of exercise. To determine how much water you need, follow these steps:

1. Weigh yourself before a workout (preferably without clothes and after using the bathroom).

2. Train for an hour at race intensity, keeping track of how much fluid you consume.

3. Weigh yourself after the workout (again without clothes, avoiding additional fluid intake before weighing in).

4. Calculate sweat loss:

   • Subtract post-workout weight from pre-workout weight.

   • Convert weight loss into fluid loss (1 lb = ~16 ounces of sweat).

   • Add the volume of fluids consumed during the workout.

For example, if you lost 1.5 pounds and drank 16 ounces of water, your total sweat loss is 1.5 x 16 + 16 = 40 ounces per hour.

2. Adjust Fluid Intake Accordingly

Once you know your sweat rate, aim to replace 80-90% of lost fluids during training sessions or races. Over-replacing (i.e., consuming 100% or more of sweat loss) increases the risk of hyponatremia. Under-replacing may lead to dehydration-induced fatigue.

• If your sweat rate is 40 ounces per hour, aim to consume 32-36 ounces per hour.

• If your sweat rate is 20 ounces per hour, aim for 16-18 ounces per hour.

This personalized approach ensures you stay hydrated without overloading your system.

Electrolytes Matter

Water alone isn’t enough—electrolytes (sodium, potassium, magnesium) play a critical role in fluid balance. Sodium is particularly important, as it helps retain water and prevents hyponatremia.

Sweat sodium concentration varies from person to person, with some athletes losing 500 mg per liter of sweat and others losing over 1500 mg per liter. To determine your sodium needs:

1. Consider a sweat test (available through sports labs) to measure sodium concentration in your sweat.

2. Use trial and error—if you experience muscle cramps, dizziness, or excessive thirst despite drinking water, you may need more sodium.

3. Aim for 500-1000 mg of sodium per hour for longer endurance sessions, adjusting based on your individual needs.

Hydration Strategies for Different Conditions

Hot & Humid Conditions

• Increase fluid intake slightly due to higher sweat losses.

• Prioritize electrolyte-rich drinks.

• Wear light, moisture-wicking clothing to aid thermoregulation.

Cold Weather Training

• Don’t neglect hydration—cold suppresses thirst, making dehydration more likely.

• Consider warm fluids or electrolyte-rich broths.

• Adjust fluid intake based on sweat loss rather than perceived thirst.

Long Training Sessions & Races

• Start hydrated—consume 16-20 ounces of fluid 1-2 hours before exercise.

• Maintain a steady intake schedule rather than drinking in large amounts at once.

• Use a combination of water and electrolyte drinks to prevent dilution of sodium levels.

Signs of Overhydration vs. Dehydration

If you experience symptoms of either, adjust hydration accordingly.

The Takeaway

Hydration is not a one-size-fits-all approach. Drinking according to your sweat rate, rather than relying solely on thirst, will help optimize performance and prevent both dehydration and overhydration. Remember to:

• Measure your sweat rate and adjust intake accordingly.

• Balance water with electrolytes to avoid sodium imbalances.

• Adapt hydration strategies to environmental conditions and race demands.

By taking a science-based approach to hydration, you can ensure your body performs at its best, whether in training or on race day.