by Thomas Incledon, PhD(c), RD, LD/LN, RPT, NSCA-CPT, CSCS
The Hydration Hype
The advertisements on television showing the sweaty athletes drinking the beverage of their choice always leaves questions in our minds about what the best drink is, how much to drink, when to drink, and if any of the commercial drinks are based on science or just marketing. There is plenty of scientific research regarding fluid replacement for survival in extreme temperatures, for prevention of heat-related illnesses, and for improved sports performance. In this article I’ll explain why the body needs fluid, how to determine your fluid loss during exercise, and what to look for in a fluid replacement. So sit back, grab a beverage of your choice, and get hydrated!
Water, Water, Everywhere
Our total body weight averages about 60% water, or 42 liters [1]. This water is divided into two major compartments in the body. The extracellular fluid, about 14 liters, is the fluid outside the cells and mostly consists of plasma, the liquid portion of the blood. The intracellular fluid, about 28 liters, is located inside the cells. Blood itself contains extracellular fluid from the plasma and intracellular fluid from the red blood cells. When our bodies try to cool down during exercise, sweat is produced (for a complete explanation of thermoregulation in the body. Approximately 10% of the water in sweat comes from the plasma, and if this loss increases, it can be detrimental to circulation, nutrient delivery, work capacity, and thermoregulation [2]. Fluid replacement maintains blood volume for circulation and this is also necessary to continue sweating in order to dissipate heat [3]. So you can see how important water is to our entire body. In addition to the water lost in sweat, the electrolyte sodium, which stimulates the thirst mechanism and helps retain urine, is also lost. But that’s not all. During endurance exercises lasting more than one hour and even during intermittent, stop-and-go exercises, our bodies use and deplete carbohydrates [4]. Carbohydrates are not only a source of energy, but also a source of water.
Why Don’t We Drink To Our Health?
Our bodies maintain normal hydration on a daily basis by drinking when our thirst stimulus is activated. However, in some physiological and psychological stress conditions, quenching our thirst mechanism isn’t enough to keep us fully hydrated. This is called involuntary dehydration [5]. It is because of this syndrome that adding sodium chloride to fluid replacement drinks has been advocated. The addition of salt will stimulate the thirst mechanism to encourage more drinking, improve the taste of a fluid replacement drink, allow the body to retain the water it already has, and replace what was lost in sweat. This becomes especially important if a well-balanced diet is not followed. Although the losses of electrolytes from sweat are minimal, they can be fully replaced in a normal meal, and therefore, adding sodium chloride to a drink is not necessary [6].
How Much Fluid Do You Lose?
It is well accepted that exercise and heat stress produce high rates of fluid loss and these can be as much as 2 liters per hour. The research on fluid replacement states that you should replenish exactly what you lose, and maybe even up to 150% more [7]. But fluid amounts will vary from person to person and even from day to day, so it’s important to determine just how much you are losing. Weighing yourself daily before exercise and then after exercise is a very practical way to determine your fluid loss. The rule of thumb is “ounce for ounce.” For every ounce of weight lost, drink an equal amount of fluid. Watch your fluid loss carefully, especially if you are exercising in high heat and high humidity with high intensity. If your body weight loss exceeds 4% of your original weight, you should reduce the intensity and duration of your exercise [8]
The Perfect Fluid Replacement
So the debate is whether to be like Mike and drink a sports beverage, or just stick with Mother Nature’s recipe and chug plain old water. There are definitely benefits to both schools of thought. Water is infinitely less expensive and readily available. In terms of thermoregulation and survival, water will not let you down. On the other hand, the American College of Sports Medicine (ACSM) recommends adding carbohydrates (30-60 g/hour in a concentration range of 4-8%) to fluid for exercise that lasts longer than one hour, since the correct proportions will not harm water delivery to the body and may enhance performance [6]. They also maintain that the addition of sodium (.5-.7 g/L of water) will enhance the taste of the drink and increase fluid retention. The commercial sport beverages try to compromise on ratios between what can give the best fluid replacement and still provide the most carbohydrates [8]. Many of the sport beverages on the market contain the appropriate ratios, but you should always read the labels. Ingesting too high of a carbohydrate concentration will delay water entering your system and may lead to dehydration. Eating nutritionally balanced meals and drinking fluid throughout the day cannot be emphasized enough. You should always prepare your body with adequate nutrients and fluids prior to exercise, and continue to consume fluid throughout exercise, so your body is never working at a deficit. Try to drink about 17 ounces of fluid 2 hours before exercise and drink on a schedule at regular intervals because the thirst mechanism isn’t very reliable for adequate hydration. But don’t go overboard with the water, either. Studies have shown that maintaining a hyperhydration state, either by ingesting glycerol or abundant amounts of water, isn’t any more beneficial for thermoregulation or performance than maintaining euhydration, or a normal fluid balance [9]. Don’t forget, though, that even a small amount of dehydration can increase cardiovascular strain and make it difficult for your body to dissipate heat. This affects not only performance issues, but life and death issues as well.
References
1. uyton, A.C. and J.E. Hall, Textbook of Medical Physiology. 1996. Ninth Edition: p. 297-313.
2. Shirreffs, S.M., Effects of ingestion of carbohydrate-electrolyte solutions on exercise performance. Int J Sports Med, 1998. 19(Supplement): p. S117-S120.
3. Cyle, E.F. and S.J. Montain, Thermal and cardiovascular responses to fluid replacement during exercise, in Perspectives in exercise science and sports medicine: exercise, heat, and thermoregulation, C.V. Gisolfi, D.R. Lamb, and E.R. Nadel, Editors. 1993, Brown & Benchmark. p. 179-223.
4. Shi, X. and C.V. Gisolfi, Fluid and carbohydrate replacement during intermittent exercise. Sports Med, 1998. 25(3): p. 157-172.
5. Greenleaf, J.E., Problem: thirst, drinking behavior, and involuntary dehydration. Med Sci Sports Exerc, 1992. 24(6): p. 645-656.
6. Convertino, V.A., et al., American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc, 1996. 28(1): p. i-vii.
7. Shirreffs, S.M., et al., Post-exercise rehydration in man: effects of volume consumed and drink sodium content. Med Sci Sports Exerc, 1996. 28(10): p. 1260-1271.
9. Sawka, M.N. and E.F. Coyle, Influence of body water and blood volume on thermoregulation and exercise performance in the heat, in Exercise and Sports Sciences Reviews, J.O. Holloszy, Editor. 1999, Lippincott Williams &Wilkins. p. 167-218.
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