Role of Whole Foods in Promoting Hydration after Exercise in Humans
Transcription
Role of Whole Foods in Promoting Hydration after Exercise in Humans
Review Role of Whole Foods in Promoting Hydration after Exercise in Humans Rick L. Sharp, PhD Department of Health & Human Performance, Iowa State University, Ames, Iowa Key words: hydration, fluid balance, exercise, electrolytes, osmolality, carbohydrate, fluid retention Various reports indicate that humans receive 20 –25% of their daily water intake from food. Fruits, vegetables and other high-moisture foods, therefore, make an important contribution to total fluid intake. In addition, co-ingestion of other nutrients and ingredients can impact drinking behavior, absorption, distribution and retention of water, all of which contribute to the person’s hydration state. Therefore, a food’s hydration value derives from the interaction between its water content and the presence of these co-nutrients and ingredients. Research is reviewed in this paper showing increased voluntary fluid intake of young boys during exercise when the beverage is flavored and contains sodium chloride and carbohydrate. Additional research on rehydration after exercise and heat exposure showed improved recovery of plasma volume and fluid status when food was ingested before consuming water in the two hours after exercise. Collectively, these findings point to an interaction between fluid intake and co-ingested nutrients in regulating human hydration during and after exercise. Key teaching points: • Voluntary fluid intake during exercise is often inadequate to prevent dehydration, requiring correction of fluid deficits after exercise. • Flavoring, energy content, and ingredient profile of hydration beverages influences voluntary fluid intake and can help ensure adequate fluid intake. • In addition to providing water, foods contribute energy nutrients and ingredients that may assist in regulating the hydration process. • Results from several studies suggest that both carbohydrate content and presence of osmotically active ingredients (electrolytes) stimulate voluntary drinking and promote fluid retention. INTRODUCTION of the volume lost during the prior exercise session. Recommended volumes greater than those lost (⬎100%) are intended to compensate for obligatory water loss from renal filtration during the hours following exercise. Drinking water, beverages, and foods are sources of dietary water for rehydration. The presence of other nutrients and properties of these sources, however, may influence the rehydration rate and the ingested water’s fate. For example, the presence of sodium chloride in the rehydration may favor the refilling of extracellular fluid (ECF) preferentially. This paper will examine the interactive effects of water and co-ingested Water losses through sweating can rise as high as 3 L/hr during heavy physical activity in hot environments and unless adequate fluid is consumed, can lead to persistent hypohydration. Consequently, guidelines for fluid intake before, during, and after exercise have been produced. Typically, such guidelines specify either plain drinking water or some type of sports beverage containing carbohydrate and electrolytes. When the goal is to replace fluid losses after exercise is completed (rehydration), the recommended volume of fluid to be consumed is generally between 100 to 150% Address correspondence to: Rick L. Sharp, PhD, 250 Forker Building, Department of Health & Human Performance, Iowa State University, Ames, IA 50011. E-mail: [email protected] Presented at the ILSI North America 2006 Conference on Hydration and Health Promotion, November 29 –30, 2006 in Washington, DC. Acknowledgements: The research presented in reference [19] was performed with financial and product support to the author from Campbell Soup Company, Camden, NJ. Conflict of Interest Disclosure: There are no conflicts of interest to declare in connection to this work. Journal of the American College of Nutrition, Vol. 26, No. 5, 592S–596S (2007) Published by the American College of Nutrition 592S Rehydration after Exercise nutrients in affecting the availability of ingested water during and after physical activity. Table 2. Water Content (% of Food Weight) of Commonly Consumed Foods. Data Were Extracted from [4] High Water Content DIETARY SOURCES OF WATER Body water balance is maintained by matching daily water loss with dietary intake of water. Metabolic water production (250 –300 mL/day) also contributes to a small degree. The Food and Nutrition Board has established an adequate intake (AI) for total water intake of 3.7 L/day for adult men and 2.7 L/day for adult women [1]. Based on results from the Continuing Survey of Food Intakes by Individuals CSFII, Heller reports that US adults receive roughly 25% of their total daily water intake from foods containing water [2]. NHANES III results (Table 1) suggest US adults generally receive approximately 19% of total daily water intake from ingesting food and 35– 40% from drinking water (both tap and bottled) [3]. Disregarding the small amount of salts and other ingredients found in drinking water, this means that 60 to 65% of the average person’s daily water intake is co-ingested with other ingredients such as carbohydrate, salts, and caffeine that may influence absorption, distribution, and retention of the ingested water. Databases showing the water content of foods are readily available [4] as shown in the examples presented in Table 2. This table shows examples of foods containing a large fraction of water such as fresh fruits and vegetables like lettuce, oranges, and tomatoes; and foods not comprised of a large fraction of water such as nuts, meats, breads, and cheese. Depending on consumer choices and access to particular foods, there can be considerable variability in the quantity of total water intake accounted for by food ingestion. Table 1. Total Daily Water Intake from Various Sources for US Adult Males and Females. Data Extracted from NHANES III Database and Published in [3] Source of Water Total Watera Drinking Waterb Water ⫹ Beveragesc Water in Food Age Range (yr) Males Females 19–30 31–50 19–30 31–50 19–30 31–50 19–30 31–50 3908 3848 1389 1292 3176 3089 732 761 2838 3101 1156 1229 2321 2523 515 574 a Total water intake is sum of plain drinking water and the water content of all foods and beverages. b Drinking water includes both tap and bottled water. c Water ⫹ beverages is sum of drinking water and all other beverages including coffee, tea, alcoholic beverages, soft drinks, and other such beverages. JOURNAL OF THE AMERICAN COLLEGE OF NUTRITION Low Water Content Food Water Content Food Water Content Lettuce, iceberg Squash, cooked Pickle Cantaloupe Orange Apple Pear 96% 94% 92% 90% 87% 86% 84% Steak, cooked Cheese, cheddar Bread, white Cookies Walnuts Corn flakes Peanuts, roasted 50% 37% 36% 4% 4% 3% 2% NUTRIENT INTERACTIONS Water absorption, distribution in body water compartments, and retention of ingested water are affected by the presence of other nutrients and ingredients in the rehydration solution or food. For example, it is well known that sodium favors refilling of the ECF [5–7]. Carbohydrate accelerates water absorption in the small intestine by stimulating carrier-mediated water movement [8]. Caffeine has a diuretic effect that may influence the net retention of ingested water [6]. Therefore, comparing water content of foods without considering the presence of the other substances that may impact ingested water availability does not provide a complete picture of a particular food or beverage’s hydration value. In a practical sense, it may be better to choose foods that both contain high water content and provide other ingredients that can facilitate the hydration process. In research studies examining water needs associated with exercise in hot environments, evidence has accumulated to suggest intaking fluid before, during, and after exercise [9]. The purpose of pre-exercise fluid is to “top-off” body water stores to accommodate future sweat losses and the purpose of ingestion during exercise is to replace sweat losses as they occur to prevent development of a hypohydrated state late in exercise. In fact, this practice is well-known to preserve thermoregulatory sweating, blunt the rise in core temperature, and prolong performance [10 –12]. Ingestion of adequate and appropriate fluids after exercise is recommended to replace any body water deficits that may have accrued during the prior exercise. In some kinds of physical activity, it is important to restore fluid balance as rapidly as possible to correct hypohydration before the next session of physical activity occurs. For example, American football players often practice twice per day during the hottest months of the year (August and September), making refilling body water stores between the two daily practice sessions a priority. Because of these considerations, much research has been done to elucidate the dosage and composition of the ideal fluid replacement beverage. Commercial sport drinks are formulated with a combination of carbohydrate and electrolytes to provide energy, facilitate water absorption, replace electrolytes lost in 593S Rehydration after Exercise sweating, and enhance flavoring to increase voluntary drinking behavior. ROLE OF DRINK FLAVOR AND SODIUM CHLORIDE IN PROMOTING DRINKING Despite availability of adequate fluids, people generally do not drink enough to fully compensate for sweat losses [7,13]. Wilk and Bar-Or [14] examined the roles of carbohydrate, flavoring, and sodium chloride in stimulating increased drinking among adolescent boys performing endurance exercise in the heat. Twelve boys aged 9 to 12 yr performed 3 h intermittent exercise in 35°C environment. During the exercise, subjects were allowed free access to either water, grape-flavored water, or a grape-flavored beverage containing 6% carbohydrate and 18 mmol/L NaCl. By the end of exercise, the subjects had consumed the largest volume from a carbohydrate-electrolyte (CE) beverage, followed by flavored water, and least with unflavored water (p ⬍ 0.05). Whole body fluid balance at the end of exercise was, likewise, significantly different among the three treatments, with fluid balance slightly negative in the unflavored water trial (⫺0.7%) and slightly positive after the CE beverage trial (⫹0.5%). Urine production was not different among the trials, implying that water retention was improved by the presence of carbohydrate and sodium chloride in the CE trial. However, because no blood samples were obtained in this study, possible effects on plasma osmolality and fluid regulating hormones cannot be confirmed (Fig. 1). In a subsequent study, Rivera-Brown [15] re-examined the question of the effect of beverage composition on voluntary drinking in adolescent males during exercise in the heat. The subjects used in the study by Wilk and Bar-Or were not heat acclimatized, thus their sweat rates were relatively low during Fig. 1. Effect of flavoring, carbohydrate, and electrolytes on voluntary fluid intake during exercise in the heat. Adapted from Wilk and Bar-Or [14]. 594S the experimental trials which left open the possibility that flavored sports drinks may not encourage increased drinking and better body water maintenance in exercising populations who are acclimatized to heat and therefore, experience higher sweat rates during exercise. Twelve boys aged 11 to 14 yr participated in the study that involved 3 h intermittent exercise outdoors in a tropical environment. On one occasion, the boys were allowed ad lib plain water intake and in the other trial were provided with a grapeflavored carbohydrate-electrolyte beverage (6% CHO, 18 mmol/L NaCl). Similar to the earlier study by Wilk and Bar-Or, voluntary drinking was significantly increased by 32% in the CE trial from approximately 1450 ml in the water trial to approximately 1900 ml in the CE trial. During the water trial, the boys reached 1% hypohydration by the end of the experimental period. CE resulted in a body water balance not different from zero by the end of exercise, suggesting fluid intake matched the sweat rate and prevented a whole body water deficit from occurring. REPLACEMENT OF FLUID LOSSES AFTER EXERCISE: ROLE OF FOOD Sodium is well established as the major extracellular ion and was shown as early as 1973 to accelerate recovery of plasma volume after exercise in the heat [5]. Subsequent studies also showed improved retention of ingested fluids when sodium was added to the rehydration beverage [16]. Improved retention, therefore, may accelerate the recovery of fluid deficits after exercise in the heat. Although not widely studied, there is some evidence to suggest that food ingestion during rehydration can accelerate the recovery of body water stores. Sproles et al [17] demonstrated improved rehydration when a 425 g meal was consumed at the beginning of a 5 h rehydration period in which a carbohydrate-electrolyte beverage was ingested. A later study by Maughan et al [18] examined the role of a meal consumed with water and compared the rehydration effectiveness of this approach with ingestion of a carbohydrate-electrolyte drink during rehydration. In this study, eight participants dehydrated to ⫺2% with exercise. In the next 60 min after exercise, the subjects ingested either a glucose-electrolyte (6% CHO, 21 mmol/L sodium) or water plus a meal. The meal consisted of rice, beans, and beef providing 63 mmoles Na⫹ and 21 mmol K⫹. The meal’s water content was calculated and subtracted from water to be ingested so that total water intake would equal 150% of the water lost in the prior exercise. Subjects were given water in equal amounts every 15 min for 60 min after exercise. Rehydration was assessed by measuring urine volume and body weight recovery periodically for the next 5 h (Figs. 2, 3). During the 6 h of rehydration, subjects experienced reduced urine production measured at 1 h and 2 h when the meal was VOL. 26, NO. 5 Rehydration after Exercise Fig. 2. Cumulative urine volume after ingestion of either a carbohydrate-electrolyte beverage or a meal plus water. Adapted from Maughan et al [18]. Fig. 3. Net fluid balance after ingestion of either a carbohydrateelectrolyte beverage or a meal plus water. Adapted from Maughan et al [18]. consumed with water. Thereafter, urine volume was similar in all trials but cumulative urine volume over the 6 h of observation was significantly lower with the meal feeding than with CE. The fraction of ingested water that was retained was significantly higher in the meal-plus-water trial (67%) than in either of the CE trials (51% and 52%). Because of the large volume of fluid ingestion during the first hour after exerciseinduced dehydration, whole body fluid balance was positive in all three trials at the beginning of the 6 h observation period (approximately ⫹500 ml in all three trials). By the end of 6 h, net fluid balance was negative after both CE trials (⫺337 ml and ⫺373 ml) but was euhydrated after the meal-plus-water trial (⫺29 ml). The authors concluded that adding electrolytes to the rehydration beverage is not necessary if the electrolytes are provided in solid food consumed during the rehydration period. This study may not have examined the possible contribution of JOURNAL OF THE AMERICAN COLLEGE OF NUTRITION energy nutrients (carbohydrate, fat, and protein) in solid food, but based on the lack of any difference in plasma osmolality it is doubtful that these nutrients had an important role in promoting increased water retention in the meal-plus-water trial. This effect was likely due to the higher total sodium dose in the meal (63 mmol) than in the CE beverage (43 mmol). In a similar study conducted in this laboratory [19], thirty males (n ⫽ 15) and females (n ⫽ 15) rehydrated after 3% bw loss from exercise and heat exposure to determine the effect of soup ingestion (Chicken Noodle Soup®, Campbell Soup Company) on the rehydration process, particularly recovery of plasma volume. Each subject participated in four trials in which fluids were given in divided doses every 20 min for a 2 h rehydration period to match the prior fluid loss (body weight change). In one trial, only plain tap water was consumed while in the other two trials, 175 ml of either chicken broth or chicken noodle soup were consumed at the beginning and 20 minutes of rehydration. Water was ingested thereafter in divided doses so that the total volume consumed equaled that lost. In a fourth trial, a CE drink was given (175 ml at beginning and 20 minutes of rehydration) followed by water. Blood samples and body weight were taken every 20 minutes and urine volume was measured at the beginning and end of the rehydration period (Fig. 4). The dehydration protocol produced an average body weight loss of 2.5% and approximately 7 to 8% decline in plasma volume. By the end of the 2 h rehydration period, plasma volume was still depressed (p ⬍ 0.05) when water was used as the sole fluid (⫺5.5%) and when a CE drink was provided (⫺4.1%). When either chicken broth or chicken noodle soup were fed at the start of the rehydration period, final plasma volume was not significantly different from the euhydration value (⫺1.0% in both). That plasma volume restoration was accelerated by feeding either chicken broth or chicken noodle soup is not surprising given the sodium dose was 41 mmol in the chicken broth trial and 59 mmol Fig. 4. Plasma volume changes during rehydration after ingestion of either water only, carbohydrate-electrolyte (CE) beverage plus water, chicken broth (Broth) plus water, or chicken noodle soup (Soup) plus water. Adapted from Ray et al [19]. 595S Rehydration after Exercise in the chicken noodle soup trial as compared to zero with tap water and only 6 mmol with the CE beverage trial. It should be noted that the absence of any CE advantage when compared to plain water contrasts with earlier studies that showed more rapid and complete rehydration with CE than with water [5,6]. However, those earlier studies fed a much larger total quantity of CE beverage, thereby providing a higher total dose of sodium than was used in the Ray et al study. For example, Costill and Sparks [5] gave 2.87 L containing 22 mmol/L Na⫹ in divided doses over a 3 h recovery period for a total sodium of 63 mmol. Likewise, Gonzalez-Alonzo et al [6] gave subjects either 1.95 L or 1.82 L of a 20 mmol/L CE drink for a total sodium dose of 39 and 36 mmol, respectively. Therefore, the total sodium dose used in the earlier studies was similar to that given in the broth and soup trials of the Ray et al study. Further, it is likely that if CE were provided as the sole rehydration beverage during the 2 h rehydration period of the Ray et al study, plasma volume restoration would have been accelerated relative to water ingestion as it was in the Costill and Sparks and Gonzalez-Alonzo et al studies. CONCLUSION AND RECOMMENDATIONS The collection of studies reviewed in this paper suggest that the combination of flavor, carbohydrate, and electrolytes encourages voluntary drinking during exercise in the heat and this helps to prevent development of hypohydration, preserve thermoregulation, and likely improve performance. In rehydrating after acquiring a fluid deficit, electrolytes (particularly sodium) accelerate recovery of both plasma volume and total body water by encouraging fluid retention of kidneys and privileging the extracellular space for water distribution. Sodium can be supplied either as a solute in the rehydration beverage or ingested as a constituent of a meal along with water in the rehydration period. Potassium is also a cation commonly included in rehydration beverages and because it is mainly distributed in the intracellular space, it may favor refilling intracellular fluid during rehydration. More research is needed to determine whether the preferential rehydration of ECF is more important in restoring thermoregulation and performance than rehydration of the intracellular fluid. Lastly, because of the presence of additional nutrients and ingredients in foods, the value of a particular food in promoting hydration is likely the result of the interaction between its water content and the amounts of other constituents that affect the absorption, distribution, and retention of water. REFERENCES 1. Food and Nutrition Board of the Institute of Medicine: Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. Washington, DC: National Academies Press, 2004. 596S 2. Heller KE, Sohn W, Burt BA, Eklund SA: Water consumption in the United States in 1995–96 and implications for water fluoridation policy. J Publ Health Dent 59:3–11, 1999. 3. Food and Nutrition Board of the Institute of Medicine: Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. Washington, DC: National Academies Press, pp. D2–D9, 2004. 4. USDA Nutrient Database for Standard Reference, Release 15. Nutrient Data Laboratory Home Page. Washington, DC: US Department of Agriculture, Agricultural Research Service, 2002. Available from: www.nal.usda.gov/fnic/foodcomp, 5. Costill DL, Sparks KE: Rapid fluid replacement following thermal dehydration. J Appl Physiol 34:299–303, 1973. 6. Gonzalez-Alonzo J, Heaps CL, Coyle EF: Rehydration after exercise with common beverages and water. Int J Sports Med 13:399– 406, 1992. 7. Nose H, Mack GW, Shi XR, Nadel ER: Role of osmolality and plasma volume during rehydration in humans. J Appl Physiol 65:325–331, 1988. 8. Fordtran JS. Stimulation of active and passive sodium absorption of sugars in the human jejunum. J Clin Invest 55:728–737, 1975. 9. American College of Sports Medicine, Convertino VA, Armstrong LE, Coyle EF, Mack GW, Sawka MN, Senay LC Jr, Sherman WM: Position stand on exercise and fluid replacement. Med Sci Sports Exerc 28:i–vii, 1996. 10. Fortney SM, Nadel ER, Wenger CB, Bove JR: Effect of blood volume on sweating rate and body fluids in exercising humans. J Appl Physiol 51:1594–1600, 1981. 11. Greenleaf JE, Castle BL: Exercise temperature regulation in man during hypohydration and hyperhydration. J Appl Physiol 30:847– 853, 1971. 12. Sawka MN, Francesconi RP, Young AJ, Pandolf KB: Influence of hydration level and body fluids on exercise performance in the heat. JAMA 252:1165–1169, 1984. 13. Adolph EF: “Physiology of Man in the Desert.” New York: Interscience, 1947. 14. Wilk B, Bar-Or O: Effect of drink flavor and NaCl on voluntary drinking and hydration in boys exercising in the heat. J Appl Physiol 80:1112–1117, 1996. 15. Rivera-Brown AM, Gutierrez R, Gutierrez JC, Frontera WR, Bar-Or O: Drink composition, voluntary drinking, and fluid balance in exercising, trained, heat-acclimatized boys. J Appl Physiol 86:78–84, 1999. 16. Maughan RJ, Leiper JB: Sodium intake and post-exercise rehydration in man. Eur J Appl Physiol Occup Physiol 71:311–319, 1995. 17. Sproles CB, Smith DP, Byrd RJ, Allen E: Circulatory responses to submaximal exercise after dehydration and rehydration. J Sports Med Phys Fitness 16:98–105, 1976. 18. Maughan RJ, Leiper JB, Shirreffs SM: Restoration of fluid balance after exercise-induced dehydration: effects of food and fluid intake. Eur J Appl Physiol Occup Physiol 73:317–325, 1996. 19. Ray ML, Brown MW, Ruden TM, Baier SM, Sharp RL, King DS: Effect of sodium in a rehydration beverage when consumed as a fluid or meal. J Appl Physiol 85:1329–1336, 1998. Received July 16, 2007 VOL. 26, NO. 5