Energy while running

Rebecca Stubbs - Everlast - photo by David Knight

Rebecca Stubbs - Everlast

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

21 Responses to Energy while running

  • […] This year though, I really could feel the hunger, probably cos I had not been doing long runs and my body has forgotten how to burn less fuel at a given pace so that it can last longer.. (if you don’t understand how it works, read more here) […]

  • Joe Guglielmo:

    I am a diabetic and I have been having trouble keeping my blood sugar up. When I finish my sugar is in the 50s even though I was 180 to 200 before I started. Is there anything I can carry with me when I run that would prevent it? Something that is easy to carry and I do not need to drink water with it. Thank you

    • Hi my problem is big. I train hard as a marathon n altra marathon runner. After my long runn 3hrs up ward or 40to 80km I fil like I wanna die naw. Filing dzy like I wanna vomite and sweting even if is not cold. Fil like I must sleep. Brithing hard. Wht can I do to prevent this. M a fast runner

  • JD:

    Joe – I am type 1 diabetic as well, currently 43 and have been in sports since I could walk. A number of years ago I went on an insulin pump, with the pump I can modify my basal input (cut back on my basal let’s say by 90%) during my run. This keeps my sugars in check over a run, however you have to then fight the after exercise high, but more on that another time. Currently I am training for my first marathon. Few things to keep in mind:
    1. Do not run while you have any fast acting insulin in your system (usually insulin such as Novolog has a 3hr peak).
    2. Before you run (within 30 minutes) eat something like a Cliff bar or half of one. They contain long burning fuel, you won’t go super high, however the burn of carbs will go over a period of time. This will help stabilize your sugars during the run.
    3. Always take something with you such a Goo or Hammer gels. These offer quick energy (22-25 carbs). Another point to this is always be prepared!!

    If you are not already, look into wearing an insulin pump, it will allow you to manually control your intake of insulin during exercise. I use the OneTouch Ping (which is water proof) One more item to look into is the Dexcom meter This meter will provide you real-time bg level readings during your exercise, it is a great way to see how your body is reacting to the exercise and you will be able to plan i.e. it will tell you that you are dropping fast, then it is time for a gel.

    Hope this helps, good luck!!


  • […] are considered a high glycemic index food, which means eating them before a workout will increase your blood sugar and top up your […]

  • Anshu:


    I’ve been quite worried recently about this whole ‘muscle breakdown’ thing. As a runner, should I be taking protein supplements to aid my diet? Currently, I run no more than an hour but will be increasing my runs to longer period as I’m gonna be entering for half-marathon in 3 months time. Thanks!


  • […] approximately 20g of carbohydrate. I need another 10g per hour to maintain my blood sugar levels (link). Whatever the level I consume I will start doing it from the beginning of the race. It has helped […]

  • Tony:

    I want to thank the group for all of this great advice. I have a 13 year old that is not only good enough to make the high school varsity team, but compete on the county level. She has been stuck with her times (Fall 5K and Spring 800 meter and one mile). She has been extremely disciplined with training and protein, but after reading here, I recognize her required glucose intake is way off. Thanks again for the help! Districts are Thursday, so we began complex carb loading tonight!

    • Phil Salmon:

      Hi Tony,

      My daughter has ben recently diagnosed as Type 1 and loves the shorter runs and also athletics.

      Can you give me any tips as to what are the best things to take dfore during and after the short runs. She is 11 yrs. old. Looking at the shorter stuff more than the mhalf marathons. She is not on a pump just injections.



  • Carol stevenson:

    I’m doing a 26 mile marathon walk in a weeks time and just want to know the best things to eat the three days up to it.
    I follow a gluten free diet

  • Sergio:

    Before I begun training at morning I measured my glucose level and it is at 130mg/dl
    After training for about 1 hour my nlood glucose increases to about 260mg/dl.
    What are the risk associated with the increase?
    How can I control or stabilize my blood sugar while training and remain in a safe range?

    Thanks in advance

  • Japhet Kipkorir:

    Hello, I’m along distant runner, and I have been trying to aiming to run good times in marathon every year. But in 2012 I did a lot of marathon (5) 2:10, 2;11, 2:13, 2;11, 2:16 and the year after my performs where not good and some marathon I was unable to finish. I usually feel that I’m in good shape till 25km then from there I will feel like fainting and in 30km I will feel week and not able to cope with the pass. What can I do to stop this from happening?


  • […] fueling for marathons (or half marathons) is critical, but here’s my understanding. [I used this article as a reference if you want to go back and read some more […]

  • […] 60 minutes, don’t worry about fueling during it.  Your body can handle it.  According to this article, the average person can store about 400g of glycogen in their muscles, and about 100g in […]

  • Timo Latruwe:

    You don’t just tell us what to do, you tell us why!
    Very helpful, thank you so much

  • Angela:

    I need some help I run in the afternoon after work and seam to run out of energy I have 1/2 of cup if oatmeal with raisincan I try something else for energy /thanks.

  • Sphiwe Nzuza:

    I need help for my running enegy i,m going to run 21 km nexst week my weight is 111 i need more information what can i take to boost my energy i,m runnng township to township marathon challenge please help me out

  • […] need to and should replenish your energy as fast as possible, after you’ve completed the half marathon. Studies have shown the body […]

  • Kerri:

    I’m running a marathon in September and I’m a new runner. Right now it takes me about 6 mins to run 1 k and it’s exhausting! It took me over 30 mins to run 5k. I had to stop a few times, I just wanted to lay on the ground and sleep. Does it get easier overtime? Any tips will me greatly appreciated! Thanks:)

  • Stephen Snoding:

    Hi Owen,

    I tend to find that when i run in the morning (6:30am) then go to work, come home and cook dinner and stuff, that by half 8/9pm i am totally wiped out.

    What can I do to stop this, or at least lessen it.

    kind regards

    Stephen Snoding

  • chrisgg:

    An elite athlete running a marathon will use roughly the same total energy as any other runner finishing the race regardless of their time. If an elite runner needs an energy boost half way through a marathon, then so should all the other slower runners need it when they have gone about the same distance, not when they gone for the same amount of time. So, although an elite athlete may need a gel after one hour of fast pace running, a slower runner, running at half the speed, might not need it for 2 hours when they have also reached halfway. Why not talk in terms of distance run than time run? In these days of massive spreads between the ability of runners from fastest to slowest, time becomes meaningless.
    Putting it another way, someone walking 3 miles for a leisurely hour will use very little glycogen compared with an elite runner completing a half marathon of 13.1 miles in an hour.

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