With warm weather on the way, fluid intake should be on the minds of most triathletes. How much fluid should you drink to prevent dehydration and optimize athletic performance? How much fluid should you drink to avoid dehydration while not putting yourself at risk of hyponatremia? These are the key questions that I aim to answer in the following discussion. To prevent confusion, I will indicate fluid volume in both ounces and milliliters, but to clarify, 30 mL is equal to 1 ounce.
Water is essential to the human body, 60-70% of our body mass is water, up to 90% of our brain mass is water, and up to 75% of muscle is comprised of water. Water is also the main component of blood the important carrier of glucose, oxygen and other nutrients. In general, your body loses 64-80 ounces of water daily through urine, feces, sweat, skin, and expired air. This water needs to be replaced by daily fluid consumption of 64-80 ounces. Another simple way to calculate daily fluid needs is to consume one milliliter of fluid per calorie eaten. However, a scientific formula based on body weight has been designed to calculate more individualized needs. For moderately active men and women, multiply body weight in pounds by 0.35 or 0.31 respectively to calculate fluid requirements in ounces per day. For example, for a 175 lb man: 175 x 0.35 = 61.25 ounces (a little over 7 cups of fluid daily,) or for a 125 lb woman: 125 x 0.31 = 38.75 ounces (almost 5 cups of fluid daily). Another, albeit much less scientific, way to determine daily fluid requirements is to evaluate your urine. Dark and concentrated urine is indicative of inadequate fluid intake. Urine should be pale yellow to clear, and copious.
Water is the best beverage choice for general daily hydration; however, endurance athletes with very high caloric requirements may substitute water for higher calorie juices, smoothies or sports drinks. Electrolyte containing beverages with 4% to 8% carbohydrate are recommended for events lasting longer than 1 hour. Though beverages with caffeine provide hydration, they are not the best choice as excessive caffeine consumption may interfere with sleep patterns and will have a mild diuretic effect.
Appropriate fluid intake before, during, and after exercise is an important predictor of athletic performance. Dehydration is linked with compromised exercise performance, while over- hydration is associated with the life-threatening electrolyte imbalance hyponatremia.
So, how much to drink?
The American College of Sports Medicine and National Athletic Trainers Association recommends that athletes should be well hydrated before beginning endurance exercise. The day before a long training session or race, aim to drink as much as 90 ounces of fluid along with a diet high in carbohydrate with at least 3 grams of sodium. Two to three hours before the event, drink 400 to 600 mL (13 to 20 ounces) and during the event aim to drink 150 to 350 mL (5 to 12 ounces) every 15 to 20 minutes (totaling 400-800 mL or 12-24 ounces per hour). Consuming more than 800 mL per hour will increase your risk for hyponatremia. Hyponatremia is the opposite of dehydration. It is a serious electrolyte disturbance resulting from water intoxication. These recommendations are general and suggest a very wide range for adequate fluid consumption. A good way to calculate your personal fluid requirements during exercise is to calculate sweat losses by weighing yourself unclothed, then exercising (put your clothes back on!) without consuming any fluids or food for one hour, and then to re-weigh after exercise without your sweaty clothes on. For every pound (16 ounces) lost, you need to drink 80% to 100% – for example, if you lose 1 pound in one hour you should aim to drink 13 to 16 ounces of fluid per hour (390-480 mL).
Post-exercise fluid intake is critical for proper recovery. Aim to consume up to 150% of weight lost during an exercise session, or 24 ounces per pound lost. Generally, athletes will lose 1-2% of their body weight – loss exceeding 2% is an indicator of dehydration. To calculate fluid replacement without weighing yourself, use the following example. A 135 lb woman will lose approximately 1.35 lbs (21.6 ounces) during exercise; she should aim to replace 100% to 150% of 21.6, or 24 ounces per 1.35 lbs = 21 to 32 ounces of fluid during recovery. The best recovery beverage is a combination of water, carbohydrate and electrolytes, such as sports drinks or juice to replace both lost fluid and glycogen.
Adequate fluid intake before, during, and after exercise will aid in peak athletic performance and recovery. With temperatures increasing as summer approaches, it is vital to consume adequate fluids to prevent dehydration. For specific hydration needs related to training level and type of event consult a Sports Nutritionist for tailored professional recommendations. In the next installment, we plan to discuss some more complicated issues surrounding hydration, including the effects of alcohol and caffeine intake, electrolytes and hyponatremia, and altitude.
American College of Sports Medicine. Position stand on exercise and fluid replacement. Med Sci Sports Exerc. 1996: 28; i-vii.
Clark N. Nancy Clark’s Sports Nutrition Guidebook 3rd Edition. Human Kinetics, Champaign, IL; 2003.
Latzka W, Mountian S. 1999. Water and electrolyte requirements for exercise. Clin Sports Med. Jul; 18 (3): 513-24.
Noakes D. 2002. IMMDA-AIMS Advisory statement on guidelines for fluid replacement during marathon running. NSA 17:1 (15-24.) Noakes D. 2003. Overconsumption of fluid by athletes. British Medical Journal. July (327): 113- 114.
Reynolds K, Water: Too much of a good thing. Water Conditioning and Purification. August, 2005; 34-35. Ryan M. Sports Nutrition for Endurance Athletes. Velo Press, Boulder, CO; 2002.