Fiber could be described as the (incredible!), edible part of the plant food that you cannot digest. Ironically, it is just as important as the part that you can!
Let’s find out why:
Fiber sources: Fiber comes exclusively from plants. Fiber cannot be derived from animal sources. Vegetables, fruits, grains and legumes are the major sources of fiber in our diet. Fiber is classified as a complex carbohydrate, just like the all too familiar starches that most of us consume daily such as bread, potatoes, rice, and so on. However, unlike starches, fiber is resistant to the action of our digestive enzymes, and is hence often referred to as a non-starch polysaccharide (multiple sugar units), or a non digestible carbohydrate.
Because of this peculiar characteristic of fiber, it goes through your digestive tract relatively intact and unchanged, providing important health benefits. That said, the bacteria in your digestive tract do ferment some of the fiber to substances called short chain fatty acids that are important for digestive health, and contribute a small amount of energy as well.
Fiber can be broadly classified into 2 types
Soluble: As implied, soluble fiber dissolves in your digestive fluid, forming a sticky, jello-like mass that can bind with bile, an emulsifying agent that is required for fat and cholesterol absorption. As bile bound to fiber gets excreted through your digestive tract, cholesterol levels decline as there is less bile available for it’s absorption. Blood cholesterol levels drop further as the liver makes more bile from cholesterol to replace the excreted bile. In addition to being viscous (dissolves in water to form a gel), soluble fiber is also fermented by bacteria in the digestive tract.
Soluble fiber is found in oatmeal, barley, legumes, certain fruits such as apples, strawberries and citrus fruits. (Figure 1). The fruits mentioned above, have a form of fiber called pectin that is gel forming and completely metabolized by colonic bacteria. Pectins are isolated and used by the food industry for thickening jelly, adding to jams etc.
Insoluble: As you may have guessed, insoluble fiber does not dissolve in water. This form of fiber is found in wheat bran, whole grains, cereals, vegetables, the outer coverings of seeds and apples, as well as the skins of root vegetables. (Figure 2). Think of how cereals containing bran remain unchanged when a liquid such as water or milk is added, whereas oatmeal gets “sticky” when cooked in water or milk. What is interesting about insoluble fiber is that it can:
- Speed up the passage of partly digested food through your digestive tract.
- Increase fecal weight ( adds bulk to stool).
Both these properties can help relieve constipation. In fact, wheat bran has a significant laxative effect as it can absorb three times it’s weight of water producing a softer, bulkier stool. Fruits and vegetables are high in soluble fiber, which can trap nutrients and delay the rate at which they leave the stomach for the small intestine. This in turn can reduce spikes in your blood sugar after a meal, and a subsequent release of insulin from your pancreas.
Degree of processing: The size of food particles and the degree to which they are processed, also impacts the intestinal response to the fiber one consumes. For e.g. coarsely ground bran has a higher water holding capacity than finely ground, thus promoting larger stools. It also speeds up the passage of semi-digested food through your gut. Including intact grains rather than pulverizing them in a mill helps significantly with delaying gastric (stomach) emptying and can promote an increased sense of fullness, helping you with your weight loss efforts if that is your goal.
Include a variety of foods with both soluble and insoluble fiber: Many foods contain both soluble as well as insoluble fiber. For e.g., the skin of the apple contains a form of fiber called cellulose, which is insoluble, while the inside of the apple contains the soluble fiber, pectin. Clearly, we need both soluble and insoluble fiber to tap into fiber’s innumerable health benefits. Please stay tuned to next week’s post as we explore the “whole truth” behind fiber’s wondrous properties.