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All About Fiber

What's Going On In Your Colon?

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Updated March 26, 2014

In this series about understanding carbohydrates, we have been focusing on what happens in our digestive systems as we eat various types of carbohydrates. We will continue this theme when it comes to fiber. Most fiber is, like other carbohydrates, made up of many glucose molecules. However, fiber does not break down into glucose before it gets to the colon, and often not even there. Even so, fiber does have effects on our digestion all along the way.

Need to review the basics about fiber? Check it out here.

The stomach: In the stomach, fiber is bulky, so it tends to make us feel full. However, insoluble fiber moves out of the stomach fast unless there is fat, protein, or soluble fiber to slow it down. Soluble fiber, especially the viscous types that hold onto water, will slow down stomach emptying, especially when eaten with lots of fluid and some fat. This is at least partly why soluble fiber tends to decrease the glycemic effect of a meal - the contents of the stomach more gradually enter the small intestine, and from there, the blood.

The small intestine: In the small intestine, it's a similar situation - the presence of insoluble fiber tends to speed "transit time" up, and the gel-like soluble fiber slows things down.

The colon: As we have seen in the other parts of this series, in the colon there is a whole other digestive world happening with the (mostly friendly) bacteria in the colon.

Life in the Colon

It's common to think of the colon as a place where water is removed from whatever is left from digesting the food, and the rest is moved along towards the toilet. But there is actually a whole world in our guts, occupied by ten times the bacteria as the numbers of all of our human cells (this includes all bacteria from the mouth to the anus). We literally could not stay alive if it wasn't for the wonderful friendly bacteria in our digestive systems, where battles are fought, helpful substances are manufactured, and the immune system is bolstered. Did you know that in "Colon World":
  • Vitamins are constructed (particularly Vitamin K and some B vitamins)
  • More minerals are absorbed into the bloodstream
  • Friendly bacteria crowd out the ones that cause disease, such as Salmonella
  • Friendly bacteria lower the levels of some toxins, such as ammonia
  • Special fats, called short-chain fatty acids, are manufactured, most of which are absorbed into the bloodstream, but some are used to feed the cells of the colon.
  • The health of colon cells, which turn over rapidly, is for the most part dependent upon the bacteria of "Colon World", which in turn is dependent upon the food we give these bacteria.
It is the short-chain fatty acids which are getting the most attention recently. It is difficult to get these in our food, so the body relies on the process going on in "Colon World" to make these fats for us. Evidence is building that they are important in keeping the cells of the colon healthy and preventing such conditions as ulcerative colitis, colon cancer, and diverticular disease. They may also help regulate cholesterol and even, to some extent, insulin responses.

What Types of Fiber Feed the Colon?

The fiber types that are most amenable to fermentation are the soluble ones - gums, pectins, etc, found in such foods as berries, beans, flax seeds, plums, apples, and oats, and in some fiber supplements, such as those using psyllium and guar gum. Oligosaccharides and resistant starch also provide fodder for the bacteria. Different "bacteria food" produces different kinds of SCFAs and other products, so it's important to get a variety of fibers in our foods.

Insoluble fiber (found in such foods as vegetables, the bran of grains e.g. wheat bran, nuts, and seeds) isn't available for much fermentation, but it is still important in the colon. Not only does it provide bulk in the stool, its tendency to "speed things along" means that the fermentation will take place all along the length of the colon, including the near the end, where the majority of colon cancer occurs. Without insoluble fiber, most of the fermentation would take place in the top part of the colon, so the colon cells there would get most of the benefit.

New Recipe: Flax Bread with Resistant Starch has lots of soluble and insoluble fiber plus resistant starch.

What are the other benefits of dietary fiber?

Besides reducing the glycemic effect of meals and contributing to colon health, there is evidence that fiber may benefit us in other ways. It seems to help lower cholesterol and triglycerides, and also may help to prevent:
  • Ulcers, particularly in the beginning of the small intestine (duodenal ulcers)
  • Diabetes
  • Heart Disease
  • Cancer
References:
  • Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients) (2005), Food and Nutrition Board, National Academy of Sciences
  • More References For This Article and This Series

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