Irritable Bowel Syndrome Message Boards | How do certain fiber supplements work?

Zara - you have asked a very interesting question - sorry to see no one has answered. Let me see if I can explain why some fibers gel and others don't.

Total fiber in the diet is no longer consider to be just SF and IF. It is now defined as the sum of the dietary fiber and functional fiber. Dietary fiber is carbohydrates and lignin that are intrinsic and intact in plants. Functional Fiber consists of isolated, nondigestible carbohydrates that have beneficial physiological effects in humans. Typically they act like SFs. The thing all fiber has in common is that is undigestible by humans and passes throught the stomach and small intestine basically unchanged. However, when it gets to the large bowel it can be fermented by colonic bacteria.

All soluble fiber and most functional fibers such as Benefiber bind or dissolve in water. However, some fibers can only bind about twice their weight in water while others can bind more than 10 times their weight in water. The greater the water binding properties of a SF the more viscous the water in which it is dissolved becomes. Also, the amount of water a SF binds is dependent on other factors such as pH, temperature, ion concentrations, etc. For example, fruit pectin, a SF, can bind a lot of water and it becomes very vicous like you see when you use it make jams and jellies. Other SFs like Benefiber don't bind as much water and don't change the viscosity of the fluid in which they are dissolved. In fact, many functional fibers are design specifically to have more or less water binding properties depending on the application they are being designed to be used in.

In summary the differences between the effect that various SF have on the water in which they are dissolved depends on the type of carbohydrates in the SF and the other ingredients in the water. Adding 5 grams of one type of SF to a glass of water may not make a noticeable change in viscosity while adding 5 grams of another type of SF may produce a gel. However, in both instances you will get 5 grams of SF that will pass through the stomach and small intestine to the colon where it is fermented by the colonic bacteria.

There is an excellent discussion, albeit a bit technical, on how fibers change the viscosity of water in this write-up

Good luck

Reference
Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids) Food and Nutrition Board (FNB)

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