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Carbohydrates and Fiber in Onions

by
author image Dr. Tina M. St. John
Tina M. St. John runs a health communications and consulting firm. She is also an author and editor, and was formerly a senior medical officer with the U.S. Centers for Disease Control and Prevention. St. John holds an M.D. from Emory University School of Medicine.
Carbohydrates and Fiber in Onions
Three large onions on a cutting board. Photo Credit volgariver/iStock/Getty Images

With more than 500 varieties, onions are among the top 10 vegetable crops in the world. Color is only one of the characteristics that distinguish different varieties of onions, which are grouped broadly into fresh and storage categories. All onion varieties contain similar concentrations of carbs, including sugars and fiber. The taste of different onions depends on the sulfur, sugar and water content.

Types of Carbs

All carbohydrates consist of sugar, or saccharide, molecules in different structural formations. Single or paired saccharide molecules have the common designation of "sugars." Glucose, fructose and lactose are examples of sugars. Starches are long chains of glucose molecules that break down into sugar in your intestines. Fiber is a third type of carb that differs significantly from the other two because your body cannot digest fiber into its component sugar molecules. Fiber passes through your digestive system intact. Onions contain carbs in the form of sugars and fiber; they do not contain starch.

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Sugars in Onions

Onions contain the sugars glucose, fructose and sucrose. The presence of these sugars is why onions caramelize when you cook them slowly over low heat. Because sugars accumulate as onions mature, green onions, also known as spring onions, contain less sugar than mature onions. Yellow, red and white onions that you find at the market are mature onions. A cup of chopped, green onions contains approximately 2.3 g of sugar, compared with a cup of chopped, mature onions, with 6.8 g of sugar. Like other nonstarchy vegetables, onions will not cause a sudden rise in your blood sugar levels because the fiber content slows sugar digestion and absorption.

Fiber in Onions

Onions are a good source of dietary fiber, especially soluble fiber. This type of fiber reacts with water and forms a gel, which absorbs cholesterol in your intestines. The fiber-cholesterol complex is excreted in your stool, helping to lower your blood cholesterol level. A cup of chopped green or mature onions contains approximately 2.7 g of fiber.

Sweet Onions

Sweet onion varieties, such as Walla Walla, Vidalia and Maui, have a milder, sweeter flavor than other onions, but they do not contain significantly more carbs. Sulfur-containing chemicals impart onions with their pungent, hot flavor. Sweet onions contain a lower concentration of these chemicals and have more water than other onion varieties, resulting in a more mild flavor. The thin skin on sweet onions also distinguishes them from yellow, white and red storage onions. Unlike these common onions, which can be stored at room temperature, sweet onions are best kept in your refrigerator.

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