Soluble fiber is one of the two types of fiber found in the plants we eat. Unlike its counterpart, insoluble fiber, soluble fiber dissolves in water, which means it creates a gel-like stool that slows digestion and affects the absorption of certain nutrients, including fat. A balanced diet includes a healthy portion of both types of fiber, which can be found in a variety of fruits, grains and vegetables.
All the plants that humans consume contain a mix of soluble fiber and insoluble fiber, although most plants typically contain more of one type than the other. Human beings cannot digest either type of fiber, which helps create a bulkier stool that moves easily through the intestinal tract. Insoluble fiber remains relatively intact throughout the digestive process because it does not break down in water, which makes it the most beneficial fiber type for creating a healthy stool. Soluble fiber does break down in water, which results in a gelatinous stool that moves more slowly through the intestines and binds with other nutrients so that they are not digested, or absorbed, into the body. This binding process is what affects the absorption of fat.
Several studies in the latter part of the 20th century found that consuming fiber does affect fat consumption. One such study published in 1992 in "The Journal of Nutrition" found that increasing fiber intake increased the amount of fat secreted in the stool. This happens because soluble fiber binds to bile acids that are secreted into the small intestine by the gallbladder to help break down fat. The fiber and bile acids then get flushed out of the system during a bowel movement. This ultimately results in less fat being absorbed into the body, as the bile acids are not present to break down the fat for absorption.
The stool created by soluble fiber can also affect the absorption of other nutrients, which in most cases is beneficial. For example, soluble fiber can prevent excess sugar and starch from being absorbed, which can help regulate your blood sugar levels and control cholesterol. Lower cholesterol levels can help prevent stroke and problems with your heart. Decreased levels of sugar, also known as glucose, in the blood can help prevent or control symptoms of diabetes. Fiber can affect the absorption of helpful minerals, such as zinc, iron and magnesium, but it is not typically cause for concern if you are receiving enough minerals in your diet. Most fiber-rich foods are also high in mineral content.
The Harvard School of Public Health recommends children and adults consume between 20 and 30 grams of both types of fiber -- commonly referred to as dietary fiber, preferably in the form of whole foods. Soluble fiber content is high in oatmeal, lentils, nuts, beans and fruits such as apples and blueberries. Insoluble fiber is more prevalent in whole wheat and grains, as well as brown rice, legumes, and vegetables such as carrots, tomatoes and cucumbers. Fruit peels are also high in insoluble fiber. If you are just starting to add more fiber to your diet, gradually increase your intake until you reach the recommended daily amount, as adding too much fiber too quickly to your diet can cause bloating and gas. Drink plenty of water to avoid constipation.