Fiber is material made by plants that’s not very well digested by people. Some types of fiber are a little better digested than others, but on the whole, most fiber passes through your gastrointestinal system intact. Dietary fiber is recommended by nutritionists because it's strongly linked to many health benefits. Gas or bloating is a common side effect of high-fiber diets.
Dietary fiber is classified as soluble or insoluble, which means it either dissolves in water or it doesn’t. Insoluble fiber, often referred to as cellulose or simply “roughage,” is the least digestible type of fiber. A small percentage of cellulose is broken down in your large intestine by friendly bacteria via the process of fermentation, but the vast majority of it passes through your system undigested. Benefits of eating insoluble fiber include cleansing the intestinal tract and combating constipation by promoting regular bowel movements. In contrast, soluble fiber such as pectin dissolves in water and becomes sticky in the intestines, which leads to attachment and removal of toxins and excess cholesterol.
Intestinal gas and flatulence are common side effects of high-fiber diets. The gas, which is mainly carbon dioxide, is a byproduct of bacterial fermentation of fiber. The better digested the fiber, the more gas that’s produced. This depends on many factors, the main one being the amount of time the fiber spends in the large intestine -- the longer the time, the longer the fermentation process. Other factors include the bacteria’s ability to ferment fiber, the type of fiber and individual differences. For example, some people just naturally deal better with high-fiber diets and experience fewer symptoms.
Daily fiber recommendations for adults range from about 21 to 38 grams, depending on gender, body size, caloric intake and age. Fibrous vegetables such as carrots, broccoli and spinach are rich sources of cellulose, whereas wheat bran, legumes and fruits such as apples and cherries are rich sources of pectin and other soluble fibers.
Insoluble fiber doesn't dissolve in water, but it attracts water in the large intestine and bulks up the stool. Drinking plenty of water with a high-fiber diet is a must because a lack of water in your system leads to constipation, reduced intestinal motility and more fermentation and gas production. Intestinal gas is also caused by certain chemicals in vegetables, especially cabbage, cauliflower, broccoli and beans. Eating too much fruit can lead to bloating because intestinal bacteria also ferment fruit sugar or fructose. Chemical residues on the outside of fruit and vegetables cause gastrointestinal problems as well. Mixing fibrous foods with dairy products might make you gassy if you’re lactose intolerant, which is the inability to digest milk sugar or lactose.
- Human Metabolism: Functional Diversity and Integration; J. Ramsey Bronk
- Contemporary Nutrition: Functional Approach; Gordon M. Wardlaw et al.
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.