Text originally from the Easterley Road Club. Original text HERE.
Before setting out on a ride, you should make sure that you have eaten enough food for the distance you will be travelling; or that you take sufficient food with you. If you are going to be racing, then this should be eaten with sufficient time beforehand to allow it to digest (say 1-2 hours). If a rider has pushed himself too far without eating enough to replace the energy used he can get what is known as the Bonk or the Hunger Knock. This is where the rider’s blood sugar levels have fallen too low which can bring on dizziness and the shakes. When you have got to this state it will take a long time to recover so it is best to slow down (you won’t have much choice!) and have something to eat, although it is better to try and eat something before you get to this state. A chocolate bar may be used as a ‘get you home’ assuming it is not too far but this should be backed up with something more substantial, say an energy drink, fruit (eg bananas), bread pudding, etc.
Food that you take with you is known as Bonk food surprisingly enough and complex carbohydrates (such as fruit, pasta, rice potatoes, bread etc) are best. Try and avoid sugary foods which cause a surge in blood sugar levels and then a dramatic fall, leaving you worse off than before. Energy drinks are good as they also replenish fluid levels at the same time, however please choose with care.
As well as eating, you must also drink (even in the winter) to stop dehydration which can have similar effects to the Bonk. Try and drink before you get thirsty as it may be too late by then. Tests have shown that a loss of body fluids of little as 2% can cause a 20% loss in cycling performance! This is especially critical when racing and in any events lasting over an hour (or when it is very hot), you should carry a bottle with you. Recommended drinks are Maxim, Isostar, Gatorade, Hi Five and PSP amongst others which can be bought in powder form in large quantities (which is much cheaper). Some of these drinks are purely glucose polymers (i.e. they only supply energy) whereas others are ‘isotonic’. Isotonic drinks include minerals and salts which match those of body fluids to replace those lost through sweating. This is helpful in avoiding cramp in hot weather.
Avoid most of the commercial ‘energy’ drinks in cans unless they are made by those manufacturers listed above as they consist mainly of sugar and have the same effect as sugary foods.
Remember try to eat and drink little but often.