Staying Hydrated
Are you drinking too little or too much?


water bottle

Staying hydrated should be a simple matter. If you’re thirsty, drink.

And if you read the latest guidelines from the International Marathon Medical Directors Association (IMMDA), that is exactly what you should do—drink when thirsty.

The concept of drinking according to thirst may seem too simple to be an accurate. For years, the message has been that by the time you feel thirsty, you're already dehydrated. Yet a few high-profile cases of overhydration, including the death of a participant in the 2002 Boston Marathon, have led to a revaluation of hydration guidelines.

According to these drink-when-thirsty advocates, our bodies can handle temporary under-hydration for up to 8 hours. Most endurance runners and cyclist feel thirsty at about two percent dehydration—the point at which performance begins to drop off.. Ultra-runners and distance cyclists can maintain performance at three percent hydration. IMMDA adds, ”The new scientific evidence says that thirst will actually protect athletes from the hazards of both over- and under-drinking.”

Flasks, Bidons, and Dehydration

Wait! Over-drinking?

Until recently, most of us worried that we weren’t getting enough to drink. We run carrying water flasks and down several cups at every aid stations. We bike festooned with multiple bidons (water bottles) or wear bladder-equipped packs. No less an authority than the American College of Sports Medicine (ACSM) has warned us of the dangers of even mild dehydration:

  • Decreased performance
  • Muscle weakness and cramps
  • Headache
  • Heartburn
  • Heart palpitations
  • Dizziness
  • Overheating
  • Irritability

Clearly, a serious shortage of fluids can cause problems. Almost every marathoner has seen someone stagger into the medical tent seemingly suffering from dehydration. Over 200 hundred years ago, the Rebbe Rashab (6th Lubavitcher Rebbe) wrote, “Even mild dehydration reduces blood volume, which reduces blood flow carrying water, oxygen and nutrients to the muscles, organs and glands. Continued loss of water will directly affect the heart and brain, which require large amounts of water and oxygen brought by blood flow.”

Mayim Chaim

There is no question that adequate hydration is essential to your wellbeing. Overall, your body is 70% water, and the water content of your blood, brain, muscles and cells is even higher.  On a daily basis, water:

  • Regulates body temperature
  • Lubricates joints and moistens tissues
  • Helps dissolve minerals and nutrients, carrying them to cells
  • l>Protects organs and tissues
  • Removes waste and toxins, lessening the burden on kidneys and liver

But how much water is enough?


As you might expect, it all depends.  Each of us is different: during an activity, you need to consider your sweat rate, the heat and humidity, and how long and hard you are exercising. You also need to consider pre-activity preparation and post-activity recovery.

The common wisdom is that most of us don’t drink enough water. The average person needs about eight 8 ounce glasses of water per day. But what’s average?

urine color chart

Another approach is to consume ounces of water equal to half your weight in pounds. So if you weigh 120 pounds, you should drink 60 ounces of water per day.

But this doesn’t take how active you are during the day, the food you consume, or the weather.

The American Council on Exercise offers the following guidelines pre-, during, and post- endurance (90+ min) activities:

  • 2 - 3 Hours Before: 17 - 20 oz.
  • 20 - 30 Minutes Before: 8 oz.
  • During: 7-10 oz every 10 - 20 minutes
  • After: 8 oz.

Urine Color

General guidelines are fine, but as we mentioned, each of us is different. A more individualized measure of hydration is the color of your urine (we apologize for the yuck factor). Use this chart to see if you are drinking enough fluids during the day to stay hydrated.

Strong smelling urine can also be a sign of dehydration—assuming you haven’t eaten asparagus or garlic.

Sweat Loss Replacement

Possibly the smartest way to ensure you are properly hydrated, especially during endurance activities, without the risk of over-hydration, is by drinking to match your sweat rate. Your sweat rate is the amount of fluid you lose during an hour of exercise under normal conditions. And that is the amount you should consume, according to the ACSM, to get all the performance benefits of being well hydrated.

Without seeming repetitious, individual sweat rates differ significantly. So we’ve provided a sweat loss tool to calculate your rate (sidebar).

Don’t Stay Thirsty, My Friends

So which approach should you take during an endurance event? Thirst or sweat loss?

In the end, it comes down to listening to your own body. If you feel uneasy relying on thirst alone, determine your sweat rate and drink accordingly. If you are thirsty or sweating bucketfuls, drink. If you don’t feel the urge to drink or your stomach starts sloshing, take a pass on the next water stop.