The body's normal liver and muscle glycogen will support the first 2 or 3 hours of exercise at 70% VO2 max. without any need for supplementation. And both a good training program to improve the form and muscle efficiency of the individual as well as riding (or exercising) at a reasonable pace will postpone the onset of glycogen depletion and fatigue.
Taking in carbohydrates during the event provides an additional source of glucose "fuel" that will extend the length of time before the bonk occurs. A well-trained cyclist will need slightly more than 1 gram of carbohydrate per minute to sustain maximum performance, and oral supplementation (started at the beginning of the exercise, not after glycogen depletion has occurred) should replace carbohydrate at that rate.
This is important in rides of greater than 2 hours duration. As a general rule, the body can utilize 60 grams of ingested carbohydrate per hour to supplement muscle glycogen stores, and the stomach can handle between one and two quarts of fluid before nausea occurs. This does put an upper limit on carbohydrate supplementation during a ride but gives you some guidelines for developing your own program. And there is no problem in using solid food supplements as well, as long as enough fluids are taken along with them.
Insulin surge - avoid the potential hypoglycemia that might occur from sugary drinks ingested in the minutes immediately before a competitive event. Although this can be seen in sedentary individuals eating sweets, rebound hypoglycemia does not appear to be a practical problem for athletes. However, choosing to err on the side of caution, most authorities recommend avoiding all simple carbohydrates for the several hours before an event, starting carbohydrate supplementation in the few minutes immediately preceding the start of the activity.
Pre Ride Meal - There is almost unanimous support for the benefits of a pre ride meal of complex carbohydrates 3 or 4 hours before the event. These carbohydrates will "top off" your muscle and liver glycogen stores, and the slow digestion and absorption of the complex carbohydrates may provide additional glucose supplements to your internal glycogen stores even after the ride has started. Studies have failed to demonstrate that using commercial energy bars or a high fat meal offer a performance advantage over a less expensive complex carbohydrate such as oatmeal.
As sugar concentration increases, the risk of nausea and bloating rises as well. Almost everyone can tolerate a 7 to 10% concentration of glucose, but many cyclists will tolerate solutions of up to 15% to 20%. And the use of complex carbohydrate polymers will allow more carbohydrates to be ingested and absorbed by minimizing the overall concentration of the solution. A good initial strategy is to drinking 400 ml of an 18% glucose polymer solution at the start of the event and then drinking 100 ml every 10 minutes.
Fluid replacement rates of 500 ml per hour are appropriate for the majority of cyclists during prolonged exercise, but rates of up to 1 to 2 liters per hour have been reported in the Tour de France. Hyponatremia is a risk with larger volumes.
There is some evidence that protein may help the absorption of carbohydrates in the immediate post ride window (several hours) that maximizes glycogen repletion in the muscles. But the most important part is not the protein, but maximizing carbohydrate intake during this time.
The athlete who is training daily, or is in a multiday event, can use this glycogen window to their advantage to get a jump on the normal repletion process and minimize the chance of chronic glycogen depletion (and the fatigue that goes along with it). There is also suggestive evidence that the muscle stiffness that occurs after vigorous exercise is related to muscle glycogen depletion, so rapid repletion may have an added benefit of minimizing this day after effect.