When I started out riding, I would quite often start to feel trembly and weak after as little as thirty kilometres. That trembly hungry feeling is called "bonking." It is to be avoided at all costs. The physiology of the condition is that the muscles have exhausted their store of glycogen, and start to switch to burning fat. Fat has more energy gram for gram than the carbohydrate based glycogen, but using it requires almost exactly twice as much oxygen as sugar. As the amount of oxygen that can be transported from the lungs to tissues by the cardiovascular system is the rate-limiting step in any exercise (getting fit means increasing this oxygen transporting ability), switching to fat metabolism generally means slowing down drastically. As you get fitter, the muscles adapt to burning more fat, and the margin between sugar and fat metabolism is smoother.
The start of my PBP preparation almost a year ago was a little 100km spin through Kent known as the Ride of the Falling Leaves. It was a crispish day, and a bit breezy, and by the time I got to the control at 50k I was very definitely in the mood for a sandwich. Two attractive young people were serving up such delicacies as roast pepper and mozzarella on poppyseeded white, so I bought two, and set off up the hill. Trying to eat while travelling uphill is a big mistake: you could choke on a crumb while you're breathing heavily, and I was concentrating so hard on not doing this that I missed a navigational arrow and shot off down a long hill in the wrong direction. After that I took my own sandwiches, so I would always have one handy on a downhill stretch.
Eating small amounts of carbohydrate little and often prevents the sugar stores in the muscle and liver from rapid depletion. You could eat anything with sugar or starch in it, including very expensive sports drinks, but I prefer to lash out 72p for a box of oatcakes. Each cake is 50 kcal, and I eat about 6 cakes every 50k, alternating with either Tunnock's caramel logs or a McVities cake bars. Some nuts and dried fruit are nice to have along for a bit of variety. All of these things have a good calorie/weight ratio and are compact enough to stuff in a pocket, or map case.
After while I listened to my body and ditched the cheese sarnies, because I found it hard to handle much fat while in the saddle. Fat provides plenty of energy and makes food taste good, but it is hard to digest. If there is a lot of fat in the stomach, the food hangs around there for longer, which is bad news if you need the energy straight away. It may even slow the absorption of water from the stomach, which could lead to dehydration. That said, it's hard to avoid fat altogether, and not to take advantage of the sterling efforts of the feeding crews at the controls seems to me to be rude and neglectful. I generally pick the highest carbohydrate option on the menu and then plan to cycle at a 15k pace for the next hour to allow my guts a bit of leeway for digestion.
Whatever I eat, I enjoy it immensely. There is nothing like the accentuated appetite after your muscles have burnt off a lot of fuel. The most mundane foods take on miraculous flavours; eating becomes intense, serious, necessary. You get hungry; your hunger is satisfied. Such simple gratifications are rare in late twentieth century Britain.
I mostly drink water on the way round, generally getting through about a litre every 50k, more if it's hot. It's definitely possible to overdo it: on the PBP I was anxiously overhydrated at the start and had to stop three times for a piss in the first 100k, which is a bit of a waste of time and effort. Still, better that than any distance in the opposite direction. Most of the water you lose from your body while cycling is from sweating. Sweat has to be manufactured by the sweat glands, which in turn get their fluid from the extra-cellular fluid, which is in turn in equilibrium with the blood stream. All of the cells in the body, including the muscles operate best within quite tight physiological parameters. If the water being lost from the cells is not rapidly replaced from the extracellular fluid and in turn the blood stream then the muscles will be working in sub-optimal conditions. By the time you feel thirsty it's too late.
Much is made (generally by those with a financial interest in their sale) of "balanced isotonic" drinks and all the rest. It is true that drinks with a little sugar and rather less less salt are more readily absorbed from the gut, and treatment with such oral rehydration solutions can be lifesaving in cholera. Cycling a few hundred or even a thousand kilometres is not getting cholera, and your guts are perfectly able to cope with plain water. If you like the taste, and you like to spend your money on such things, splash out, otherwise, find the nearest tap.
©Douglas Carnall 1999