You will probably have seen film clips of exhausted marathon runners staggering about in a very distressed state.
You may have heard this described as athletes 'hitting the wall' or 'bonking'.
 Cyclists say that this is when 'The man with the hammer gets you'.
It is what happens when you totally deplete your glycogen stores causing your blood sugar levels to fall dangerously low.
It can happen very suddenly if you don't eat and drink enough when you are exerting yourself.
If you have experienced this scary condition, you won't want to experience it again.
The feelings of exhaustion and disorientation can be extreme.
If you have never experienced this, then read the article and try and make sure that it doesn't happen to you!  This article isn't supposed to scare you off exercising.
It is supposed to encourage you to take your dietary needs more seriously if you are into endurance sports.
Fat burning At low exercise intensity, most of your energy comes from burning fat.
Even a slim person has enough body fat to fuel many hours of such exercise.
Unfortunately, this process cannot take place fast enough to fuel more intense efforts.
That's why the body has a separate energy system for that based on something called glycogen, a glucose-based compound.
Glycogen burning Glycogen is your body's 'premium-grade' fuel.
You need that for your more intense efforts, for example -- sprinting or hill-climbing.
It is stored in the muscles and liver and the amount is somewhat limited.
You could easily burn through this in about 2 hours of hard effort which is why marathon runners often 'hit the wall' at about the 20 mile mark if they don't train hard enough before the event, or take in calories while they are running.
How to avoid 'hitting the wall' The fitter you are, the less stress a given physical demand will place on your body.
For example, an unfit person might be burning a lot of glycogen to cycle up a moderate hill, whereas a fit cyclist might be able to get most of his/her energy for that effort from stored fat.
That spares glycogen for when it might be needed later.
It is crucial to eat and drink sensibly when exercising vigorously for long periods of time.
Even if you are trying to lose weight, don't skimp on the calories when working hard.
Hitting the wall won't help you! It is possible to top up the energy supplies on the move.
Some people manage this from food alone, but I find it hard to stomach much food when working hard.
I prefer to get most of my 'fuel' calories from energy drinks.
I'm a cyclist and I mix 2 parts water to one part orange juice in my drinks bottles.
The fructose in the juice supplies some energy.
If I'm riding for more than about 3 hours, I always add maltodextrin carbo-powder to my drinks and research shows that fructose works well in conjunction with that.
It also makes it taste better.
For rides over about 5 hours, I would eat a couple of sandwiches, some malt loaf, or fig rolls, that kind of thing.
Make sure that you settle on a routine that works for you, and get used to it before taking on a big athletic challenge.
You don't want to find yourself halfway round a marathon course or a century bike ride only to start suffering stomach cramps or bloating from that new brand of energy drink that you thought you'd try out!