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:
rlsharp@iastate.edu
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
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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
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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].
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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
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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].
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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.
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Received July 16, 2007
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