Nutrition
Before Competition
The main nutrient to watch in this context
is, of course, carbohydrate; glycogen (the
body's carbo store) is the limiting fuel for
endurance exercise.
Eating a diet high in
carbohydrates while training should ensure
that you have good muscle glycogen stores
prior to competing. Available evidence shows
that topping up with extra carbohydrate just
before the action has a
negligible effect on events of short or
moderate duration (less than 60 minutes).
It's like adding an extra gallon of petrol
to a car with a full tank
before a short journey. But if
your initial glycogen levels are low (eg, if
you're participating in a tournament taking
place over a week) and/or the exercise lasts
90 minutes or longer, pre-exercise
carbohydrate may improve your performance.
The type of food and its timing is crucial to whether it's helpful or
harmful. Some basic physiology should help
explain why this is. Exercising muscles burn
fat and glucose - the glucose is obtained
both from the glycogen stores in the muscles,
and from glucose circulating in the blood.
The liver has the task of masterminding
blood glucose levels and trying to ensure
that they don't dip too low (causing
hypoglycaemia, resulting in weakness,
dizziness and nausea). If the liver
registers that blood glucose levels are
dropping, it can release glucose into the
blood from its own glycogen store. If blood
glucose levels rise (eg, after
eating food containing
carbohydrate) the hormone insulin is
released, which forces glucose out of the
bloodstream and into storage as glycogen.
The relevance of this to pre-exercise
nutrition is as follows. During a fasting
period (such as overnight), liver glycogen
stores will be lowered. If you compete in a
morning event without having eaten anything
since the night
before, you're starting at a
disadvantage. Although your muscle glycogen
will still start off high, once it begins to
be used up (after an hour or so) there's a
reduced amount of blood glucose supplied by
the liver to turn to. You can avoid this
problem by
eating a high-carbohydrate meal
1-4 hours
before exercise. The available
evidence suggests that the optimal amount is
somewhere between l-4g of carbohydrate per
kg of body weight. The mount of carbohydrate
should be less the nearer to competing to
avoid gut problems. ('Carbohydrates' in
Berning, J. and Nelson-Steen, S., 'Sports
Nutrition for the 90s', Aspen, Maryland,
1991).
Avoiding the hypoglycaemic backlash
The insulin response is more difficult to
pin down. Some carbohydrates cause a more
marked surge in blood glucose (and hence
insulin) than others. Measurements have been
made and foods categorised according to
their 'glycaemic index' (see table). A
food's glycaemic index (GI) gives an
indication of the degree of blood-glucose
surge to expect. High-GI foods (which
include glucose itself and bread, bananas)
bring about a marked and immediate rush of
glucose into the bloodstream, whereas lowGI
foods (eg, beans, lentils) release glucose
at a slow and steady rate over a much longer
time period. There's a potential problem
with high-GI foods - the high insulin
response can can actually lead to an over-enthusiastic
hoarding away of blood glucose, leading in
turn to a net blood-sugar drop and its
unwelcome effects. Some individuals seem to
be more susceptible to this problem than
others, however, which may help to explain
why research studies have come up with
apparently contradictory information in this
area.
For some time in sports nutrition it has
been accepted wisdom that sugar should not
be consumed within the 60-15 minutes prior
to exercise for fear of a hypoglycaemic
backlash. Several early studies found that
runtime to exhaustion was shorter by about
20-25 per cent after athletes consumed 2-3oz
of glucose within an hour
before an endurance test. However,
a number of more recent studies have not
found this effect.
For example, research carried out at the
Gatorade Sports Science Institute in
Illinois found that pre-exercise feedings of
carbohydrate (6 per cent sucrose/glucose
solution or a 20 per cent maltodextrin/glucose
solution) did not result in hypoglycaemia,
or adversely affect sensory or physiological
responses during 50 minutes of moderate-intensity
cycling. Exercise was started at the time of
the individual's peak insulin response (usually
somewhere between 20-45 minutes after
carbohydrate consumption). Although blood
sugar did drop when exercise commenced, it
did not reach significantly low levels and
returned to baseline values after 30 minutes
of exercise. The volunteer cyclists were not
aware of any subjective problems, such as
perceived muscle weakness or nausea, during
the exercise ('Glycaemic and Insulinemic
Response to Pre-exercise Carbohydrate
Feedings', Seifert, J., et al, International
Journal of Sport Nutrition, 4, pp
46-53,1994).
Meanwhile, other research has found that the
best food to take an hour
before activity is carbohydrate
which has a low glycaemic index. Scientists
at the University of Sydney tested out four
different pre-race feeds: ( 1 ) boiled
lentils (2) baked potatoes (3) a sports
drink comprising glucose dissolved in water,
and (4) plain water. Volunteers cycling to
exhaustion averaged 117 minutes with the
lentils, compared with 108 minutes for the
sports drink and only 97 minutes with
potatoes. There seemed to be a link to blood
glucose levels - after 90 minutes, blood
glucose was about 20 per cent higher for the
lentil eaters compared to those who took the
sports drink ('Carbohydrate Feeding
before Exercise: Effects of
Glycaemic Index', International Journal of
Sports Medicine, vol 12 (2), pp
180-186,1991). So there may be a benefit
from a pre-exercise meal of lentils or the
like if 1) you're going to be active for
over an hour, 2) you think you suffer from
reactive hypoglycaemia, and 3) you are
unable to take in any extra carbs as you go.
Can't stand lentils? Check the table for
other low-GI foods.
Another strategy which should suit
everybody, whether or not you have a
tendency to hypoglycaemia, is to take in
carbohydrate 5-10 minutes
before exercise of an hour or
more. If you're exercising at an intensity
greater than 50% V02max, the insulin
response to glucose ingestion is suppressed.
Choose something that will be absorbed
quickly either a sports drink or a high-GI
food (solid food isn't recommended for
runners so close to racing - take the risk
only if you know your gut can handle it!).
Running
Sprinting You won't benefit from extra carbs
before competing, as glycogen
stores aren't a limiting factor. However, it
might be worth taking some sodium
bicarbonate
before racing. The research is
split down the middle - about half the
studies have found a benefit, the other half
no effect. No serious detrimental effects
have been found, however, apart from nausea
in susceptible individuals. A dose of 300mg
per kg of body weight has been found
effective in some studies for exercise
lasting between 30-120 seconds (eg, 800m
track). It' s probably best to take this 1-3
hours
before racing ('Bicarbonate
Loading', Heigenhauser, G. and Jones, N. In
Lamb, D. and Williams (eds), 'Ergogenics,
Enhancement of Performance in Exercise and
Sport,' Brown & Benchmark, lowa, 1991).
Middle Distance Depending on your level of
fitness and the terrain involved (eg, hills
vs flat running), glycogen may or may not be
a limiting factor. Make sure you've had a
high-carb meal 2-3 hours beforehand if
possible. Taking extra carbohydrates on
board in the 10 minutes
before racing probably won't
hinder your performance - but it' s doubtful
it will have any positive effect either!
Caffeine
before running may improve your
capacity to 'kick' at the end of a race - a
study found that two cups of strong coffee
brought about a significant improvement over
1500m. For peak absorption, drink about an
hour
before exercise ('Effect of
Caffeinated Coffee on Running Speed,
Respiratory Factors, Blood Lactate and
Perceived Exertion During 1500m Treadmill
Running', British Journal of Sports
Medicine, 26(2), pp 116-120,1992).
Endurance If you're running for over an
hour, extra carbs
before and during a run may well
help your performance. The amount of
carbohydrate ingested four hours prior to
performance should be based on body weight.
Several studies have used 4-5g/kg with good
results. For an athlete who weighs 60kg (132
pounds), the recommended amount would be
240-300g. The carbohydrates could be
consumed in any of several forms, including
fluids such as juices or glucose polymer
solutions, or solid carbohydrates such as
fruits or starches. The fibre content should
be minimised to prevent possible intestinal
problems during exercise. If carbohydrate is
consumed approximately one hour
before competing, 1-2g/kg has
been found to enhance performance in several
studies. Both glucose polymers and foods
with a low glycaemic index have been used
successfully. If carbohydrate is consumed
immediately
before exercise, ie, within 10
minutes of the start, about 50-60g of a
glucose polymer in a 40-50 per cent solution
has been used effectively in some studies.
A blight of many distance runners is gut and
bowel problems. If you suffer from this,
choose your foods carefully. It' s probably
wise to avoid food high in fibre, and you
should certainly avoid fatty foods. Try
liquid meals - choose between those
formulated for athletes, for convalescents
(check for a high carbohydrate content) or
make your own by blending milk, skimmed milk
powder and fruit. |
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