The Case for Fiber

Fiber, fiber, fiber—all we hear about is fiber. What is fiber anyway and why is it so important? How do we make sure we’re eating enough fiber?

Fiber is basically the structure of plant material. Due to its complex structure, fiber passes right through our small intestine without being digested or absorbed. The most common forms of fiber come from green vegetables and whole grains. Typically, vegetables that grow above the ground like wheat, corn and oats have more fiber than vegetables that grow below the ground like potatoes and rice. After all, they have to stand up. This requires structure. Since the walls of fiber contain complex structures like cellulose that evade enzymatic digestion, fiber passes right through the small intestine intact and then enters the colon for elimination.

The colon functions much better if it contains a healthy amount of fiber. There is less spasm in the walls of the colon and things pass through much easier. As a result, there is much less constipation and less diverticulosis. We also develop fewer colon polyps if we have a healthy intake of fiber.

Once in the colon, fiber comes in contact with the trillions of bacteria that inhabit it. Some of us have bacteria (Firmicutes) that have the ability to digest some of this fiber. Unfortunately, the bacteria digest the fiber into fatty acids that can then be absorbed through the wall of the colon and into the circulation. This is another potential cause of weight gain. Some patients actually can gain weight by consuming too much fiber. Most of us cannot do this though and fiber is very healthy for us as a result.


Fiber is not calorie-free. It can be a significant source of calories.

All fiber sources are not the same. They can be divided into two categories: soluble and insoluble. The can also be divided on the basis of fermentability by bacteria. Soluble fiber is more likely to be fermentable, whereas insoluble is most likely not fermentable. Therefore even though all forms of fiber are nutritionally good for you, soluble fiber tends to interact with the bacteria in your colon more whereas insoluble fiber does not. Soluble fiber can also lower your cholesterol, whereas insoluble fiber less so. It is mostly a bulk agent and assists you with your bowels.

So from a health point of view, soluble fiber wins out and is better for you. If you are just looking to improve your bowels, insoluble fiber may be preferable since it is less gassy. What are the sources of soluble and insoluble fiber? I’ve listed them below. You will note that some items exist in both categories.

Soluble fiber sources

  • Legumes (beans and peas)
  • Oats
  • Rye
  • Barley
  • Fruits: prunes, plums, berries, bananas, and the insides of apples and pears
  • Vegetables such as broccoli
  • Root vegetables such as sweet potatoes and onions
  • Psyllium
  • Flax seeds
  • Nuts, with almonds being the highest in dietary fiber

Insoluble fiber sources

  • Whole grain foods
  • Wheat and corn bran
  • Legumes such as beans and peas
  • Nuts and seeds
  • Potato skins
  • Vegetables such as green beans, cauliflower, zucchini
  • Some fruits including avocado and unripe bananas
  • The skins of some fruits, including kiwifruit and tomatoes


All fiber is not equal.

If you are trying to lose weight, then the best form of fiber to eat comes from green vegetables! Another reason why “V” is the mainstay of the PVC diet! Veggies have much fewer calories than whole grains. Remember, if you’re trying to lose weight, use green vegetables as your major source of fiber rather than whole grains.

How much fiber is the right amount?

This is a common question I am asked. My patients all want to know how many grams of fiber to eat. I usually tell them to forget the grams and look at their stool. (I can’t believe how many people don’t do this).

If you are passing large formed stools, then you are probably eating enough fiber. If instead you are passing skinny narrow stools or ribbons and balls (scyballous stools), then you are not eating enough fiber and need to increase it. Remember, diverticulosis is a common problem in people who do not ingest enough fiber. You can avoid getting them by starting early and eating fiber every day.