Most foods that contain carbohydrates can cause gas. By contrast, fats and proteins cause little gas (although certain proteins may intensify the odor of gas).
The sugars that cause gas are raffinose, lactose, fructose, and sorbitol.
- Raffinose — Beans contain large amounts of this complex sugar. Smaller amounts are found in cabbage, Brussels sprouts, broccoli, asparagus, other vegetables, and whole grains.
- Lactose — Lactose is the natural sugar in milk. It is also found in milk products, such as cheese and ice cream, and processed foods, such as bread, cereal, and salad dressing. Many people, particularly those of African, Native American, or Asian background, have low levels of the enzyme lactase needed to digest lactose. Also, as people age, their enzyme levels decrease. As a result, over time people may experience increasing amounts of gas after eating food containing lactose.
- Fructose — Fructose is naturally present in onions, artichokes, pears, and wheat. It is also used as a sweetener in some soft drinks and fruit drinks.
- Sorbitol — Sorbitol is a sugar found naturally in fruits, including apples, pears, peaches, and prunes. It is also used as an artificial sweetener in many dietetic foods and sugarfree candies and gums.
Most starches, including potatoes, corn, noodles, and wheat, produce gas as they are broken down in the large intestine. Rice is the only starch that does not cause gas.
Dietary fiber is carbohydrate that is indigestible in the small intestine and reaches the colon relatively intact. In the colon, certain bacteria digest fiber (fermentation), which produces gas. Dietary fiber can be classified as either soluble or insoluble.
Soluble fiber dissolves in water and becomes a soft gel. It is found in oat bran, beans, barley, nuts, seeds, lentils, peas, and most fruits. Insoluble fiber does not dissolve or gel in water. It absorbs liquid and adds bulk to stool. Cellulose (found in legumes, seeds, root vegetables, and vegetables in the cabbage family), wheat bran, and corn bran are examples of insoluble fiber.
High fiber substances containing both soluble and insoluble fibers have the properties of both. They include oat bran, psyllium, and soy fiber. Methylcellulose is a semi-synthetic fiber. It is soluble and gel forming, but not fermentable.
Types of fiber differ in the speed and extent to which they are digested in the GI tract, and in the process of fermentation. The solubility and fermentation of a particular fiber affects how it is handled in the GI tract. However, the effect of identical fibers varies from person to person.
A gradual increase in dietary fiber can modify and improve symptoms. But individual responses vary and too much of a type of fiber can worsen symptoms. It may be necessary to try different types of fiber. With any dietary fiber it is best to start low and go slow.