Fiber refers to the parts of plant foods that humans cannot digest. Good sources include bran cereals, beans, vegetables, fruit and whole grains. People who eat a lot of fiber have a better-functioning gastro-intestinal system than those who eat too little. Fiber increases the bulk and frequency of bowel movements, and its lack is a common cause of constipation. It also may help reduce the risk of colon and rectal cancer.
The optimum diet should provide at least 40 grams of fiber a day. You can achieve this by increasing your consumption of fruits (especially berries), vegetables (especially beans), and whole grains. Ready-made cereals can be good fiber sources, but check the labels to make sure they give you at least four, and preferably five, grams of dietary fiber per one-ounce serving.
Unfortunately, for all the good it does, fiber does have the inconvenient and embarrassing side-effect you've been experiencing - flatulence. This occurs for the same reason that beans produce gas: when bacteria in the gut attack and digest these complex carbohydrate molecules, methane gas is released. Individuals differ greatly in their tolerance to the side effects of increased fiber consumption; sometimes, you can avoid it by increasing your fiber intake slowly. Or, if bran gives you gas, eat less of it, take smaller amounts more frequently, or concentrate instead on eating more fruits, vegetables and whole-grain products. Taking bran as a supplement may cause two other problems. It may interfere with the absorption of minerals from foods, and it may impede bowel movements if you do not drink large amounts of water with it. Therefore, I do not usually recommend the use of fiber supplements in the form of pills, powders or straight wheat bran. I do suggest that individuals who are frequently constipated try powdered psyllium seed husks with lots of water.
For most people, the best advice concerning fiber is simply to eat more fresh vegetables and fruits as well as products made from whole grains. If you eat fewer products made from white flour and enjoy more whole-grain foods of all sorts along with your vegetables and fruits, you will not have to worry about getting enough fiber in your diet.
Andrew Weil, M.D.