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There seems to be a lot of misunderstanding about the benefits of fiber. This is because the dozens of research studies that have been published on fiber have given us mixed messages. Recently, though, a consensus seems to have emerged around a few basic facts. These make it easy to separate the wheat from the chaff.
The most important finding that is consistent across all studies is that people who eat more fiber-rich foods get fewer heart attacks. They also have less risk of developing diabetes and colon cancer, and over time they are about ten pounds lighter.
Lentils and beans, nuts, fruits and vegetables are the best sources of fiber. The problem is that they are also rich in antioxidants, vitamins, lignans, minerals and phytoestrogens, so no one knows for certain that it is the fiber that is doing all that good.
Here's why: these foods contain two kinds of fiber - soluble and insoluble - that have different effects in the body. Soluble fiber lowers cholesterol, but insoluble fiber does not. Soluble fiber also lowers blood sugar much more than the insoluble kind. But the benefits of high-fiber foods are the same no matter what kind of fiber they provide.
The colon cancer connection is equally confusing: people who eat a lot of high-fiber foods have 40% less risk of colon cancer, but in two clinical trials, feeding people extra fiber did not cut their risk of developing colon polyps at all.
Another source of confusion is so-called fiber foods. Yogurts, juices and many prepared foods claim to be high in fiber, but it is usually something called inulin. This is a soluble fiber, but it is not viscous enough to stick to sugar or cholesterol, so it doesn't have much benefit at all. It may be helpful because it feeds probiotic bacteria that improve gut health, but this has never been studied.
So for now, the best advice is to avoid taking extra fiber. Just eat more high-fiber foods. They seem to have a lot of other things going for them too.