Energy while running
How your body stores energy
Your body also stores energy as carbohydrates, fats and proteins; but the way energy is stored is not directly related to where the energy came from in the first place.
Carbohydrate is stored as glycogen in the muscles and liver. You can store about 400g of glycogen in your muscles, and about 100g in your liver (though, as we shall see, these can be increased with training). This means you can store about 2000 kcal as glycogen – enough energy to run or walk about 20 miles.
You store fat all over your body, particularly just beneath the skin and around your internal organs. The human body is, on average, about 15-25 per cent fat (about 10kg for a 60kg person) – though athletes tend to have a rather lower percentage body fat than the rest of the population.
Proteins are used as the body’s building materials and are not generally used as the main source of energy; but your body can break down and use the energy from proteins in extreme circumstances.
How your body uses energy during exercise
Your body mainly uses carbohydrates and fats to provide energy during exercise. The precise mixture depends on the amount and type of exercise you do, how well trained you are, and on your inherited physiology.
The energy system used for all exercise lasting more than about 1½ minutes is called aerobic – which means that is uses oxygen. For short burst of intense energy, the body can use anaerobic systems which don’t require oxygen, but which can only be sustained for short periods and which (for all but very short bursts of energy) produce a lot of lactic acid.
The aerobic energy system primarily uses a combination of carbohydrates and fat (but can also use proteins). The proportion of carbohydrates and fats used depends on the intensity of the exercise. For low intensity exercise, your muscles mainly draw their energy from fat. But as your effort level goes up and your muscles have to generate energy more quickly, so you have to increase the proportion of energy that comes from carbohydrates. The proportion of energy provided from carbohydrates rises as exercise intensity increases from around 10% to up to 90%.
The carbohydrates used during exercise this are stored as glycogen both in the muscles, and around the liver. These stores are limited, and as they begin to run out the proportion of energy provided by fat rises (though you cannot metabolise fat unless there is at least some carbohydrate as well). Once your glycogen stores have run out, your body begins to break down muscle proteins to provide energy and to maintain blood sugar levels.
If you exercise very intensely, however, your aerobic metabolic system will not be sufficient to provide all the energy you need. At this point, your anaerobic metabolism is used to provide extra energy from glycogen. While this has the advantage of being very efficient at producing energy, and requiring no oxygen, it has the disadvantage of being very limited, and producing lactic acid as part of the process.
During training, you are aiming to increase your body’s ability to turn fuel into energy by:
- teaching your body to become more efficient at converting fat into energy, so that you can exercise more intensively while still burning fat, thus protecting your (much more limited) stores of glycogen. This is one reason why the “long, slow, distance run” is a key part of training for a marathon.
- increasing your body’s ability to store glycogen, especially in the muscles, increasing your total carbohydrate stores.
Carbo-loading before a long run
For long races (longer than a half marathon), you need to ensure your glycogen stores are fully topped up at the start line. To do this, some marathon runners indulge in what is known as “carbo-loading” for the three days before the marathon. This means increasing the intake of carbohydrates – for example, pasta – in the run‑up to the race, while cutting back on running (and hence reducing glycogen consumption).
Many years ago it was thought that carbo-loading should be preceded by a period of carbo-depleting, in which the body was starved of carbohydrates. This was thought to encourage the body to store carbohydrates during the loading phase. Subsequent studies showed that this is not effective, and it is no longer thought to be helpful to deplete your glycogen before the carbo-loading phase.
Carbo-loading should be accompanied by an increase in water consumption, partly because water is needed for the body to store glycogen, and partly because you should be increasing your water consumption anyway before a long race.
Suggestions for carbo-loading
breakfast cereal with raisins; thick slices of toast with honey; banana; bagel with peanut butter; pancakes
jacket potato with tuna, sweetcorn or cottage cheese; rice salad with chicken or beans and vegetables; banana sandwich; fresh fruit
pasta with tomato sauce; fresh vegetables; fruit salad
toast with honey; low-fat yoghurt or fromage frais; low fat rice pudding
As we saw in Chapter 6, the speed with which carbohydrates are absorbed into the blood stream is measured by the glycaemic index (GI). In the six hours before you run, you should aim to eat high glycaemic index foods, such as bread, raisins and sugar, to increase your blood sugar and top up your body’s glycogen stores. Examples of suitable meals on the day of the race are bread with honey, a baked potato with a low fat filling, or pasta with tomato sauce.
Why you need to take in energy on long runs
If you run for less than an hour, you are unlikely to need to eat or drink while you are exercising. But for runs longer than an hour, your performance is likely to be improved if you top up your fuel as you go.
During the first hour of exercise, most of your energy comes from glycogen stored in your muscles. After about an hour, the muscles begin to draw their fuel from the blood sugar, which is in turn supplied by glycogen stored in your liver, as well as from their own stores. Your liver glycogen levels are also finite. So when your liver glycogen is depleted, your blood sugar level falls and you are unable to carry on exercising. This low blood sugar (called “hypoglycaemia”) induces a feeling of tiredness and light-headedness, and your legs begin to feel very heavy. Marathon runners know this as “hitting the wall” or getting “the bonks”. It is a quite distinct (and unpleasant) feeling, and it has the same effect on your running performance as a large bear climbing on to your back.
But if you can replenish your blood sugar while you are exercising, you will not deplete your liver glycogen stores so rapidly, with the result that you can carry on for longer before you hit the wall. That is why it is a good idea to take energy on runs of more than 2 hours.
How much energy you need while running
Research shows that the average person’s muscles can take up about 30g to 60g an hour of carbohydrate from the bloodstream.[i] So consuming more carbohydrate than this will not improve energy output or reduce fatigue. It takes about 30 minutes for the carbohydrate which you eat or drink to reach your bloodstream, so you need to start consuming carbohydrates before you begin to feel tired. The best strategy is to begin soon after you start your run.
For example, if you consume about 500ml an hour of an isotonic sports drink containing 7g of carbohydrate per 100ml, then you will consume about 35g of carbohydrate per hour, which will roughly maintain your blood sugar levels.
An alternative to using sports drinks to maintain your blood sugar levels is to use gels. These are sachets sold in specialist running and fitness shops, which contain a sugary syrup designed to be taken during long runs and races. Each sachet contains around 20g of carbohydrate, so you can usefully take 2-3 each hour.
The main advantage of gel sachets is that they are easy to carry in a pocket, or tucked into your waistband. This means that you can take them on training runs as well as during a race, so that you can train using the same energy source as that which you will be using on race day itself. (By contrast, you can’t easily do this with a sports drink, since you would have to carry a lot of sports drink on your training runs.)
The gel sachets should be washed down with water (otherwise they are too concentrated and sickly), so most runners take them as they approach a water aid station. To get the right concentration, you need to wash down a full sachet with about 250ml of water (more than a full paper cup). If this is too much water to drink in one go while you are running, then take less than a full sachet at each water station.
Well known brands of gels include Squeezy™, Power Shots™, Cliff Shots™, Science in Sports™, and Gu™. They are all broadly the same (though some also contain caffeine, which I personally like) so try them all and see which you find most palatable. If you decide to use gels during a race, make sure that you use them on your long training runs as well, so that you can find out before the race if they are going upset your stomach.
Energy after running
After you have finished running, you should aim to restock your carbohydrate stores as quickly as possible. This will reduce the risk of illness. In addition, the more quickly you restock your glycogen stores, the more you develop your body’s ability to store energy in this form.
In general, the principles for consuming energy after running are:
- aim to consume carbohydrates quickly – preferably within half an hour, and certainly within 2 hours; you may well not feel like eating straight away, but you should try to force yourself;
- consume high glycaemic index carbohydrates which can be absorbed by the body quickly, such as a bagel with peanut butter, or a sports drink;
- drink plenty of water as well, since this is needed for storage of carbohydrates and you will need it to offset dehydration;
- try to accompany the carbohydrates with some protein and fats, since this aids absorption of the carbohydrates. A baked potato with beans or tuna is ideal.
In other words, the best foods to eat immediately after a long run are roughly the same as you should eat before it: plenty of high glycaemic carbs with a little protein, washed down with lots of water.
[i] Coggan, A. R. and Coyle, E. F. (1991), ‘Carbohydrate ingestion during prolonged exercise: effects on metabolism and performance’, in J. Holloszy (ed.), Exercise and Sports Science Reviews, vol 19 (Baltimore: Williams & Wilkins), pp 1-40