Fiber 101: Soluble Fiber vs Insoluble Fiber (2013 Food List Update)

We all know the benefits of fiber! Fiber not only promotes gut health, but also helps reduce the risk of developing chronic gastrointestinal diseases, such as constipation, hemorrhoids and diverticulosis. A high-fiber diet is also linked to a lower risk of developing cancers, especially colon and breast cancer. In addition, it helps lower LDL cholesterol and total cholesterol. Furthermore, high-fiber foods generally have a lower glycemic index value, an important element in managing Type 2 diabetes.

Types of Fiber: Soluble and Insoluble

Both soluble and insoluble fiber are undigested. They are therefore not absorbed into the bloodstream. Fiber content is often listed under “Total Carbohydrate” on a Nutrition Facts label. Because it is mostly undigested, it provides anyway between 0 to 2 calories per gram. Soluble fiber forms a gel when mixed with liquid, while insoluble fiber passes through our intestines largely intact.

Insoluble Fiber

Functions

• move bulk through the intestines
• control and balance the pH (acidity) in the intestines

Benefits

• promote regular bowel movement and prevent constipation
• remove toxic waste through colon in less time
• help prevent colon cancer by keeping an optimal pH in intestines to prevent microbes from producing cancerous substances

Top Insoluble Fiber Foods

• Wheat bran, 11.3 grams of insoluble fiber per 1/2 cup
• All Bran cereal, 7.2 g per 1/3 cup
• Most beans (1/2 cup)
  • Kidney beans, 5.9 g
  • Pinto beans, 5.7 g
  • Navy beans, 4.3 g
• Lentils, 4.6 g per 1/2 cup
• Shredded Wheat cereal, 4.5 g per cup
• Most whole grains. Bulgur, for instance, contains 4.2 grams of insoluble fiber in 1/2 cup
• Flax seeds, 2.2 g per 1 tbsp
• Vegetables (1/2 cup)
  • Okra, 3.1 g
  • Turnip, 3.1 g
  • Peas, 3 g

Soluble Fiber

Functions

• bind with fatty acids
• prolong stomach emptying time so that sugar is released and absorbed more slowly

Benefits

• lower total cholesterol and LDL cholesterol (the Bad cholesterol) therefore reducing the risk of heart disease
• regulate blood sugar for people with diabetes

Top Soluble Fiber Foods

• Purple passion fruit, 6.5 g of soluble fiber per 1/2 cup
• Psyllium husk powder, 3 g per 1/2 Tbsp
• Metamucil powder, 2.4 g per serving
• Oat/Oat bran, 2.2 g per 3/4 cup
• Beans (1/2 cup)
  • Black beans, 2.4 g
  • Navy beans, 2.2 g
  • Kideny beans, 2 g
• Soy
  • Tofu, 2.8 g per 3/4 cup
  • Edamame, 1.5 g per 1/2 cup
• Vegetables (1/2 cup)
  • Avocado, 2.1 g
  • Brussels sprouts, 2 g
  • Sweet potato, 1.8 g
  • Asparagus, 1.7 g
  • Turnip, 1.7 g
• Fruit
  • Dried figs, 1.9 g per 1/4 cup
  • Orange, 1.8 g, medium size
  • Fruit with skin, like pear, apricots, and nectarine, ~ 1.2 g
• Flax seed, 1.1 g per 1 tbsp

An Ideal Ratio of Soluble and Insoluble Fiber?

An average diet contains 1:3 soluble fiber to insoluble fiber ratio. In a small study published in 2017, UC Davis researchers compared the difference of 3:1 soluble to insoluble fiber ratio versus a 1:3 ratio. They tested on satiety, glucose response, insulin response, and more. Results showed that 3:1 soluble to insoluble fiber diet significantly increased satiety and decreased hunger. In addition, insulin response was the lowest in the 3:1 soluble to insoluble group. In other words, the body responded to the diet with secreting less insulin, suggesting that insulin sensitivity may have been improved. This result suggested that a greater proportion of soluble fiber may have better metabolic benefits.

Despite the above results, don’t worry too much about choosing a specific type of fiber. Many foods such as oat, oat brans, psyllium husk and flax seed are rich in both insoluble and soluble fiber. Eating enough fiber is more important! The recommended intake of fiber is 25 g to 38 g per day. If you eat at least 5 servings of fruits & vegetables as well as at least 6 servings of grain products per day (at least 3 of which are whole grains), you are very likely meeting the fiber requirements.

Reference: Fiber Content of Foods in Common Portions by Harvard University Health Services, dated May 2004, archived here.