Dietary fiber is divided into two general categories based on how it mixes with water: soluble and insoluble. Most of the health benefits associated with eating dietary fiber, such as keeping blood cholesterol and glucose levels from rising too much, are related to soluble fiber such as pectin. On the other hand, water-insoluble fiber types are helpful for digestion and combating constipation. Most plant foods contain a combination of soluble and insoluble fibers.
Insoluble fibers are primarily what give plants their cell structure. Herbivores such as cows are able to digest and metabolize insoluble fibers into glucose sugar, which they use for energy, but human digestive tracts break down only a very small percentage of insoluble fiber. Consequently, between 70 and 90 percent of insoluble fiber passes through your body completely undigested. Insoluble fibers don’t dissolve in water, but they attract water in your intestines, which bulks up your stool, promotes regular bowel movements, combats constipation and reduces the risk of hemorrhoids. Foods rich in insoluble fiber include flaxseed products, whole-grain cereals and breads, nuts and vegetables such as celery, broccoli and carrots.
Lignin is a type of insoluble fiber that fills the spaces within the cell walls of plants. It provides strength to cell walls and is important for conducting water in plant stems. Lignin is completely indigestible by animal enzymes, although some fungi and specialized bacteria can degrade it. All lignin passes through your body completely intact. For comparison, over 90 percent of water-soluble pectin is digested in your large intestine by “friendly” bacteria.
Cellulose is the main structural component of plant cell walls and algae, which makes it the most common organic compound on Earth. Cellulose is primarily responsible for bulking up the stool because it strongly attracts water from the intestinal mucous membranes. Perhaps surprisingly, your body can digest between 30 and 50 percent of cellulose fiber, so it’s a source of some usable energy. On the other hand, the slow bacterial fermentation of cellulose in your large intestine often causes gassiness and bloating.
Hemicellulose is a category of insoluble fiber that includes xylan, arabinoxylan and xyloglucan. Hemicellulose fibers run next to cellulose fibers in plants. As a group, hemicellulose is between 50 and 80 percent digestible by people, depending on the amount and efficiency of the friendly intestinal bacteria.
If you increase the amount of insoluble fiber in your diet, don’t forget to drink more water. Ironically, consuming too much fiber without drinking enough water actually causes constipation, which is what most people are trying to avoid by eating fibrous foods.
- Encyclopedia of Human Nutrition; Benjamin Caballero et al.
- Biochemical Pathways; Gerhard Michal
- Principles and Practice of Phytotherapy: Modern Herbal Medicine; Simon Mills and Kerry Bone
Sirah Dubois is currently a PhD student in food science after having completed her master's degree in nutrition at the University of Alberta. She has worked in private practice as a dietitian in Edmonton, Canada and her nutrition-related articles have appeared in The Edmonton Journal newspaper.