Fiber and Obesity

Fiber encompasses both dietary fiber and added fiber. Dietary fiber is defined by the National Academy of Sciences as nondigestible carbohydrates and lignin that are intrinsic and intact in plants, while added fiber includes isolated, nondigestible carbohydrates that have beneficial physiological effects in [Page 276]humans. Sources of dietary fiber include fruits, vegetables, whole grains, and legumes/nuts, while sources of added fiber include fiber supplements found in pill or functional food form. Both types of fiber have beneficial health effects.

Experts suggest that fiber is beneficial in managing chronic diseases such as cardiovascular disease, diabetes, colon cancer, and obesity. The current recommendation for total fiber intake for young men is 38 grams per day and for young women is 25 grams per day. Mean intake for fiber is 16.5 to 17.9 grams per day for men and 12.1 to 13.8 grams per day for women, significantly lower than the recommendations.

There appears to be an association between fiber intake and obesity. Populations that consume higher fiber report lower obesity rates. Additionally, fiber intake is inversely associated with body weight as well as body fat. Controlled studies have examined both individuals consuming whole foods with dietary fiber and individuals on diets with fiber supplements. One study found that supplementing a low-calorie (1,200 kcal/day) diet with four grams of fiber per day produced a significantly greater loss in body weight than individuals without fiber supplements. However, another study using a fiber supplement could not detect any differences in hunger, satiety, or body weight between individuals with or without fiber supplements.

Fiber is thought to reduce body weight through several means. First, fiber-rich foods require increased chewing, which results in saliva and gastric acid production. This saliva and gastric acid production may result in stomach distention (i.e., a feeling of fullness). Additionally, soluble/viscous fibers, which absorb water in the stomach, are thought to delay gastric emptying time and consequently promote satiation and a prolonged feeling of fullness. After a meal, these soluble fibers may suppress glycemic and insulinemic responses, which may reduce the rate of return of hunger. This is also supported by evidence that a large intake of fiber at breakfast results in reduced food intake at lunch.

Fiber may also exert its effect by blocking or limiting the absorption of macronutrients, which would decrease net caloric intake, resulting in weight maintenance. Experts think that fiber effects the secretion of certain gut hormones, such as cholecystokinen (CCK). CCK is secreted after a meal from cells in the upper small intestine, and controls gut motility, gallbladder contraction, and secretion of pancreatic enzymes. Consequently, if fiber can modulate CCK secretion, this will be another means in which fiber promotes weight maintenance.

While evidence suggests that increasing fiber intake is beneficial for individuals seeking to maintain a healthy weight, it is not desirable to consume an excessive amount of fiber. Although no tolerable upper intake level for fiber has been established by the National Academy of Sciences' Dietary Reference Intakes, it is important to note that overconsumption of fiber can lead to decreased availability of certain vitamins and minerals.

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