burke (2007) nutritional strategies for the marathon

8/9/2019 Burke (2007) Nutritional Strategies for the Marathon

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C O N F E R E N C E P A P E R

Sports M ed 2CO7; 37 4^5): 3

0112-1642/O7/CXXM-O344/S4

O 2007 Adls Da ta Infarm atton BV. Al l r ights res

Nutrition Strategies for the arathon

Fuel for Training and Racing

Louise M Burke

Dep artment of Sports Nutrition, A ustralian Institute of Sport, Belconnen, Australian Capital

Territory, Australia

bstr ct

Muscie glycogen provides a key fuel for training and racing a marathon.

Carbohydrate 'loading' can enhance marathon perfonnance by allowing the

competitor to run at their optimal pace for

a

longer period before fatiguing. For the

well trained runner, this may be achieved by tapering exercise over the final days

before the m arathon and ensuring carbohydrate intakes of 10-12 g/kg/day over the

36-48 hours prior to the race. Sports nutrition guidelines recommend that the

runner consumes sufficient carbohydrate to promote restoration of muscle glyco-

gen between training sessions. This strategy should allow the runner to 'train

harder' and recover optimally between workouts. A recent hypothesis suggests

that runners might 'train smarter' by training w ith low glycogen stores, since this

might promote greater stimulation of the training response. However, there is no

evidence that a low carbohydrate diet enhances the outcomes of training or

provides benefits as a depletion phase prior to carbohydrate loading. In fact, a low

carbohydrate diet may even impair performance if carried out for extended

periods. If there are benefits to m anipulating glycogen stores for som e workouts,

this is likely to happen as the natural outcome of the periodisation of the high-

volume programme of an elite runner.

To run 42.195km is one of the ultimate chal-

lenges in spo rt. Therefore, it is not surprising that as

the science of sports nutrition has evolved over tlie

past 30 years, marathon runners have been quick to

put new knowledge into practice in the field. This

review will focus on muscle glycogen as a fuel for

training and racing, noting how the marathon helped

to popularise the discovery of this important exer-

cise fuel.

1

C arbohydrate Loading

marathon, the term 'hitting the wall', which

scribes this overwhelming fatigue, has become p

of everyday jargo n. Carbohyd rate loading, or sup

compensating the muscle glycogen stores in pre

ration for prolonged exercise, resulted from stud

undertaken in the late 1960s. ' Using percutane

biopsy techniques to examine fuel utilisation

enzyme activities in the muscle, Scandinavian spo

scientists found that several days of low-carbo

drate eating depleted muscle glycogen and redu

cycling endurance compared with the results asso


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ion Ron H ill, and later by recreational runners

joined the running boom of the 1970s and

A modified version of carbohydrate loading was

asevere depletion or glycogen stripping

3

days of high carbohyd rate intake and taper, was

ere measured after

and 3 days

gh carbohy drate intake (10 g/kg body

ss per day) in well trained m ale athletes.'^' After 1

ntent increased significant-

intake. This

ycogen storage, showing

1.2 Gender a nd C orbohy drate Loading

that female a thletes fail to super-compensate muscle

glycogen stores compared with males and fail to

show a performance benefit following carbohydrate

loading.'" However, this is at least partly exp lained

by the relatively smedler amounts of dietary carbo-

hydrate and restricted energy intakes of many fe-

male a thletes. Indeed, when female athletes are pro-

vided with sufficient energy and carbohydrate in-

take,

they are able to achieve a significant increase

in glycogen storage, similar to that seen in a male

population.'^' Menstrual status of female athletes

may affect glycogen storage, with greater storage

occurring during the luteal rather than the follicular

phase. Further studies of gender differences in the

achievement of, and response to, carbohyd rate load-

ing are warranted. In the meantime, it is likely that

the main challenge related to carbohydrate-loading

practices of females is the issue of adequate die

carbohydrate and energy.

1,3 Carbo hydrate Loading a nd

iVlarathon Performanoe

Theoretically, carbohydrate loading can enhance

performance in sporting events that would otherw ise

be limited by glycogen depletion. Although a half

marathon event is too short to benefit from super-

compensated carbohydrate stores, carbohydrate

loading has been shown to enhance o verall perform-

ance of a 30km cross-country run, a 30km treadmill

run in trained men and a 25km treadmill run in

moderately trained men.f* Where perfonnance en-

hancements have been found, carbohydrate loading

was associated not with an increase in overall run-

ning speed but with maintenance of race pace during

the last part of the run compared with the control

trial or control group.'*' Even when carbohydrate

loading did not cause a statistically significant en-

hancement of total running time, participants were

found to run faster over the last 5km than in a

control trial. Therefore, even though field studies


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u

2.Fat Ada pta tion : a Twist on

Pre Loading Depletion

Marathon runners should have a high capacity for

fat oxidation during exercise as a legacy of their

training. However, this capacity can be further up-

regulated by as little as 5 days of training while

following a low-carbohydrate (4 weeks) may

impair training outcom e and cause som e health risks

(for a review, see Burke and Kiens'^').

Research during the 1990s showed that the meta-

bolic changes created by fat adaptation are robust. In

fact, they persist for at least 24 hours despite aggres-

sive tactics to increase carbohydrate availability

(e.g. restoration of muscle glycogen and the con-

sumption of carbohydrate before and during exer-

cise). In effect, fat adaptation could be viewed as a

slightly extended 'dep letion pha se' prior to carbohy-

drate loading. Such a combination of dietary strate-

gies would seem the perfect preparation for am ia-

thon, simultaneously o ptimising carbohydrate stores

while maximising the capacity for fat oxidation.

Curiously, the effect on performance is unclear.'^'

The apparent failure of translation of metabolic

changes has been variously explained as a failure of

scientists to detect small changes in perfonnance

that might be worthwhile in real-life sport, or the

existence of 'responders' and 'non-responders' to

fat adaptation strategies.

There is now evidence that what was initially

viewed as glycogen sparing may be, in fact, a dow n-

regulation of carbohydrate metabolism or 'glycogen

impairment'. Fat adaptation/ carbohydrate restora-

tion strategies are associated with a reduction in the

activity of a key enzyme regulating carbohydrate

that when fat adaptation/carbohydrate restorat

are applied to exercise protocols that mimic a r

life race (i.e. self-pacing, and the interspersing

high-intensity and moderate-intensity exerci

there is a compromised ability to perfonnance hi

intensity sprints. Although a marathon is viewed

an endurance event, the critical activities in a r

(i.e.

the breakaway, the surge up a hill or the sp

to the fmish line) are all dependent on the runn

ability to work at high intensities. With grow

evidence that this critical ability may be impaired

now seems clear that fat adaptation or pre-load

depletion strategies should not be undertaken

marathon runners.

3. Training an d iVIuscie G lyc og en

According to the current guidelines for spo

nutrition,'^ the everyday diet of a marathon run

should provide enough carbohydrate to cover

fuel costs of their training programme and rest

glycogen between workouts. This is likely to be

the range of 7-12 g/kg/day according to volume

intensity of training sessions. However, data p

taining to nutrient-gene interactions and the cellu

signalling pathways that promote muscle adap

tions to training have recently suggested that tra

ing with low or moderate glycogen levels may

celerate the transcription of several important gen

In support of this, one study has shown that train

with low glycogen levels might accelerate train

outcom es. In this study, untrained subjects achie

greater increases in muscle enzyme content

exercise enduranc e in the leg trained with a proto

promo ting depleted glycogen stores, than the con

lateral leg, which undertook the same volume

training in a glycogen-recovered state.'' ' Howev

other chronic studies of diet and training interv

tions in well trained athletes, including runners

have shown that higher carbohydrate intakes t

allow greater glycogen recovery are associated w

fewer symptoms of overtraining during high-v


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burke (2007) nutritional strategies for the marathon

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