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The Soluble Fiber in Onions

By Sandy Keefe

Although fiber intake in the United States dropped in the early 1900s with the introduction of processed foods, many Americans are becoming increasingly aware of the role fiber plays in heart and bowel health. Dietary fiber comes from plant foods, so dairy and meat products don’t help meet your daily requirements. Fruits, grains and vegetables, such as onions, provide varying amounts of soluble and insoluble fiber.


Soluble fiber grows inside the cells of onions and other plant foods, while insoluble fiber is found in the walls of those plants. When you eat onions, the water, vitamins and minerals from the food flow through the walls of your intestine and into your bloodstream to meet your nutritional needs. Both soluble and insoluble fibers remain inside your digestive tract to serve other purposes. Soluble fiber dissolves in water and then combines with bile acids to produce a gel that’s passed in your stool. Insoluble fiber doesn’t dissolve, but contributes to the bulk and consistency of your stool.

Fiber Content

Although onions may not be the first food that comes to mind when you think about dietary fiber, they are surprisingly high in soluble fiber. A one-half cup serving of cooked onions contains 1.5 g of total fiber, and 1.2 g of that total is soluble fiber.

Heart-Healthy Benefits

As soluble fiber combines with bile acids, your liver uses stored cholesterol to create more bile acids. It then pulls cholesterol from your blood to replace lost stores, reducing both LDL, or “bad” cholesterol, and total cholesterol levels in your blood. Over time, lower cholesterol levels limit the formation of plaque inside your artery walls. Healthier arteries lower your risk factors for coronary heart disease, heart failure, stroke, high blood pressure and heart attacks.

Boosting Fiber Content

Combine onions with other vegetables high in soluble fiber to boost your intake. Choose dark, leafy vegetables, such as kale, collard greens, Swiss chard or dandelion greens. Legumes, including dried peas, beans and lentils, are packed with fiber. Cruciferous, or gas-producing, vegetables, including broccoli, cabbage, cauliflower and Brussels sprouts, offer plenty of heart-healthy soluble fiber.


While the soluble fiber in onions promotes heart health, preparation methods may cancel out some of those benefits. Deep-fried onion rings and onion straws are high-calorie foods made with unhealthy fats that increase your blood cholesterol. Fresh sliced onions are low in calories and fat-free, but a large burger smothered with cheese and onions won’t help your heart.

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