why cant I eat fiber?? 
Fiber - comes only from plants: Leafy vegetables and grains. Great for your digestive system: it can't be digested and any fiber ingested goes out the chute; acts as a medium for other waste as it goes through your chute. Fiber, ideally, should be a significant portion of your diet because it maintains colon health. Vegetarians have the best digestive system and they are "regular" and usually don't have to wipe.
Fiber fills your stomach and there is very little energy from plants from a energy to weight ratio. Because it fills your stomach, your body will start digesting whatever is in it BUT there isn't very much energy from plants. And because energy is spent digesting, you'll have less energy to do other things (like playing the piano).
I don't know why some of you mentioned constapation with fiber and ingesting it before a performance. Fiber makes your bowel movements regular. Constapation would occur because with the increase of fiber, there was not enough intake of water. But digestion takes approximately 10 hours from one end to the other so I do not see how eating brocoli before a performance would cause constapation.
Protein provides energy but protein is the most difficult to digest because of its structural complexity. Monosaccharides are the easiest to digest and derive energy from but are only good for very short term energy. Disaccharides and Polysaccharides (complex sugars) will provide a more even release of energy. Scientists have an Index for it: the Glycemic Index - a scale of 1-100 with 100 being the highest glycemic level. Depending of the intended performance, the foods should be chosen to aide in it. The lower the index, the longer the sustained energy. The higher, the more easily burned.
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What you eat is important but HOW you eat it is just as important. Do you chew thoroughly? If you don't (almost all of us don't) then it takes more time to digest and/or it won't digest completely. If you eat during a performance, you should eat only
small bites - one or two - of food at a time distributed evenly throughout the performance. Having less food in your stomach increases the thoroughness of digestion.
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Another factor about energy is that energy for your brain is the least effective when taken orally. THe best source of energy comes from certain fat stores. Since playing is as much about how your brain functions, it's best to supply your brain with the most effective energy source. Unfortunately, this source is also the most difficult to tap. - Sorry, but I have no idea about how to tap into it but for me, it starts tapping late at night when I'm in bed my mind starts running on and on and my body is not tired. This is also the time that I am most creative and mind most active and the time I also want to practice the most. Unfortunately, it's 2am and I'm not supposed to wake people up so early.
BUT! If you really want to experience this "enlightenment" (It really is an enlightenment - just read on), stop eating. Water is acceptable as it is not food. You go on a fast for a few days. Because your body is not intaking energy, you'll burn all of the easy access energy thus relying mostly on your energy stores. You will feel like crap for the first two or three days and hungry all the time but after this time, you won't feel hungry at all. Instead, you'll see things so clearly and you are totally relaxed. Stress hormones have stopped running through your system so you wont feel any stress. Things are so bright and clear you'll be amazed - this will be the most clear point in your entire life. You'll feel as if you've reached a level of enlightenment... then once run out of your stores, then your body starts to shut down. At this point, START EATING! Otherwise, you'll die of starvation. Keep in mind that once you start eating, you'll end up gaining more weight than before you started the fast as your system has tuned itself to store as much energy as possible - eating an apple is like eating two or three apples. But this effect will go away in a few weeks as long as you don't go on another fast.
I'm sure there are other ways to to the above, like yoga or meditating or something, but I've never tried yoga and the last time I tried meditating was half a lifetime ago and it wasn't as enlightening.
Topic: What to eat before a performance