- Studies suggest that an increase of 5 to 10 g per day in soluble fiber intake—two to four extra servings of fruits and vegetables—reduces cholesterol levels by about 5%.
For many years, fiber (the indigestible component of plant foods) was thought to be useful only for adding bulk to the diet to prevent constipation. But the shift in the diets of Western societies from ones based on whole grains, vegetables, fruits, and legumes to diets based on meats, refined grains, and processed foods has been associated with an increase in the incidence of coronary heart disease as well as diabetes; several studies point to a lack of dietary fiber as a primary cause. Some debate has ensued over whether fiber has a protective effect or is simply a marker for a healthy diet. But in recent studies, fiber has emerged as an independent factor for the prevention of disease.
Both types of fiber—soluble (sometimes called viscous) fiber, which dissolves in water and forms a gel-like substance, and insoluble fiber, which does not dissolve in water—are important for disease prevention. Most plant foods contain some of each type, but often one or the other predominates. Soluble fiber is found in legumes, barley, oats, and most fruits, while wheat and other whole grains and some vegetables contain insoluble fiber.
The two types of fiber exert different effects in the intestine. Soluble fiber binds bile acids and removes them in the stools. By absorbing many times its weight in water, insoluble fiber increases stool bulk and helps wastes pass more easily and rapidly through the digestive tract.
The connection between fiber and heart disease has focused on the effect of soluble fiber on blood cholesterol levels. In the liver, cholesterol is used to make bile acids. Soluble fiber binds with bile acids in the intestines and removes them in the stool. The liver responds by converting more cholesterol into bile acids. The resulting fall in cholesterol in liver cells leads them to take up more LDL (“bad”) cholesterol from the blood.
Studies suggest that an increase of 5 to 10 g per day in soluble fiber intake—two to four extra servings of fruits and vegetables—reduces cholesterol levels by about 5%. Other studies have shown that fiber intake directly affects the risk of fatal and nonfatal heart attacks.
Other research indicates that soluble fiber is more strongly associated with a reduced risk of heart attack and heart disease death than insoluble fiber. However, the effect of soluble fiber on blood cholesterol levels does not fully account for the protective effect of dietary fiber. This finding opens the possibility that fiber may work in additional ways—by affecting the body’s use of glucose and insulin, for example, or by reducing triglyceride levels.
