Is there an optimum diet for the cyclist?

There is overwhelming evidence that adequate dietary carbohydrates are needed for maximum performance. At least 10 grams per kilogram of body weight per day. What is unclear is whether more carbohydrate (beyond 600 to 700 grams per day) will provide additional benefits.

And Fat?
If you are interested in multi-day endurance events, there may be some advantage to several weeks of a moderate fat intake equivalent to 30% of total Calories. But there is no evidence this helps in single day, high performance (%VO2max greater than 60%) activities and there may be long term health consequences. As total Caloric needs increase, the only reason to consider a high fat (more than 15 to 20% of total Caloric needs) diet would be maintenance of a positive Caloric balance IF carbohydrates alone were not meeting the challenge.

And finally, there is NO evidence that more than 2 grams per day of protein are beneficial in endurance, sprint, or power training and performance.

There are three additional practical points for the cyclist to remember.

First, the body's normal liver and muscle glycogen will support the first 1 or 2 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.

Second is that taking in carbohydrates during the event provides an additional source of glucose "fuel" that will extend the length of time before the bonk occurs. This becomes 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.

Finally, eating a high carbohydrate diet for several days prior to the event will maximise your internal glucose (glycogen) stores and will prolong the duration of activity before fatigue occurs.