Vol. 85, Issue 5, 1941-1948, November 1998
Effect of beverage osmolality on intestinal fluid absorption
during exercise
C. V.
Gisolfi,
R. W.
Summers,
G. P.
Lambert, and
T.
Xia
Departments of Exercise Science and Internal Medicine,
University of Iowa, Iowa City, Iowa 52242-1111
 |
ABSTRACT |
To determine how
osmolality of an orally ingested fluid-replacement beverage would alter
intestinal fluid absorption from the duodenum and/or jejunum
during 85 min of cycle exercise (63.3 ± 0.9% peak
O2 uptake) in a cool environment
(22°C), seven subjects (5 men, 2 women, peak
O2 uptake = 54.5 ± 3.8 ml · kg
1 · min1) participated in
four experiments separated by 1 wk in which they ingested a water
placebo (WP) or one of three 6% carbohydrate (CHO) beverages
formulated to give mean osmolalities of 197, 295, or 414 mosmol/kgH2O. CHO solutions also
contained 17-18 meq Na+ and
3.2 meq K+. Nasogastric and
multilumen tubes were fluoroscopically positioned in the gastric antrum
and duodenojejunum, respectively. Subjects ingested a total of 23 ml/kg
body mass of the test solution, 20% (370 ± 9 ml) of this volume 5 min before exercise and 10% (185 ± 4 ml) every 10 min thereafter.
By using the rate of gastric emptying as the rate of intestinal
perfusion (G. P. Lambert, R. T. Chang, D. Joensen, X. Shi, R. W. Summers, H. P. Schedl, and C. V. Gisolfi. Int. J. Sports Med. 17: 48-55, 1996), intestinal absorption was determined by segmental perfusion from the duodenum (0-25 cm) and jejunum (25-50 cm). There were no differences
(P > 0.05) in gastric emptying (mean
18.1 ± 1.3 ml/min) or total fluid absorption (802 ± 109, 650 ± 52, 674 ± 62, and 633 ± 74 ml · 50 cm1 · h1
for WP, hypo-, iso-, and hypertonic solutions, respectively) among
beverages; but WP was absorbed faster
(P < 0.05) from the duodenum than in
the jejunum. Of the total volume of fluid ingested, 82 ± 14, 74 ± 6, 76 ± 5, and 68 ± 7% were absorbed for
WP, hypo-, iso-, and hypertonic beverages, respectively. There were no
differences in urine production or percent change in plasma volume
among solutions. We conclude that total fluid absorption of 6%
CHO-electrolyte beverages from the duodenojejunum during exercise,
within the osmotic range studied, is not different from WP.
water absorption; duodenum; jejunum; human
| |
INTRODUCTION |
THE ROLE OF SOLUTION osmolality in intestinal fluid
absorption remains somewhat controversial. Discrepancies in the data
can be attributable in part to species differences, the segment of the
intestine studied, the composition of the solution employed, and
extrapolation of the results from direct perfusion into the intestine
to oral ingestion of the same solution (23, 24). In an attempt to
resolve many of these issues, we recently designed a technique to
evaluate both gastric emptying and intestinal absorption after the oral
ingestion of different rehydration beverages (13). In our most recent
investigation using this technique (14), we evaluated fluid and solute
absorption from the first 75 cm of the proximal small intestine. We
assumed that the distance from the pyloric sphincter to the ligament of
Treitz (0-25 cm) represented the duodenum and that the next 50 cm
beyond the ligament of Treitz represented the jejunum (25-50 and
50-75 cm). Intestinal fluid was sampled just (3-5 cm) beyond
the pyloric sphincter and at 25, 50, and 75 cm beyond this site. The
results of this study revealed significant differences in water flux in
the duodenum and the first 25 cm of the jejunum after the oral
ingestion of a water placebo and an isotonic carbohydrate-electrolyte
(CHO-E) beverage. There were no differences between these two beverages in the last 25-cm segment (i.e., 50-75 cm). Moreover, when total water flux for the first 50 cm or the entire 75 cm was analyzed, there
were no significant differences between the two beverages studied.
In an early investigation (23) to evaluate the effects of osmolality on
intestinal fluid absorption, we studied three solutions with the same
CHO and electrolyte content, but, by manipulating the form of CHO,
solution osmolality varied from 186 to 403 mosmol/kgH2O. This study was
performed at rest with a multilumen tube positioned so that the test
segment included the last 10-15 cm of duodenum and the first
25-30 cm of jejunum. The results showed no significant difference
in fluid homeostasis among beverages as evaluated by changes in plasma
volume. However, if fluid movement across the 10-cm mixing segment of
the multilumen tube was also included in the analysis, fluid absorption
of the hypotonic solution was 17% greater than of the hypertonic
solution and produced a more rapid increase in plasma volume. These
experiments, conducted at rest, implied that a hypotonic solution would
have a marked advantage over a hypertonic solution during exercise,
when plasma osmolality rises.
Thus the purpose of this study was to reevaluate these same solutions
and water after their oral ingestion during cycle exercise at 65% peak
oxygen uptake
(
O2 peak) and
to simultaneously measure gastric emptying and intestinal absorption
from the duodenum and the first 25 cm of the jejunum independently.
This design focuses on fluid replenishment during prolonged exercise
and addresses the role of the duodenum independent of the jejunum in
fluid absorption. It addresses the following three questions.
1) How important is the intestinal
segment studied? 2) How much of an
orally ingested beverage can be absorbed in the first 50 cm of the
intestine? 3) What is the influence
of beverage osmolality on intestinal absorption from these two
different intestinal sites during exercise? We hypothesized that the
hypotonic beverage would yield the greatest fluid absorption on the
basis of its osmotic advantage in the duodenum without sacrificing any
absorption decrement in the jejunum because it contained an equal
amount of CHO and electrolyte to the other beverages studied.
| |
METHODS |
Subjects and infusion solutions.
Five men and two women (age = 28 ± 4 yr; height = 181 ± 1 cm;
body mass = 80 ± 4 kg;
O2 peak = 54.5 ± 3.8 ml · kg1 · min1)
served as subjects in this study. They all received a thorough physical
examination and provided signed, informed consent before participating.
All experiments were performed in accordance with the guidelines for
the use of human subjects in research and were approved by the local
Human Use Committee.
O2 peak was
determined by using a graded protocol on a electronically braked cycle
ergometer (The Bike, Cybex, Ronkonkoma, NY). A workload corresponding
to 60-65%
O2 peak was
determined and served as the exercise intensity for the experimental trials.
Each subject ingested a water placebo and three 6% CHO-E solutions
containing two or three forms of CHO (Table
1). All CHO solutions contained glucose and
fructose as transportable substrates but in different forms to yield
different osmolalities. The hypotonic solution contained fructose as
sucrose and glucose in the free form and as maltodextrin in the
combined form. The isotonic beverage contained fructose as sucrose and
glucose in the free form. The hypertonic solution contained fructose
and glucose as free monosaccharides, requiring no digestion before
transport. Each of the CHO solutions also contained 17-18 meq
Na+ and 3 meq
K+. All beverages also contained 1 mg/ml polyethylene glycol 3350 (PEG), a nonabsorbable marker for
determination of water flux. Experiments were performed 1 wk apart
after an 8-h fast.
Measurement of gastric emptying and intestinal absorption.
The technique employed to simultaneously determine gastric emptying and
intestinal absorption has been described in detail (13, 14). Briefly, a
nasogastric (NG) tube (50 in., no. 14 Fr, Levin) is attached to a
multilumen tube (Arndorfer, Greendale, WI) with a small rubber band and
positioned fluoroscopically, so that the NG tube is located in the
gastric antrum and the first sampling port of the multilumen tube is
located 2-3 cm past the pyloric sphincter. Gastric emptying was
determined by using a modified, repeated double-sampling technique (13)
in accordance with the methods of George (5) and Beckers et al. (1).
The multilumen tube utilized in this study had additional sampling ports located 25 cm and 50 cm from the initial port, just beyond the
pyloric sphincter. Thus net fluid and solute fluxes were determined from the duodenum (0- to 25-cm segment) and from the first portion of
the jejunum (25- to 50-cm segment).
During each 10-min interval of the experiment, intestinal fluid was
collected at a rate of 1 ml/min from the proximal and 25-cm sampling
sites and by constant syphonage at the 50-cm sampling site. Net water
flux was calculated for each interval according to the following
equations (2)
where
GER is the gastric emptying rate;
E is the flow
rate entering a given segment (ml/min);
L is the flow
rate leaving a given segment (ml/min);
N is the net
water movement across the wall of the segment of intestine studied
(ml/min); and
p is the total
sampling rate from the proximal collecting sites of the two segments
studied. [PEG]s,
[PEG]p, and
[PEG]d are the concentrations of the nonabsorbable marker in the stomach, at the
proximal site of each segment, and at the distal site of each segment,
respectively. Net water flux from the 25- to 50-cm segment was
calculated by subtraction after determination of net flux in the 0- to
25- and 0- to 50-cm segments, respectively. Solute flux was calculated
by multiplying the solute concentration at the proximal and distal
sampling sites (of the 0- to 25- and 0- to 50-cm segments) by the flow
rates entering and leaving the segments. Net movement of solute was
determined by subtraction (2). Solute flux in the 25- to 50-cm segment
was calculated as described for water flux. Fluxes of
Na+ and
K+ were doubled to account for
movement of the accompanying anion. All results were calculated after a
35-min equilibration period to allow a steady state to be reached (2,
25). Steady-state PEG values from the 50-cm sampling site for each
solution are shown in Fig. 1. Samples were
collected during the equilibration period but were not used in data
analysis. Analytical recovery of PEG averaged 82 ± 1% from
solutions containing 1 mg/ml PEG.
View larger version (20K):
[in this window]
[in a new window]
|
Fig. 1.
Polyethylene glycol 3350 (PEG) concentrations from 50-cm sampling site
after 35-min equilibration period. There were no differences over time
for any solution. Values are means ± SE;
n = 5 subjects for water placebo.
|
|
Protocol.
After the NG and multilumen tubes were placed and a venous catheter was
inserted in a superficial arm vein, the subject sat for 20 min to
stabilize plasma volume. At the end of this period, blood and urine
samples were collected, rectal temperature was obtained with a clinical
thermometer, nude body weight was obtained, and the subject donned his
or her cycling clothes. The subject then mounted the stationary bike, a
heart rate monitor (Polar Vantage XL, Polar USAF, Stamford, CT) was
attached, and stomach contents were aspirated through the NG tube.
After the stomach was aspirated, the subject drank an initial bolus of
test solution equal to 20% of the total volume ingested (23 ml/kg body
mass). Mean total volume ingested was 1,850 ± 89 ml, and the
initial bolus volume averaged 370 ± 18 ml. Five minutes after ingestion of the initial bolus, exercise commenced for 85 min. At
10-min intervals thereafter, additional volumes of test solution were
ingested equal to 10% (185 ± 9 ml) of the total volume consumed.
Experiments were performed at 22°C with a wind velocity of ~0.6
m/s produced by a fan placed in front of the subject. Blood was sampled
every 20 min to determine changes in plasma volume, osmolality,
Na+,
K+, and glucose. Heart rate was
monitored every 15 min. Rectal temperature, body mass, and urine volume
were recorded postexercise. Sweat rate was calculated from the change
in nude body mass corrected for fluid ingestion, phenol red injection,
and stomach, intestinal, urine, and blood sample volumes.
Analytic procedures.
Phenol red concentration in the stomach samples was measured
spectrophotometrically at 560 nm after dilution (0.3-ml sample to 5 ml
deionized water) and alkalinization with 1 ml borate buffer (pH 9.2)
(5, 22). All samples and standards were analyzed in duplicate with
deionized water as a reference blank. PEG in the intestinal samples was
determined by the method of Hyden (11) as modified by Malawar and
Powell (15). Osmolality was measured by using freezing-point depression
(Multi-Osmette, Precision Systems, Natick, MA),
Na+ and
K+ concentrations by flame
photometry (model IL 943, Instrumentation Laboratory, Lexington MA),
and CHO (i.e., glucose and fructose) by high-performance liquid
chromatography (Dionex DX-500 System, Sunnyvale, CA). Samples
containing sucrose were hydrolyzed with 8.75 N trifluoracetic acid to
liberate glucose and fructose before measurement, and samples with
maltodextrin were hydrolyzed with 
-amylase and amyloglucosidase.
This provided a more accurate determination of CHO flux in the
intestine as flux values of sucrose and maltodextrin would not
necessarily represent movement of the solute from the lumen but
possibly only digestion to monosaccharides (i.e., glucose and fructose)
with subsequent absorption. Hemoglobin and hematocrit were determined
in quadruplicate by using the cyanmethemoglobin and microcentrifugation
methods, respectively. Percent change in plasma volume was calculated
from hemoglobin and hematocrit values by using the formulas of Dill and
Costill (4).
Statistical analysis.
Data were tested for normality with the Shapiro-Wilk test and were
found to be normally distributed, with the exception of the
Na+ and
K+ flux data, in which appropriate
transformations were made to normalize the data. Repeated-measures
analysis using the SAS/STAT MIXED procedure (21) was utilized for all
comparisons, except for comparing PEG concentrations within a solution
over time, in which a one-way analysis of variance with repeated
measures was used. Pairwise comparisons of values between solutions and within solutions at different gastrointestinal sites or time points was
performed by using Bonferroni's method. The level of significance was
set at P < 0.05. All data are
reported as means ± SE.
| |
RESULTS |
Eight subjects began the experiments, but one vomited during the first
trial and opted not to continue the study. This person had successfully
completed numerous other studies in the past. Another subject was ill
during the water placebo experiment and did not complete it but
finished all other trials without gastrointestinal distress. There were
no differences in thermoregulatory response, heart rate, urine
production, fluid absorbed in the whole 50-cm test segment, or fluid
retained (from that absorbed in the 50-cm test segment) during the
different trials (Table 2). If water absorption values obtained after equilibration are extrapolated to
represent the entire 90-min experiment, 75 ± 4% of the fluid presented (ingested minus aspirated) to the intestine (1,396 ± 102 ml) was absorbed in the first 50 cm of the intestine. This absorbed
volume (1,021 ± 54 ml) accounts for 73% of the total volume of
sweat produced during the 90-min experiment.
Gastric emptying.
Figure 2 shows the complete gastric
emptying curves for the different beverages. Ingesting water, the
isotonic beverage, or the hypotonic beverage resulted in rapid gastric
emptying and equilibration at an average gastric volume of ~340 ml.
Thus, at an average gastric emptying rate of 17 ml/min or 170 ml every 10 min, subjects basically kept pace with average fluid ingestion of
185 ml every 10 min. In contrast, the hypertonic beverage delayed gastric emptying and resulted in a significantly higher gastric volume
compared with the other three beverages. However, after equilibration,
average gastric emptying of 19 ml/min or 190 ml every 10 min basically
kept pace with ingesting 185 ml every 10 min, and gastric volume
equilibrated at ~575 ml. Thus the higher gastric volume compensated
for the higher osmolality of this beverage to yield approximately the
same gastric emptying rate.
View larger version (29K):
[in this window]
[in a new window]
|
Fig. 2.
Complete gastric emptying curves for 4 beverages studied. GER (ml/min),
mean gastric emptying rate after 35-min equilibration period; solid
lines, gastric emptying over each 10-min period; dashed lines, fluid
ingested every 10 min. Beverages were ingested in ~30 s. Hypertonic
beverage was maintained at a higher (P < 0.05) gastric volume throughout experiments. Values are means (SEs
are deleted for clarity); n = 5 subjects for water placebo. GER values were used as perfusion rates for
measurement of water and solute fluxes in intestine.
|
|
After the 35-min equilibration period, mean gastric volume measured
before fluid ingestion every 10 min was significantly greater during
ingestion of the hypertonic beverage than of all other beverages (mean
values = 507 ± 84, 266 ± 49, 231 ± 18, and 264 ± 42 ml for hypertonic, isotonic, hypotonic, and WP, respectively), but mean gastric emptying rate was not significantly different among
trials (see values in Fig. 2). There were no differences (P > 0.05) in gastric volume (Fig.
2) or gastric emptying for any beverage over time after the
equilibration period .
Osmolality.
Osmolalities among the different solutions were formulated to be
significantly different (Table 1) and remained so in the stomach and in the first 25 cm of the duodenum (Fig.
3, Table 3).
With regard to osmolality in the 25- to 50-cm segment (jejunum), WP was
significantly (P < 0.05) lower than
that of the hyper-, iso-, and hypotonic beverages, the hypertonic
beverage was significantly greater than the isotonic beverage, and the
isotonic beverage was significantly greater than the hypotonic
beverage. The change in osmolality from the duodenum to the jejunum is
attributable to increased electrolyte concentration (Table 3, Fig.
4), CHO digestion, and net water flux (Fig.
5). In the case of the hypertonic beverage, the decrease in osmolality from the stomach to the jejunum is
attributable to greater total solute flux than net water flux. The rise
in osmolality of the hypotonic beverage from the stomach to the
duodenum is attributable to maltodextrin digestion,
Na+ secretion, and greater water
than solute absorption. Osmolality of the isotonic beverage did not
change from stomach to duodenum, but, in the jejunum, it rose
significantly above values observed in the stomach and duodenum but
only by 10 mosmol/kgH2O. The
significant rise in osmolality of the WP from stomach to duodenum is
attributable to Na+ secretion and
greater water absorption than net solute flux.
View larger version (22K):
[in this window]
[in a new window]
|
Fig. 3.
Mean osmolality in stomach and intestinal segments. Solutions were all
significantly different from each other before ingestion, in the
stomach, in 0- to 25-cm segment, and in 25- to 50-cm segment except
hypertonic vs. isotonic and isotonic vs. hypotonic solution in 25- to
50-cm segment. Values are means ± SE;
n = 5 subjects for water placebo.
 Significantly different from solution.
# Significantly different
from stomach. * Significantly different from 0-25 cm
segment. All P < 0.05.
|
|
View larger version (22K):
[in this window]
[in a new window]
|
Fig. 4.
Mean sodium concentrations in stomach and intestinal segments studied.
all, All solutions having the symbols noted at that site. Hypertonic,
isotonic, and hypotonic solutions were not different from each other
before ingestion or in the stomach but were all significantly different
from water placebo at these points. In 0- to 25-cm segment, there were
no differences. In 25- to 50-cm segment, water placebo was
significantly different from other solutions. Values are means ± SE; n = 5 subjects for water placebo.
Significantly different from solution.
# Significantly different
from stomach. * Significantly different from 0- to 25-cm segment.
All P < 0.05.
|
|
View larger version (31K):
[in this window]
[in a new window]
|
Fig. 5.
Intestinal water absorption in 2 segments studied. Values are means ± SE; n = 5 subjects for water
placebo. * Significantly different from water placebo.
Significantly different from 0- to 25-cm segment. All
P < 0.05.
|
|
Water and solute fluxes.
Within the duodenum, fluid absorption during ingestion of the WP was
significantly greater than absorption during ingestion of the CHO-E
beverages. Although not statistically significant, the data were
reversed in the jejunum, i.e., fluid absorption was significantly
greater from all three CHO-E beverages compared with the WP. There were
no differences in fluid absorption among the CHO-E beverages in either
intestinal segment, but net fluid absorption of the WP significantly
fell in the jejunum compared with the duodenum (Fig. 5). When total
fluid absorption from both segments was combined, there were no
differences among beverages (802 ± 109, 650 ± 52, 674 ± 62, and 633 ± 74 ml · 50 cm1 · h1
for WP, hypo-, iso-, and hypertonic solutions, respectively).
In the duodenum, total solute flux for the CHO-E beverages was greater
(P < 0.05) than with ingestion of
the WP beverage (Fig. 6). As each of the
CHO-E beverages moved from the duodenum to the jejunum, total solute
flux during ingestion of the hypertonic and isotonic beverages fell
significantly, whereas there was no difference in total solute flux
from the hypotonic beverage (Fig. 6). The latter observation was
associated with a significantly lower CHO flux from the hypotonic
beverage compared with the hyper- and isotonic beverages (Table
4). Absolute concentrations of solutes at
the various sampling sites are shown in Table 3.
View larger version (32K):
[in this window]
[in a new window]
|
Fig. 6.
Total solute flux in 2 segments studied. Values are means ± SE;
n = 5 subjects for water placebo.
* Significantly different from water placebo.
Significantly different from 0- to 25-cm segment. All
P < 0.05.
|
|
Plasma volume, electrolytes, and osmolality.
In support of the observation that there were no differences in total
fluid absorption within the first 50 cm of the small intestine among
beverages, there were no differences in plasma volume over time among
beverages (Fig. 7). Moreover, there were no
differences (P > 0.05) in plasma
Na+ or
K+ concentrations among beverages,
but K+ concentration increased
significantly above rest after 40 min of exercise. Plasma osmolality
before exercise for all trials averaged 285 ± 2.08 mosmol/kgH2O, indicating the
subjects were euhydrated before each experiment, and rose significantly
to an average value of 291 ± 0.4 mosmol/kgH2O during exercise.
There were no differences (P > 0.05)
among beverages at any time point. Plasma glucose concentration was not
significantly different among trials but declined during ingestion of
the WP.
View larger version (23K):
[in this window]
[in a new window]
|
Fig. 7.
Percent change in plasma volume during each experiment. Values are
means ± SE; n = 5 subjects for
water placebo.
|
|
| |
DISCUSSION |
On the basis of previous studies, we hypothesized that the oral
ingestion of a hypotonic beverage would increase overall intestinal absorption and plasma volume during exercise. However, the results of
this study indicate that oral ingestion of a beverage in the osmolality
range of virtually 0 (WP) to 414 mosmol/kgH2O (Table 1) does not
significantly affect overall intestinal absorption or fluid homeostasis
during 85 min of exercise at 60-65% maximal O2 uptake. This conclusion is in
contrast with intestinal perfusion studies, primarily of the jejunum
conducted at rest (9, 10), but is consistent with other studies that
included fluid absorption from both the duodenum and jejunum (19, 23).
Gastric emptying.
The effect of osmolality on gastric emptying has been reviewed (3, 12,
17). The results of the present study are consistent with recent
reports that osmolality has only a modest effect on gastric emptying
(14, 16, 18, 19). We did not observe a statistically significant effect
of osmolality on gastric emptying when comparing the WP (osmolality
virtually 0 mosmol/kgH2O) with the
three 6% CHO beverages ranging in osmolality from 197 ± 2 to 414 ± 2 mosmol/kgH2O. The lack of
a difference in gastric emptying between water and the 6% CHO
beverages supports the findings of Mitchell et al. (16) and Costill
(3), which showed that the gastric emptying of CHO beverages ranging in
concentrations from 5 to 10% were not different from that of water.
The significantly larger gastric volume of the hypertonic beverage must
have compensated for its high osmolality because it produced a similar
gastric emptying rate compared with the other beverages.
Water and solute absorption.
The significant differences observed in water and solute absorption in
the duodenum (0- to 25-cm segment) and jejunum (25- to 50-cm segment)
are attributable to differences in beverage composition, beverage
osmolality, and the "leakiness" of the mucosal membrane in the
segments studied. In the duodenum (0- to 25-cm segment), significantly
more of the WP was absorbed than any of the CHO-E beverages primarily
because of the osmotic superiority of the WP beverage. Despite net
Na+ secretion (Table 4), net water
absorption occurred presumably because water was moving down an osmotic
gradient. Santangelo and Krejs (20) also observed net water absorption
in the presence of Na+ secretion.
The osmotic gradient that drives water absorption is thought to be the
gradient from intestinal lumen to villus tip (6, 7). When the
small intestine of an anesthetized cat was perfused with water, the
villus tips became hypotonic, but the osmotic gradient from lumen to
villus tip still promoted water absorption (7). If fluid absorption
occurs along the entire length of the villus, the osmotic gradient at
the villus base could be as high as 187 mosmol/kgH2O because the villus
base has an osmolality equal to that of the plasma (6). The high rate
of water absorption observed in the duodenum, which was also found in
two other reports (see Ref. 14), was no doubt facilitated by the
leakiness of the duodenum compared with the jejunum and more distal
portions of the small intestine.
In the duodenum, mean osmolality values for the CHO-E beverages were
all significantly different from each other (333 ± 7, 266 ± 3, and 237 ± 4 mosmol/kgH2O, for
hyper-, iso-, and hypotonic beverages, respectively), but there were no
significant differences in water flux among these beverages. In terms
of osmolality, all of these solutions would be expected to generate an
osmotic gradient in the villus that would promote water absorption.
Hallback et al. (6) found that, when the lumen of human intestine was
exposed to an isotonic glucose-electrolyte solution, an osmotic
gradient along the length of the villus was revealed with an osmolality of ~700 mosmol/kgH2O at the tip
and an osmolality that equated with plasma osmolality at the base. This
observation helps to explain fluid movement in the present study down
an osmotic gradient from luminal osmolality values that ranged from 300 to 350 mosmol/kgH2O for the
different 6% CHO-E solutions.
In addition to osmotic forces, there were significant differences in
total solute flux among these beverages (Fig. 6), which may have been
the more important force driving water movement. Another factor that
could have attenuated net water absorption of the hypotonic CHO
beverage was digestion of the maltodextrins. Although not significantly
different, total solute flux for the hypotonic beverage was only 3.5 ± 0.7 mmol · cm1 · h1 compared with 6.9 ± 1.1 and 6.9 ± 1.3 mmol · cm1 · h1 for the hyper- and
isotonic beverages, respectively. This difference is attributable in
part to the larger amount of a second transportable CHO (fructose) in
the hyper- (2.75% fructose) and isotonic (2% fructose) beverages
compared with the hypotonic (only 1% fructose) beverage (24). The
greater osmotic gradient for the hypotonic beverage may have
compensated for the greater digestion and lower total solute flux of
this beverage compared with the other CHO-E beverages to produce a
nonsignificant difference in net water flux among CHO-E beverages. In
this segment, water absorption from the WP was 26.7 ± 3.7 ml · cm1 · h1.
In the jejunum (25- to 50-cm segment), the differences observed in
water and solute flux are attributable to differences in beverage
composition, osmotic forces, membrane leakiness, and net fluid
transport that occurred in the duodenum. As observed in a recent
preliminary report (14), the present study shows that fluid absorption
during ingestion of the WP fell significantly from a mean value of 26.7 to 3.7 ml · cm1 · h1
in passage from the duodenum to the jejunum and was also lower than
values observed for all three CHO-E beverages. The marked reduction in
net fluid flux from duodenum to jejunum is attributable first to a
marked increase in luminal osmolality from 100 ± 12 to 175 ± 13 mosmol/kgH2O, when this beverage
passed from the duodenum into the jejunum, thus decreasing the osmotic
gradient driving absorption. In addition, the jejunum is a more
resistant membrane than the duodenum; and the volume flow of fluid
entering the jejunum from the duodenum must have been reduced because
53% of the WP ingested was absorbed in the duodenum. Last, the lower
total solute flux from the WP reduced its fluid absorption compared
with the three CHO-E beverages (Fig. 6). This difference in solute flux is due to the presence of CHO in the latter beverages and a greater capacity for glucose uptake in the jejunum compared with the duodenum (8). There were no differences in fluid absorption among the three
CHO-E beverages. This is attributable to the fact that jejunal luminal
osmolality and total solute flux among the beverages were similar.
Fluid balance.
Although there were differences in fluid absorption rate in the two
intestinal segments studied while subjects were drinking the different
beverages, total fluid absorbed in the first 50 cm of the intestine was
not different among beverages and there were no significant differences
in plasma volume, plasma osmolality, urine volume, or sweat production
among beverages during the 85-min exercise sessions. Of the total
volume of fluid ingested (1,396 ± 102 ml), 68, 76, 74, and 82%
were absorbed in the first 50 cm of the small intestine from the
hyper-, iso-, hypotonic, and WP beverages, respectively (Table 2). Thus
ingestion of neither the hypotonic CHO-E beverage nor the WP, which was
also hypotonic, conferred any advantage in terms of fluid balance
during exercise. This conclusion is further supported by the lack of
any difference in plasma electrolyte concentrations or in plasma
osmolality among trials.
We conclude that in normal, healthy, euhydrated adults exercising at
~65% O2 peak total
fluid absorption in the proximal small intestine from 6% CHO-E
beverages (within the osmotic range studied) is not different from a WP
beverage, despite significant differences in net water and solute
fluxes among beverages in the duodenum and the jejunum. The lack of an
osmotic effect among 6% CHO-E beverages is attributable to the limited
osmotic gradient in the duodenum, driving fluid either from lumen to
blood, in the case of the hypotonic beverage, or from blood to lumen,
in the case of the hypertonic beverage. On the other hand, the osmotic gradient in the duodenum while the WP was being ingested was fourfold greater than that for the CHO-E beverages and is considered the primary
mechanism causing the significantly greater water absorption from the
WP in the duodenum. On the basis of the changes in plasma and urine
volumes, fluid homeostasis during 85 min of cycle exercise was not
different among the beverages studied.
| |
ACKNOWLEDGEMENTS |
The authors thank the subjects who participated in the experiments,
Dr. Bridget Zimmerman for statistical analyses, and the clerical
expertise of Joan Seye for preparation of the manuscript.
| |
FOOTNOTES |
This research was supported by a grant from the Gatorade Sports Science Institute.
Address for reprint requests: C. V. Gisolfi, Dept. of Exercise Science,
N414 Field House, Univ. of Iowa, Iowa City, IA 52242-1111.
Received 23 January 1997; accepted in final form 15 July 1998.
| |
REFERENCES |
1.
Beckers, E. J.,
N. J. Rehrer,
F. Brouns,
F. T. Hoor,
and
W. H. M. Saris.
Determination of total gastric volume, gastric secretion and residual meal using the double sampling technique of George.
Gut
29:
1725-1729,
1988[Abstract/Free Full Text].
2.
Cooper, H.,
R. Levitan,
J. S. Fordtran,
and
R. J. Ingelfinger.
A method for studying absorption of water and solute from the human small intestine.
Gastroenterology
50:
1-7,
1966[Medline].
3.
Costill, D. L.
Gastric emptying of fluids during exercise.
In: Perspectives in Exercise Science and Sports Medicine. Fluid Homeostasis During Exercise, edited by C. V. Gisolfi,
and D. R. Lamb. Indianapolis, IN: Benchmark, 1990, chapt. 3, p. 97-127.
4.
Dill, D. B.,
and
D. L. Costill.
Calculation of percentage changes in volumes of blood, plasma, and red cells in dehydration.
J. Appl. Physiol.
37:
247-248,
1974[Free Full Text].
5.
George, J. D.
New clinical method for measuring the rate of gastric emptying: the double sampling test meal.
Gut
9:
237-242,
1968[Free Full Text].
6.
Hallback, D.-A.,
L. Hulten,
M. Jodal,
J. Lindhagen,
and
O. Lundgren.
Evidence for the existence of a countercurrent exchanger in the small intestine in man.
Gastroenterology
74:
683-690,
1978[Medline].
7.
Hallback, D.-A.,
M. Jodal,
and
O. Lundgren.
Villous tissue osmolality, water and electrolyte transport in the cat small intestine at varying luminal osmolalities.
Acta Physiol. Scand.
110:
95-100,
1980[Medline].
8.
Harig, J. M.,
K. H. Soergel,
J. Barry,
and
K. Ramaswamy.
Brush border membrane vesicles formed from human duodenal biopsies exhibit Na+-dependent concentrative L-leucine and D-glucose uptake.
Biochem. Biophys. Res. Commun.
156:
164-170,
1988[Medline].
9.
Hunt, J. B.,
E. J. Elliott,
P. D. Fairclough,
M. L. Clark,
and
M. J. G. Farthing.
Water and solute absorption from hypotonic glucose-electrolyte solutions in human jejunum.
Gut
33:
479-483,
1992[Abstract/Free Full Text].
10.
Hunt, J. B.,
E. J. Elliott,
and
M. J. G. Farthing.
Efficacy of a standard United Kingdom oral rehydration solution (ORS) and a hypotonic ORS assessed by human intestinal perfusion.
Aliment. Pharmacol. Ther.
3:
565-571,
1989[Medline].
11.
Hyden, S.
A turbidimetric method of determination of higher polyethylene glycols in biological materials.
Ann. Roy. Agric. Coll. Sweden
22:
139-145,
1955.
12.
Lamb, D. R.,
and
G. R. Brodowicz.
Optimal use of fluids of varying formulations to minimize exercise-induced disturbances in homeostasis.
Sports Med.
3:
247-274,
1986[Medline].
13.
Lambert, G. P.,
R. T. Chang,
D. Joensen,
X. Shi,
R. W. Summers,
H. P. Schedl,
and
C. V. Gisolfi.
Simultaneous determination of gastric emptying and intestinal absorption during cycle exercise in humans.
Int. J. Sports Med.
17:
48-55,
1996[Medline].
14.
Lambert, G. P.,
R. T. Chang,
T. Xia,
R. W. Summers,
and
C. V. Gisolfi.
Absortion from different intestinal segments during exercise.
J. Appl. Physiol.
83:
204-212,
1997[Abstract/Free Full Text].
15.
Malawer, S. J.,
and
D. W. Powell.
An improved turbidimetric analysis of polyethylene glycol utilizing an emulsifier.
Gastroenterology
53:
250-256,
1967.
16.
Mitchell, J. B.,
D. L. Costill,
J. A. Houmard,
M. G. Flynn,
W. J. Fink,
and
J. D. Beltz.
Effects of carbohydrate ingestion on gastric emptying and exercise performance.
Med. Sci. Sports Exerc.
20:
110-115,
1988[Medline].
17.
Murray, R.
The effects of consuming carbohydrate-electrolyte beverages on gastric emptying and fluid absorption during and following exercise.
Sports Med.
4:
322-351,
1987[Medline].
18.
Neufer, P. D.,
D. L. Costill,
W. J. Fink,
J. P. Kirwan,
R. A. Fielding,
and
M. G. Flynn.
Effects of exercise and carbohydrate composition on gastric emptying.
Med. Sci. Sports Exerc.
18:
658-662,
1986[Medline].
19.
Ryan, A. J.,
G. P. Lambert,
X. Shi,
R. T. Chang,
R. W. Summers,
and
C. V. Gisolfi.
Effect of hypohydration on gastric emptying and intestinal absorption during exercise (Abstract).
FASEB J.
9:
1692,
1995.
20.
Santangelo, W. C.,
and
G. J. Krejs.
Absorption of pure water by the human gastrointestinal tract (Abstract).
Gastroenterology
88:
1569,
1985.
21.
SAS Institute.
SAS/STAT Software: Changes and Enhancements Through Release 6.11. Cary, NC: SAS Institute, 1996, chapt. 18, p. 531-656.
22.
Schedl, H. P.,
and
J. A. Clifton.
Solute and water absorption by the human small intestine.
Nature
199:
1264-1267,
1963[Medline].
23.
Shi, X.,
R. W. Summers,
H. P. Schedl,
R. T. Chang,
G. P. Lambert,
and
C. V. Gisolfi.
Effects of solution osmolality on absorption of select fluid replacement solutions in human duodenojejunum.
J. Appl. Physiol.
77:
1178-1184,
1994[Abstract/Free Full Text].
24.
Shi, X.,
R. W. Summers,
H. P. Schedl,
S. W. Flanagan,
R. T. Chang,
and
C. V. Gisolfi.
Effects of carbohydrate type and concentration and solution osmolality on water absorption.
Med. Sci. Sports Exerc.
27:
1607-1615,
1995[Medline].
25.
Sladen, G. E.,
and
A. M. Dawson.
Effects of flow rate on the absorption of glucose in a steady state perfusion system in man.
Clin. Sci. (Colch.)
36:
133-145,
1969[Medline].
J APPL PHYSIOL 85(5):1941-1948
8570-7587/98 $5.00
Copyright © 1998 the American Physiological Society
Top
Abstract
Introduction
![<A><AC>Q</AC><AC>˙</AC></A><SUB>E</SUB> = GER · [PEG]<SUB>s</SUB>/[PEG]<SUB>p</SUB> − <A><AC>S</AC><AC>˙</AC></A><SUB>p</SUB>](/content/vol85/issue5/fulltext/1941/img001.gif)
![<A><AC>Q</AC><AC>˙</AC></A><SUB>L</SUB> = <A><AC>Q</AC><AC>˙</AC></A><SUB>E</SUB> · [PEG]<SUB>p</SUB>/[PEG]<SUB>d</SUB>](/content/vol85/issue5/fulltext/1941/img002.gif)
