A third type of cardiovascular benefit from bananas involves their fiber content. At about 3 grams per medium banana, we rank bananas as a good source of fiber. Approximately one-third of the fiber in bananas is water-soluble fiber. For one medium-sized banana, this amount translates into 1 gram of soluble fiber per banana. Soluble fiber in food is a type of fiber especially associated with decreased risk of heart disease, making regular intake of bananas a potentially helpful approach to lowering your heart disease risk.