When we need energy our body breaks up a substance called ATP (adenosine triphosphate).
ATP is 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?
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!
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 provides 3.75 kcal/g
protein provides 4 kcal/g
fat provides 9 kcal/g
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.
see the section on carbohydrates to find out how to achieve a full fuel tank
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 frequently 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 is the only fuel that can power intense exercise for prolonged periods, yet its stores within the body are relatively small.
If you do not 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.
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.