Most athletes are aware of the importance to be well hydrated when exercising and during races. This is even more critical when events are longer than 60 minutes or have high temperature and humidity levels. If hydrated properly your body will be able to maintain physiological functions needed for an increased effort, essentially increasing your endurance.
Being well hydrated and aware of your hydration level is one matter, but it is very important that athletes also understand you can hydrate too much as well. Excessive fluid consumption combined with low sodium intake during exercise can lead to a life threatening condition (fairly rare occurrence) known as exercise-induced hyponatremia. Hyponatremia refers to a severe reduction of sodium concentration in your blood. This is something where too much of something is not always a good thing and can turn very bad quickly.
To know how much water is too much during exercise is typically associated if you are gaining weight during exercise (replacing more fluids than you sweat). If matching sweat loss, your body will remain balanced. The problem of hyponatremia occurs when athletes force themselves to drink well beyond their sweat loss. By drinking too much water, and doing so too quickly, it will cause the sodium concentrations in your blood to drop quickly due to the fact that you can drink faster than you can urinate (void excess fluids). Due to the inability of the body to relieve the excess fluids your body cannot keep up with maintaining safe levels of sodium in the blood.
Typical symptoms that hyponatremia provides is swollen extremities (fingers, toes), swollen ankles, headache, fatigue, confusion, seizures, bloating and swelling of the brain. The population that has been considered the most at risk is small female endurance athletes that excessively drink water during exercise.
The ironic thing is that you can be both dehydrated and hyponatremic at the same time. Typically, this occurs in athletes that have a heavy salt loss in their sweat. Each athlete's salt needs are different. If an athlete that loses about 8 liters of sweat that is high in sodium loss and only replaces it with 5-6 liters of fluid, it is likely they are both hyponatremic and dehydrated.
This condition and fluid balance is critical to not only success in training but also for your own health. Monitoring and understanding your fluid needs by evaluating your sweat loss is imperative (I have provided instructions below on how to determine your fluid needs). The goal is to try and match only fluid loss during exercise. If you are gaining weight during exercise, typically you are taking in too much fluids.
Another way to avoid this condition from occurring is to monitor your hydration status. This can be done through monitoring your urine color and the volume of urine voided. A well hydrated athlete will have a urine color of lemonade vs. someone who is not hydrated well which will resemble an apple juice like color. As you become dehydrated, your body will try to hold in more fluids which means your urine volume will be smaller. So if your urine is dark and low volume you are more than likely dehydrated.
Sodium and electrolyte supplementation during exercise is something to consider as well. Every liter of sweat has an estimated 1 gram of sodium lost. Each person is different in how much of each electrolyte they excrete through sweat so everyone's needs are different as well. Sodium being the primary electrolyte in sweat can account for large losses. Taking in sodium supplements during exercise will help maintain fluid balance levels and blood volume. It can also help reduce the amount of urine excreted.
Ideally, your goal is to maintain your hydration level. Know your sweat rate and understand what it will take to stay hydrated throughout your workouts and day. Do your best to mitigate dehydration but do not over drink.
CALCULATE YOUR SWEAT RATE: To begin, record your nude body weight prior to exercising (target a 60 min run). When you are finished exercising, dry yourself off the best you can and record your nude bodyweight again. Record what and how much you consumed of fluids during your exercise. Subtract your pre-exercise weight from your post-exercise weight and add the amount of fluid you consumed to that number. This will give you the amount of fluid you lost while exercising. Then you need to divide that number by the amount of hours you exercised for and that will equal your sweat rate. Be sure to record the weather conditions as well, as to see how this may fluctuate.