Fluid Replacement and Exercise

By Howard LeWine, M.D.
Brigham and Women's Hospital


The Growing Popularity of Long-Distance Running
Why Change the Fluid Recommendations?
How Our Bodies Maintain Fluid Balance
The Danger of Drinking Too Much Water
The New Guidelines


Sport and athletic associations have stopped promoting the old mantra of "drink as much as possible." Instead, they have stressed the importance of proper fluid intake before, during and after exercise.


The Growing Popularity of Long-Distance Running


The first running of the New York Marathon took place in 1970; 55 people finished, while more than 36,500 finished the 2004 marathon. In 2002, 450,000 runners finished marathons in the United States. Similar increases have occurred for other long-duration events such as 100-mile bicycle races and triathlons.


As more people have entered the 26-plus-mile races, the degree of supervised training and fitness levels has diminished, as evidenced by an increase in the average time to finish from about 3½ hours to well over 4 hours. With 30 million adult runners in the United States and many millions more exercising regularly, organizations such as the American College of Sports Medicine and USA Track & Field realize the need for a clear message about proper hydration.


Why Change the Fluid Recommendations?


The impetus to change the guidelines comes from different sources. We now have a better understanding of exercise-related symptoms of weakness and light-headedness and an appreciation of the danger of drinking too much plain water.


Research done in the 1960s suggested that dehydration was the main reason for heat exhaustion and heat stroke. So the natural conclusion was that athletes should always drink as much as possible to avoid dehydration. More recent research has revealed that heat-related illness has much more to do with how quickly people produce heat, the efficiency of their cooling mechanisms, and how well the surrounding environment can absorb the body heat. Heat exhaustion and heat stroke occur when the speed of heat production outpaces the ability to lose heat. Hydration is only one factor that affects this balance.


In the past, most symptoms related to prolonged exercise, such as light-headedness, were immediately attributed to dehydration. Certainly dehydration can be the cause of feeling dizzy. However, a likely alternative reason for being light-headed is from standing still after a long run or bicycle ride. This causes blood to pool in the legs, decreasing blood pressure (called postural hypotension) and blood flow to the brain. Following any prolonged strenuous activity, you should continue to move your legs for several minutes until your heart rate has recovered. Another misconception is that drinking extra fluid enhances athletic performance. You need adequate intake of fluids and calories before and during prolonged exercise, but overdoing it can impair performance and potentially be dangerous.


How Our Bodies Maintain Fluid Balance


The amount of water and salt in our bodies is balanced by an intricate system of hormones regulated by the kidneys and nervous system. During exercise, we increase our heart rate and volume of blood pumped out of the heart with each beat. This is called cardiac output. With strenuous exercise, cardiac output can increase from 5 to 25 quarts per minute. Our bodies need a full tank of salt and water within our bloodstream to make this happen.


Meanwhile, as body temperature rises during exercise, we sweat to release the heat. Also, we breathe faster and harder which causes more loss of water. The body’s response to the fluid loss is to tell our kidneys to hold on to all of the fluid they can. One of the key hormones that makes this happen is called ADH, or anti-diuretic hormone. ADH levels rise in response to fluid loss by telling the kidneys not to excrete water. So the amount of water relative to salt content in the blood rises, lowering the concentration of sodium.


The Danger of Drinking Too Much Water


Some people, especially women, have an exaggerated ADH response with prolonged strenuous exercise, such as a marathon. By drinking too much water without salt or electrolytes, susceptible individuals can develop dangerously low sodium blood levels (severe hyponatremia).


Mild hyponatremia that quickly corrects itself is quite common with vigorous prolonged exercise and probably does not decrease exercise performance. Severe hyponatremia is fortunately very rare but can result in brain swelling and death.


The New Guidelines


Attention to proper hydration starts before you exercise. Drink 16 ounces of fluid over 15 to 30 minutes ending two hours prior to your planned exercise start time. If you have been eating normally with at least a little salt, then plain water is fine. If not, consider a sports drink or adding a pinch of salt and sugar to the water.


For moderate exercise lasting up to an hour, you can usually rely on thirst to drive your fluid intake. The exception is exercising outdoors in very hot or cold weather, when you should drink a little extra beyond thirst.


If you exercise regularly for longer than one hour at a time or plan to compete in a long-distance event, you should make a more accurate assessment of your personal fluid needs. A simple estimate can be made by measuring change in body weight. Empty your bladder and get on the scale in dry clothing. Exercise for one hour at your usual or expected pace. Measure the exact amount of water that you drink during the test hour. Empty your bladder again and dry off any excess sweat. Put on the same dry clothes you wore for the initial weight and get back on the scale.


The amount of fluid you need per hour of exercise equals the number of ounces you drank plus the difference in ending vs. starting weight. (One pound equals 16 ounces of fluid.)


After you finish competing or working out, pay attention to thirst and keep drinking until urine color returns to normal — usually a pale yellow.


Regarding the type of fluid to drink, sports drinks or homemade solutions with a little sugar and salt are not necessary unless you plan to exercise for more than a couple of hours. Some people, however, feel more energized using these drinks with even less strenuous workouts.




Howard LeWine, M.D. is chief editor of Internet publishing, Harvard Health Publications. He is a clinical instructor of medicine at Harvard Medical School and Brigham and Women's Hospital. Dr. LeWine has been a primary care internist and teacher of internal medicine since 1978. /AetnaInteliHealth