Exercise requires energy - so where does this energy come from
and how can we replace it?
When we need energy our body breaks up a substance called ATP
(adenosine triphosphate) - a high energy molecule consisting of
three phosphates attached by energy bonds to adenosine. Energy is
released by breaking off a phosphate from ATP to form ADP
(adenosine diphosphate). This is a continual cycle - ADP is
converted back into ATP. But how is ATP made?
2(i). Energy Systems
Three systems in the body create ATP energy: These systems work
simultaneously but the contribution from each depends on the type
of exercise - its intensity and duration.
1. ATP-CP
The sprint system - provides enough energy for a 5-6 second sprint
- and doesn't require oxygen (anaerobic). CP (creatine phosphate)
is another high energy molecule where the phosphate can be broken
off very quickly - releasing energy - and used to convert ADP back
to ATP. The muscles don't have a large store of CP so it's used up
fast. Hence why some athletes use creatine supplements to maximise
their muscle stores.
2. Anaerobic
The high power system - provides energy for a 90 second power
burst. This system is the fast anaerobic (without oxygen) breakdown
of glucose for energy but only provides 2 molecules of ATP along
with a waste product called lactic acid - too much of this causes
muscle fatigue.
3. Aerobic
The endurance system - how long you can keep going depends on how
fit you are! This system is the slow aerobic - so this time keep
breathing in that oxygen - breakdown of glucose for energy and
provides a massive 38 molecules of ATP- that's nearly 20 times more
than the anaerobic system! The aerobic system can also use fat to
produce ATP energy. Endurance training can make the muscles use fat
more efficiently - now there's a good reason to improve your
endurance fitness!
2(ii). Energy Fuel
Carbohydrate, fat and protein are the three main energy fuels
for exercise. Each of these nutrients are found in differing
amounts in foods and are broken down in the body to provide a
certain quantity of energy - measured as kilocalories (kcal) per
gram (g):
- carbohydrate 3.75 kcal/g
- protein 4 kcal/g
- fat 9 kcal/g
Hence 1g of fat releases more than twice as much energy as 1g of
carbohydrate or protein - but this doesn't mean it's the best
energy fuel for exercise!
The preferred energy fuel for the muscles is glucose, especially
as exercise intensity increases. Glucose is formed from the
breakdown of carbohydrates (sugars and starches) in your diet and
is stored as glycogen in the muscles and liver. However the body
can only store a limited amount - a person weighing 70kg will store
around 450g or 1700 kcal of glycogen. If you want to keep training
efficiently you need to keep your glycogen stores topped up - see
the section on carbohydrates to find out how to achieve a full fuel
tank.
2(iii). Which Fuel?
The amount of each fuel - carbohydrate, fat and protein - you
use during exercise depends on various factors:
- dietary intake
- fitness level
- type of exercise
- training intensity
- length of work-out
- frequency of training sessions
Anaerobic activities only use glucose, whereas aerobic
activities use all three fuels - but protein is used to a lesser
extent than glucose and fat.
During low-intensity exercise, which uses less than 300 kcal
each hour, you use a greater proportion of fat, a smaller
proportion of glucose and fewer calories. As you increase the
exercise intensity, your body will gradually use less fat, more
glucose and more calories. Therefore, most of the fuel during
moderate and high intensity exercise (using more than 500 kcal each
hour) will come from glucose.
If you continue to exercise aerobically for a longer period,
your body will gradually use more fat and less glucose in an
attempt to conserve the limited glucose stores. The fitter you are,
the more efficiently your muscles use fat and the longer you can
work out.
It's that simple - the longer you work out, the more frequent
you train, the more calories you use. But remember one step at a
time - gradual is the key to help you start an exercise programme
and stick with it.
Consequently, carbohydrate is the most important nutrient for
exercise, because it's the only fuel that can power intense
exercise for prolonged periods, yet its stores within the body are
relatively small. If you don't restock your glycogen stores
sufficiently, you will run out of fuel after only a few days of
training or you will find you feel fatigued.
2(iv). Fatigue
Runners call it 'hitting the wall', cyclists call it 'bonking' -
but what is fatigue and why does it happen?
You become sluggish, reaction time slows down, co-ordination and
balance starts to go, concentration dwindles and you feel
light-headed - all signs that fatigue is setting in.
Why? The main cause of fatigue is due to running out of those
vital glucose stores (glycogen) - although dehydration will also
result in fatigue - see the section on fluids. During anaerobic
activities, fatigue is initially due to CP depletion and the build
up of lactic acid, but repeated bouts of this type of activity will
also result in glycogen depletion.
Therefore, if you want to exercise longer and harder you need to
start off each training session by having a full tank of glucose -
how? - by eating a diet that's rich in carbohydrates.
