Distance Running

Chapter 42
Distance Running
JOHN A. HAWLEY, ELSKE-JEANNE SCHABORT AND
TIMOTHY D. NOAKES
Introduction
Many athletes steadfastly believe that there
exists a single nutritional ingredient that will
suddenly transform them into world champions.
Yet, to the best of our knowledge, the only substances that may confer such advantages to a
competitor are already on the International
Olympic Committee’s list of banned substances.
But, ever alert to commercial opportunities, marketers of nutritional supplements for athletes
continue to make extravagant claims for the
performance-enhancing effects of their products.
Sports practitioners need to be aware that there
exist few controls to regulate the extent of the
claims that such nutritional companies can make
for the effectiveness of their products. Unlike the
pharmaceutical industry, which requires that all
product claims be substantiated by the results of
costly, controlled clinical trials, no such control is
required in the marketing of nutritional supplements for sport. It is therefore not surprising to
find that athletes are often confused about the
extent to which nutrition can improve
performance.
In this review we analyse the scientific basis
for the nutritional practices of athletes, with
special reference to distance runners. We believe
that although nutrition is important for success,
it is only part of a balanced approach. As
Olympic gold and silver marathon medallist
Frank Shorter has said: ‘You don’t run 5 minutes
a mile for 26 miles on good looks and a secret
recipe.’
550
What do athletes eat and what should
they eat during training?
The major nutritional concern for athletes is the
excess energy expended during strenuous training, which, if not matched by an increased
energy consumption, will inevitably result in a
reduced training capacity and a drop in performance. Top-class athletes undergoing strenuous
training can have daily energy expenditures two
to three times greater than untrained, weightmatched individuals. This greater energy expenditure may exceed nutritional intake if only
normal eating patterns are maintained and could
explain the nibbling patterns of eating among
athletes.
The macronutrient intakes of well-trained
male and female middle- and long-distance
runners reported in a review of published
studies (Hawley et al. 1995a) are summarized in
Tables 42.1 and 42.2. A mean weighted carbohydrate (CHO) intake of 48% of total energy consumption was reported by both male and female
runners. However, in contrast to male distance
runners who usually consume sufficient energy
to meet daily training requirements, the energy
intake of many of the female athletes was lower
than would be expected, given their workload.
Among female athletes, iron, zinc, vitamin B12
and calcium intakes were also below the recommended daily allowance.
It has long been proposed that the optimum
diet for athletes, especially for endurance
runners, should contain up to 70% of energy
distance running
551
Table 42.1 The dietary intakes of well-trained male distance runners. Adapted from Hawley et al. (1995a).
Energy:
MJ (kcal)
CHO:
g (%)
[g ◊ kg-1]
Fat:
g (%)
[g ◊ kg-1]
Protein:
g (%)
[g ◊ kg-1]
144 (15)
Athletes
n
Body mass:
kg
Runners
10
—
17.3 (4121)
526 (50)
161 (35)
Endurance runners
56
69
13.5 (3226)
403 (50) [5.84]
122 (34) [1.76]
Distance runners
35
—
12.64 (3020)
374 (50)
115 (34)
119 (16)
Distance runners
10
—
12.7 (3034)
372 (49)
115 (34)
129 (17)
Marathon runners
4
61
13.84 (3309)
361 (44) [5.92]
137 (37) [2.24]
158 (19) [2.59]
Marathon runners
19
64
14.9 (3570)
487 (52) [7.6]
128 (32) [2.0]
128 (14.5) [2.0]
78 (16) [1.40]
Table 42.2 The dietary intakes of well-trained female distance runners. Adapted from Hawley et al. (1995a).
Energy:
MJ (kcal)
CHO:
g (%)
[g ◊ kg-1]
Fat:
g (%)
[g ◊ kg-1]
Protein:
g (%)
[g ◊ kg-1]
Athletes
n
Body mass:
kg
Endurance runners
18
52
9.00 (2151)
296 (55) [5.69]
62 (29) [1.19]
86 (16) [1.65]
Distance runners
17
—
8.48 (2026)
252 (48)
87 (38)
74 (14)
Distance runners
18
50
8.95 (2135)
224 (47) [4.48]
89 (39) [1.78]
74 (14) [1.48]
Distance runners
41
55
7.84 (1870)
—
—
70 (15) [1.27]
Distance runners
9
53
9.21 (2193)
333 (59) [6.28]
66 (27) [1.25]
73 (13) [1.38]
Marathon runners
19
53
9.62 (2295)
248 (44) [4.68]
99 (40) [1.86]
80 (16) [1.50]
US distance runners
51
52
14.9 (3570)
323 (54) [6.2]
89 (33) [1.7]
81 (13) [1.5]
from CHO, with approximately 15% from fat and
15% from protein. However, the proportional contribution of CHO, fat and protein to athletes’
diets may not be vastly different from that of
non-athletes. At first, this paradox may seem
confusing. This is because the amount of dietary
CHO consumed should not be determined
merely as a proportion of total energy intake, but,
instead, should rather be based upon the absolute
amount of CHO consumed relative to the
athlete’s body mass (BM). Often a ‘low’ percentage of dietary CHO when calculated from a
higher than normal total energy intake results in
adequate CHO provision. For example, Costill
(1986) reported that a sample of 22 longdistance runners consumed a diet containing
only 50% CHO. At first sight, this CHO intake
might be considered too low for distance
runners, but as their total energy intake was
nearly 50% higher than would be expected for
individuals of similar size, a more than adequate
amount of CHO (375 g · day–1 or 5.7 g · kg–1 body
mass · day–1) was being consumed.
The question as to what constitutes an ‘adequate’ CHO intake has been addressed by
several recent studies (Lamb et al. 1990; Simonsen
et al. 1991; Sherman et al. 1993). The general consensus is that providing athletes are ingesting
5–6 g CHO · kg–1 body mass · day–1, they are probably not compromising their training capacity.
Acute supplementation of the normal diets of
trained athletes with additional CHO will,
however, elevate muscle glycogen stores and
improve performance in competition (for review,
552
sport-specific nutrition
see Hawley et al. 1997b). Further, when an athlete
requires rapid recovery from training, a CHO
intake of 8–10 g · kg–1 body mass · day–1 is
recommended.
Muscle glycogen stores and the effect
of CHO loading on metabolism
The glycogen content of skeletal muscle of
untrained individuals consuming a mixed diet is
around 80 mmol · kg–1 wet weight muscle. For
individuals involved in regular endurance
training and consuming a similar diet, muscle
glycogen content is somewhat higher, at approximately 125 mmol · kg–1 wet weight muscle,
although this figure will obviously depend on
when the measurement was taken in relation to
the last training session. After several days
of a high (8 g · kg–1 body mass) CHO diet and a
reduction in training, the muscle glycogen
content may be elevated to values around
175–200 mmol · kg–1 wet weight. There is some
evidence that trained athletes who habitually
consume a moderate- to high-CHO diet (ª 6 g
CHO · kg–1 body mass · day–1) do not increase
their muscle glycogen contents to the same
extent as untrained individuals (Hawley et al.
1997a). Indeed, if well-trained athletes consume
a moderate- to high-CHO diet, muscle glycogen
‘supercompensation’ can occur on a day-to-day
basis. In this respect, Costill et al. (1981) have previously reported that muscle glycogen content
was not significantly different when trained
runners consumed either 525 or 650 g CHO ·
day–1, suggesting that the extent of muscle glycogen supercompensation is not further increased
by the ingestion of very large (> 600 g · day–1)
quantities of dietary CHO.
The mechanism(s) explaining the ergogenic
effect of CHO loading still needs to be established. One possibility is that the higher muscle
glycogen content may delay the onset of fatigue
resulting from muscle glycogen depletion during
exercise. Alternatively, the increased availability
of muscle glycogen could slow the rate of liver
glycogen depletion because it would reduce the
muscle’s demand for blood glucose. Liver glyco-
gen sparing would depend on the rate of hepatic
glycogenolysis, which seems to be accelerated by
a high liver glycogen content after CHO loading.
Effect of carbohydrate loading on
running performance
The results of studies which have examined the
effects of CHO loading and CHO restriction on
running performances, are summarized in Tables
42.3 and 42.4. Although there are many laboratory studies which demonstrate a positive relationship between pre-exercise muscle glycogen
stores and endurance performance for both
cycling and running (for review, see Hawley et al.
1997b), to the best of our knowledge only one
study has evaluated this effect in the field.
Sherman et al. (1981) examined the effects of
either low-, moderate- or high-CHO diets on
muscle glycogen content and utilization during a
half-marathon event (20.9 km) in six trained
runners. They found that large differences in preexercise muscle glycogen contents of the runners
had no influence on subsequent performance. In
fact, running times were generally a bit slower
when athletes started the trials with higher levels
of muscle glycogen. Perhaps of physiological
interest was that the absolute amount of muscle
glycogen left at the end of the three runs was
similar regardless of the initial muscle glycogen
content.
The results of Sherman et al. (1981) were subsequently confirmed in the laboratory by Madsen
et al. (1990). They reported that 25% higher starting muscle glycogen contents did not improve
treadmill run time to exhaustion at 75–80% of
.
maximal oxygen uptake (Vo2max.). In agreement
with the data of Sherman et al. (1981), the total
amount of muscle glycogen utilized during the
two treadmill runs was similar. Perhaps the
most important finding was that at the point of
‘exhaustion’, muscle glycogen content was still
relatively high in all subjects. These studies
strongly suggest that CHO loading has no
benefit to performance for athletes who participate in moderate-intensity events lasting up to
90 min.
distance running
553
Table 42.3 Effects of carbohydrate loading on moderate intensity running lasting 60–90 min. Adapted from
Hawley et al. (1997b).
Muscle glycogen
(mmol ◊ kg-1 wet weight)
Dietary treatment
Pre-exercise
Postexercise
Performance
measure
Results
A: 3 days LCHO (1.5 g ◊ kg-1 BM,
CHO ◊ day-1, then 3 days
7.7 g ◊ kg-1 BM CHO)
B: 3 days HCHO (5.0 g ◊ kg-1 BM,
CHO ◊ day-1, then 3 days
7.7 g ◊ kg-1 BM CHO)
C: 6 days NORM (5.0 g ◊ kg-1 BM,
CHO ◊ day-1)
A: 208 ± 30
A: 102 ± 39
20.9-km run
A: 83 ± 15 min (n = 6 M)
B: 203 ± 28
B: 96 ± 17
B: 83 ± 9 min
C: 159 ± 13
C: 96 ± 28
C: 83 ± 15 min
A: NORM
A: 135 ± 28
A: 101 ± 32
B: 3 days 50% CHO
3 days 70% HCHO
B: 168 ± 19
B: 129 ± 40
Run to exhaustion
at
. 75–80%
Vo2max.
A: 70 ± 20 min (n = 3 M, 3 F)
B: 77 ± 30 min
BM, body mass; F, female; HCHO, high carbohydrate intake; LCHO, low carbohydrate intake; M, male; NORM,
normal diet; 1 mmol ◊ kg-1 wet weight = 4.3 mmol ◊ kg-1 dry weight.
All values are mean ± SD.
In contrast, there is some evidence to indicate
that elevating pre-exercise muscle glycogen contents extend endurance time in events lasting
longer than 90 min (Table 42.4). Evidence for the
important role of muscle glycogen in continuous
endurance exercise also comes from studies of
the effects of high-CHO diets on running times
.
to fatigue at 70–75% of Vo2max.. The largest
increases in running endurance were found in an
investigation by Galbo et al. (1967). In that study,
the subjects ingested extreme diets with either a
low (10%) or a high (77%) CHO content. Compared with the low-CHO diet, the high-CHO
diet increased muscle glycogen content by about
150% and subsequently extended running times
to exhaustion by approximately 66%.
In addition to increasing running times to
fatigue, CHO loading may also improve running
performance during prolonged exercise in which
a set distance must be covered as quickly as possible (i.e. in a race situation). Karlsson and Saltin
(1971) reported that the consumption of a diet
high in CHO for several days before exercise
resulted in improvements of about 6% in race
times during a 30-km event. Interestingly, the
twofold higher starting muscle glycogen contents did not increase the initial running speed
but, instead, allowed the athletes to maintain a
fast race pace for longer. Williams et al. (1992)
have also reported a similar finding. They
observed that a high-CHO diet before exercise
increased the speed over the last 5 km of a 30-km
treadmill running time-trial and improved
overall performance by approximately 2%.
Fluid and energy replacement during
distance running
The pioneering studies showing that CHO
ingested during prolonged exercise could
enhance endurance performance were conducted on runners competing in the 1924 and
1925 Boston Marathon. The results of these investigations clearly highlighted the importance of
CHO loading before and CHO ingestion during
prolonged, steady-state running. Unfortunately
for the athletic community, these findings were
completely ignored. So too, it seems, were the
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sport-specific nutrition
Table 42.4 Effects of carbohydrate loading on prolonged running lasting longer than 90 min. Adapted from
Hawley et al. (1997b).
Muscle glycogen
(mmol · kg-1 wet weight)
Dietary treatment
Pre-exercise Postexercise
A: 4 days LCHO (10.5% CHO, 76% fat,
13.5% protein)
B: 4 days HCHO (77% CHO, 13.5% protein,
9.5% fat)
A: 45 ± 19
A: 35 ± 21
B: 112 ± 61
B: 75 ± 19
A: NORM
Trial 1: 4.6 ± 1.3 g · kg-1 BM CHO · day-1
Trial 2: 5.1 ± 1.4
B: COMPLEX CHO
Trial 1: 4.6 ± 1.3 g · kg-1 BM CHO · day-1
Trial 2: 7.7 ± 1.8
C: SIMPLE CHO
Trial 1: 4.0 ± 0.7 g · kg-1 BM CHO · day-1
Trial 2: 7.0 ± 1.2
—
—
A: 3.5 days 6.1 g · kg-1 BM CHO · day-1 (pasta)
B: 3 days 2.4 g · kg-1 BM CHO · day-1 and
depletion exercise, then 3.5 days
11.2 g · kg-1 BM CHO · day-1 (pasta)
C: 3.5 days 6.3 g · kg-1 BM CHO · day-1
(beverage)
D: 3 days 2.5 g · kg-1 BM CHO · day-1 and
depletion exercise, then 3.5 days
11.6 g · kg-1 BM CHO · day-1 (beverage)
A: 103 ± 49
B: 130 ± 47
Performance
measure
Run to
exhaustion
at
.
70% Vo2max
Run to
exhaustion
at
.
70% Vo2max
Results
A: 64 ± 16 min (n = 7 M)
B: 106 ± 13 min
A Trial 1: 119 ± 19 min
(n = 15 M, 15 F)
Trial 2: 122 ± 22 min
B Trial 1: 106 ± 24 min
Trial 2: 133 ± 46 min*
C Trial 1: 114 ± 16 min
Trial 2: 141 ± 27 min*
—
Run to
exhaustion
at
.
75% Vo2max
A: 153 ± 49 min (n = 14 M)
B: 169 ± 30 min
C: 107 ± 32
C: 139 ± 26 min
D: 150 ± 44
D: 168 ± 27 min†
A: NORM
B: HCHO — 3 days no CHO, then 3 days
9 g · kg-1 BM CHO · day-1
A: 100 ± 39
B: 194 ± 66
A: 29 ± 33
B: 105 ± 72
30-km
running race
A: 143 ± 20 min (n = 10 M)
B: 135.3 ± 18 min
A: NORM
Trial 1: 5 ± 1 g · kg-1 BM CHO · day-1
Trial 2: 7 days 5.4 ± 0.8
B: HCHO
Trial 1: 5.1 ± 0.8 g · kg-1 BM CHO · day-1
Trial 2: 3 days 8.6 ± 1.3
4 days 6.9 ± 1.2
—
—
30-km
treadmill run
A: Trial 1: 135.3 ± 14.1 min
(n = 12 M, 6 F)
Trial 2: 135.3 ± 14.1 min
B: Trial 1: 137.5 ± 16.5 min
Trial 2: 134.9 ± 16.5 min‡
BM, body mass; F, female; HCHO, high carbohydrate intake; LCHO, low carbohydrate intake; M, male; NORM, normal
diet; 1 mmol · kg-1 wet weight = 4.3 mmol · kg-1 dry weight.
All values are mean ± SD.
* B2 > B1, C2 > C1 (P < 0.01).
† D > C (P < 0.05).
‡ 1.9% improvement; increase in speed over last 5 km during Trial 2 (P < 0.001).
early investigations showing the importance of
adequate fluid replacement during prolonged
exercise in the heat. In fact, the earliest reference
to fluid replacement during long-distance
running is found in the 1953 International
Amateur Athletic Federation (IAAF) Handbook
controlling marathon events. The handbook
stated that ‘refreshments shall only be provided
by the organizers of a race after 15 km’, and that
‘no refreshments could be carried or taken by a
distance running
competitor other than that provided by the organizers’. As water was the only drink available to
runners, it was clear that the IAAF had also
ignored the pioneering studies conducted in the
1920s showing the benefits of CHO ingestion
during long-distance running.
In the 1960s, the IAAF modified their rules
slightly, so that by 1967 refreshments were available after only 11 km of a race. Although competitors could now make up their own drink, only
water was provided by the race organizers.
Between 1960 and 1970, the notion that water
was more important than CHO replacement
during exercise gained popularity. This was
because studies showed that runners who were
the most dehydrated after distance races had the
highest postrace rectal temperatures. Indeed,
the belief that fluid replacement alone was of
primary importance for optimizing performance
during prolonged exercise was promoted to
such an extent that CHO ingestion was actively
discouraged. As a result of the perceived
importance of fluid replacement, the IAAF again
altered their rules in 1977 to allow runners to
ingest water earlier and more frequently during
competition.
The question of whether water or CHO
replacement should be practised during
endurance exercise was not really resolved until
the late 1970s and early 1980s, when commercial
Fig. 42.1 Endurance events
provide an opportunity for intake
of fluids and substrate—usually
carbohydrate—during the event
itself. Photo © Allsport / G. Prior.
555
interests in the US revived research into the value
of CHO ingestion during exercise. These laboratory controlled studies conducted on cyclists
confirmed the findings reported some 50 years
earlier, which demonstrated that the ingestion
of CHO-containing solutions enhanced performance and endurance during prolonged exercise
(for review, see Coggan & Coyle 1991). Today, the
consumption of CHO–electrolyte beverages is
advocated by the IAAF in all races of 10 km and
longer. But, the exact amounts that should be
consumed to provide sufficient fluid, CHO and
electrolytes to replace sweat and energy losses
during exercise remain to be established.
Fluid loss and replacement
Fluid loss during exercise is determined principally by the athlete’s sweat rate, which is proportional to their metabolic rate and the prevailing
ambient temperature. Estimated sweat rates for
endurance runners, along with their rates of fluid
intake and measured weight losses, are shown in
Table 42.5. One study conducted in the 1960s
reported very low rates of fluid intake during a
32-m race (150 ml · h–1), with resultant weight
losses of more than 2.4 kg and significantly elevated postrace rectal temperatures (Wyndham
& Strydom 1969). Largely on the basis of this
single finding, it was recommended that runners
556
sport-specific nutrition
Table 42.5 The rate of fluid loss and fluid ingestion
during various long-distance running races. Adapted
from Noakes et al. (1995).
Race distance
(km)
Fluid intake
(l · h-1)
Estimated
sweat rate
(l · h-1)
Weight loss
(kg)
32
42*
56
67
90
0.15
0.4 ± 0.2
0.5
0.4
0.5
1.35
1.1 ± 1.1
0.9
0.8
0.85
2.4
2.4 ± 0.3
2.0
2.4
3.5
The total fluid intake of runners was determined as the
sum of their individual intakes, as reported at various
recording points during the race. Sweat rate was
estimated from the rate of water loss, minus estimated
respiratory losses. Total weight loss was determined as
the sweat loss, plus metabolic fuel loss plus fluid
intake minus urine output.
* 42-km values are means ± SD of the average values
from seven studies on male subjects. Female sweat
rates were lower than those for males over distances
of 42 km (0.6 l · h-1 vs. 1.1 l · h-1) and 67 km (0.5 l · h-1 vs.
0.8 l · h-1).
needed to drink ‘at least 900 ml of fluid per hour
during competition in order not to collapse
from heat stroke’ (Wyndham & Strydom 1969).
Modern studies show that runners do not voluntary consume much more than 500 ml · h–1 during
distance races. In contrast to these moderate
rates of fluid intake, sweat rates are invariably
around 1.0–1.2 l · h–1 during events lasting 2 h or
more (for review, see Noakes 1993).
One explanation for the failure of runners to
match their fluid intake to their fluid losses
during exercise is that they develop symptoms
of ‘fullness’ when they attempt to drink fluid at
high rates. Feelings of abdominal fullness may, in
part, be due to limited rates of fluid absorption,
as duodenal and jejunal perfusion studies show
the maximum rate of water absorption occurs
from isotonic solutions containing glucose, and
is limited to about 0.8 l · h–1 (Davies et al. 1980).
Similarly, in studies in which sufficient fluid was
ingested to match fluid losses during exercise,
not all of the ingested fluid appeared in the extracellular or intracellular fluid pools. Thus, the
maximum rate of fluid absorption by the small
bowel during exercise may be less than the high
rates of fluid loss incurred by some athletes
during more intensive exercise, leading to progressive or ‘involuntary’ dehydration (Noakes
1993).
An alternative hypothesis is that man, unlike
other mammals, may develop progressive dehydration during exercise because of the sodium
chloride (NaCl) losses in sweat. Large sodium
losses attenuate the rise in serum osmolality
during
exercise-induced
dehydration
in
humans, and since thirst is regulated by changes
in both serum osmolality and plasma volume,
dipsogenic drive in dehydrated humans ceases
before either fluid or sodium losses are fully
replaced. Ingestion of NaCl solutions also terminates drinking prematurely by restoring plasma
and extracellular volumes before intracellular
fluid losses have been replaced. The practical significance of this observation is that whether
dehydrated humans drink plain water or NaCl
solutions, they tend to stop drinking before they
are fully rehydrated. These complex interactions
may explain why some humans are unable to
prevent the development of ‘involuntary’ dehydration during prolonged exercise. Additionally,
the rapid alleviation of the symptoms that indicate drinking, such as dryness of the mouth,
may also cause premature cessation of drinking
before full rehydration has occurred.
Carbohydrate ingestion and oxidation
during exercise
Runners are often confused as to the optimum
fluid replacement regimen to enhance their performance. Although the addition of high (> 15 g
per 100 ml) concentrations of CHO to fluid
replacement beverages may impair intestinal
fluid absorption, inadequate CHO ingestion
impairs performance by limiting the rates of
CHO oxidation late in exercise. Accordingly,
recent attention has focused on strategies to opti-
distance running
mize the rate of CHO ingestion and its subsequent oxidation by the working muscles during prolonged exercise. In this regard, gastric
volume along with solute energy content and
osmolality are critical determinants of the rate of
gastric emptying during exercise. With regard to
gastric volume, the maximum rate at which CHO
and water can be delivered to the intestine from
an ingested solution is strongly influenced by the
average volume of fluid in the stomach. This, in
turn, is governed by the drinking pattern of the
athlete. The principal findings of studies that
have simultaneously measured rates of gastric
emptying and the oxidation of CHO solutions
that have been ingested in repeated doses during
exercise are, firstly, that the amount of a solution
emptied from the stomach is at least double the
amount that is ultimately oxidized by the active
muscles and, second, provided sufficient is consumed, the peak rates of ingested CHO oxidation
557
rise to approximately 1 g · min–1 after 70–90 min
of exercise (for review, see Hawley et al. 1992).
An interesting observation from those studies
which have fed runners CHO during exercise is
that when there is a performance improvement,
it coincides with a faster running pace over the
latter stages of a race or trial. This effect is similar
to that seen when subjects CHO-load. That is, the
additional CHO does not allow athletes to run
faster, but merely resist fatigue and maintain a
given pace for longer (for review, see Maughan
1994). Unlike submaximal cycling, CHO ingestion during steady-state running has been shown
to result in muscle glycogen sparing (Table 42.6).
Although this effect seems limited to the type I
fibres, it could potentially have a profound influence on performance during long-distance races.
More research specifically related to running
needs to be conducted to confirm the results of
these preliminary studies.
Table 42.6 The effects of carbohydrate ingestion on distance running performance.
Performance
measure
Dietary treatment
Drinking regimen
Results/comments
Reference
A: Placebo 4 h prior,
CHO solution at
start and during
B: CHO meal 4 h
prior, water at
start and during
Placebo: 10 ml · kg-1
BM fluid
CHO meal: 2 g CHO ·
kg BM
At start (8 ml · kg-1
BM) and every
5 km (2 ml · kg-1
BM): water or
6.9% CHO
solution
30-km treadmill
time-trial
A: 121.7 ± 13.0 min*
Chryssanthopoulos
et al. (1994)
A: Water
B: 5% CHO
250 ml fluid
immediately prior,
150 ml fluid every
5 km
30-km road race
A: 131.2 ± 18.7 min
B: 128.3 ± 19.9 min†
Tsintzas et al. (1993)
A: Water
B: Glucose solution
(50 g CHO + 20 g
glucose)
C: Fructose solution
(50 g CHO + 20 g
fructose)
250 ml fluid before
warm-up, 5 min
prior to trial, then:
150 ml at 5-km
intervals
30-km treadmill
time-trial
A: 129.3 ± 17.7 min‡
B: 124.8 ± 14.9 min‡
Williams et al. (1990)
B: 121.8 ± 11.4 min*
C: 125.9 ± 17.9 min‡
Continued
558
sport-specific nutrition
Table 42.6 Continued.
Dietary treatment
Drinking regimen
Performance
measure
A: Water
B: 7% CHO solution:
glucose polymer/
fructose solution
200 ml fluid at time
0, 30, 60 and
90 min
Results/comments
Reference
2-hour treadmill
run
. at 60–65%
Vo2max.
CHO solution
abolished rise in
plasma cortisol and
decreased exerciseinduced rise in FFA
Deuster et al. (1992)
A: Water
B: 5.5% CHO–
electrolyte
solution
5 min prior to
60-min treadmill
exercise: 8 ml · kg-1
run
. at 70%
BM, then: 2 ml · kg-1
Vo2max.
BM after 20, 40 min
CHO ingestion
resulted in muscle
glycogen sparing in
type I muscle fibres
Tsintzas et al. (1995)
A: Water
B: 5.5% CHO
solution
C: 6.9% CHO
solution
8 ml·kg-1 BM CHO
solution ingested
prior to exercise,
then: 2 ml · kg-1 BM
ingested at 20-min
intervals during
first hour;
thereafter, water
until exhaustion
Run until
exhaustion
.
at 70% Vo2max.
A: 109.6 ± 31.8 min
B: 124.5 ± 26.6 min§
Tsintzas et al. (1996)
A: Placebo
B: 7% CHO solution
(glucose polymer/
sucrose)
250 ml prior to
exercise and 125 ml
at 15-min intervals
during exercise
Run until
exhaustion
.
at 80% Vo2max.
A: 92 ± 27 min
B: 115 ± 25 min§
Wilber and Moffatt
(1992)
A: Placebo
B: Sucrose (81 ± 18 g)
C: Caffeine
(384 ± 13 mg)
D: Sucrose (72 ± 22 g)
and caffeine
(396 ± 29 mg)
200 ml 60 min before,
250 ml prior to the
start, 250 ml after
45 min
Run until
exhaustion
.
at 80% Vo2max.
A: 39.45 ± 11.19 min
B: 58.29 ± 15.25 min||
C: 53.02 ± 9.16 min||
Sasaki et al. (1987)
A: No fluid
B: Water
C: 5% CHO solution
(glucose polymer
and fructose)
240 ml at 15, 30, 45,
60 and 75 min
Run until
exhaustion
.
at 85% Vo2max.
A: 56.0 ± 4.39 min
B: 78.25 ± 4.93 min¶
C: 102.3 ± 7.28 min¶
Macaraeg (1983)
A: Placebo
B: 7% CHO–
electrolyte drink
400 ml 30 min prior
to exercise and
250 ml at 5-km
intervals during
the run
40-km run in the
heat: 35 km
training pace +
5 km race pace
A: 24.4 ± 4.2 min
B: 21.9 ± 2.8 min**
Millard-Stafford
et al. (1992)
C: 121.4 ± 29.7 min
D: 56 : 58 ± 11 : 10 min||
* Not significant.
† B < A (P < 0.01).
‡ Not significant overall; however, significant decrease in running speed (P < 0.05) over last 10 km of trial A from
4.14 ± 0.55 to 3.75 ± 0.86 m · s-1.
§ B > A (P < 0.05).
|| B, C, D > A (P < 0.05).
¶ B, C > A (P < 0.01).
** B < A (P < 0.03).
distance running
Conclusions and recommendations
for optimal fluid replacement
during distance running
The principal aims of fluid ingestion during distance running are to improve performance by:
• limiting any dehydration-induced decreases
in plasma volume and skin blood flow;
• limiting any rise in serum sodium osmolality
or serum osmolality;
• diminishing progressive rises in rectal
temperature;
• decreasing the subjective perception of effort;
and
• supplementing endogenous CHO stores.
Although it has been assumed that the
optimum rate of fluid ingestion is the rate that
closely tracks the rate of fluid loss, the exact composition of the solution that will optimize electrolyte and fluid replacement of the extracellular
space has not been established. Furthermore, the
rates of fluid ingestion needed to replace the high
(> 1 l · h–1) sweat rates typically induced during
prolonged exercise probably exceed the maximal
intestinal absorptive capacity for water. Most
runners will not be able to achieve such fluid
intakes without great difficulty. However, fluid
consumption can be maximized during distance
running by paying careful attention to the temperature and palatability of the drink and the
addition of electrolytes, particularly sodium, to
the beverage.
CHO ingestion during distance running is recommended whenever the exercise is of sufficient
duration or intensity to deplete endogenous
CHO stores. If CHOs are ingested frequently
enough and in appropriate volumes, it appears
that, with the exception of fructose:
• the type of CHO consumed does not greatly
influence the rate of gastric emptying of isoenergetic solutions;
• there are no physiologically important
differences in the rates of CHO oxidation resulting from repeated ingestion of a variety of
mono-, di- and oligosaccharides during exercise;
and
• all ingested CHOs are oxidized at a maximum
559
rate of approximately 1 g · min–1 after the first
70–90 min of exercise.
The reason for similar peak rates of ingested
CHO oxidation from different CHOs is because,
in all likelihood, it is the prevailing concentrations of glucose and insulin normally present
during prolonged, moderate-intensity exercise
that set the upper limit for the rate of glucose
uptake and oxidation by skeletal muscle
(Hawley et al. 1995b).
The following practical guidelines are suggested for runners participating in prolonged,
moderate-intensity exercise of up to 6 h duration:
• Immediately before exercise or during the
warm-up, the athlete should ingest up to
5 ml · kg–1 of body mass of cool, flavoured water.
• For the first 60–75 min of exercise, the athlete
should ingest 100–150 ml of a cool, dilute (3.0–
5.0 g per 100 ml) glucose polymer solution at
regular intervals (10–15 min). It seems unwarranted to consume CHO in amounts much
greater than 30 g during this period, as only 20 g
of ingested CHO are oxidized in the first hour of
moderate-intensity exercise, irrespective of the
type of CHO consumed or the drinking regimen.
• After about 90 min of exercise, the concentration of the ingested solution should be increased
to 7–10 g per 100 ml, to which 20 mEq · l–1 of
sodium should be added. Higher sodium concentrations may not be palatable to most athletes,
although they may be beneficial. Potassium,
which may facilitate rehydration of the intracellular fluid compartment, could also be included
in the replacement beverage in small amounts
(2–4 mEq · l–1). For the remainder of the race, the
athlete should consume 100–150 ml of this
solution at regular (10–15 min) intervals. Such
a drinking regimen will ensure optimal rates
of both fluid and energy delivery, thereby limiting
any dehydration-induced decreases in plasma
volume, and maintaining the rate of ingested
CHO oxidation at approximately 1 g · min–1 late
in exercise.
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