Onions are pungent, flavorful vegetables that add several key nutrients to your meal, while providing minimal calories. They are full of powerful antioxidants that help ward off harmful free radicals, which are damaging compounds that feed on healthy cells. Onions are also full of fiber, particularly soluble fiber; while beneficial, this fibers slows digestion. Enjoying onions in small portions may help prevent gas or other digestive problems associated with eating onions.
Types of Fiber
Fiber is a type of indigestible carbohydrate that travels through your digestive tract relatively intact. Foods rich in fiber -- like fruits, vegetables and whole grains -- provide soluble and insoluble fiber. While certain foods may be higher in one over the other, most of these foods have some of each type of fiber. Onions have higher concentrations of soluble fiber. This type of fiber attracts water in your gut, creating a gel-like substance that slows digestion. Onions also have small amounts of insoluble fiber, which stays intact through your digestive tract and sweeps out waste.
Fiber Content
For a healthy digestive tract, you need to consume 14 g of dietary fiber for every 1,000 calories you eat, says the Linus Pauling Institute. For example, if you normally have around 2,000 calories per day, make sure you include 28 g of fiber in your diet. Fiber content in onions varies slightly depending on the variety. A 1/4 cup serving of green scallions provides .7 g of fiber, 1/2 cup of raw red onions offers 1 g and 1/4 cup of sauteed yellow onions contains .4 g of fiber. Since almost all of the fiber in onions is soluble, onions are sure to slow down your digestive tract.
Digestive Effects
Soluble fiber in onions is in the form of fructans, a type of fiber that is beneficial in preventing certain colorectal cancers, says Erin Peabody of the Agricultural Research Service. While slowing digestion, soluble fiber allows nutrients to fully absorb and helps keep your bowel movements regular. On the other hand, soluble fiber ferments in your colon, the lower part of your intestines. This normal fermentation feeds and benefits the normal flora of bacteria in your gut. During fermentation, you may experience gas, bloating or cramping, which should resolve on its own.
Additional Considerations
Storage of onions may alter the fiber content slightly. Researchers at the University of Wisconsin-Madison published a study in the "Journal of AOAC International" in 2000 looking into fiber content of onions after storage. According to the study, onions in the United States are harvested once per year and then stored and distributed for 12 months. Researchers observed that after normal yearlong warehouse storage, insoluble fiber in Spanish onions increased slightly. While insoluble fiber may increase over time, onions continue to have high levels of soluble fiber, thus slowing digestion.
References
- National Digestive Diseases Information Clearinghouse; Gas in the Digestive Tract; January 2008
- Linus Pauling Institute; Fiber; Jane Higdon; December 2005
- Agricultural Research Service; Onions with a Nutritional -- Not Pungent -- Punch; Erin Peabody; March 2007
- "Journal of AOAC International"; Changes in Content and Composition of Dietary Fiber in Yellow Onions and Red Delicious Apples During Commercial Storage; J.A. Marlett, et al.; July-August 2000
- USDA National Nutrient Database for Standard Reference
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