Review articleDietary fiber and body weight
Introduction
The increasing prevalence of obesity in the United States population and associated morbidity compels us to develop dietary strategies to combat the problem. Although it is well known that excess calorie intake is the primary dietary cause, alterations in food patterns or nutrients must be considered. Dietary guidance universally recommends diets higher in fiber for health promotion and disease prevention, but there are inconsistencies in the literature on the relation of dietary fiber to body weight. The first goal of this review is to describe the challenges in measuring dietary fiber intake in studies and describe recent developments in dietary fiber definitions and recommendations. Second, epidemiologic studies on the role of fiber in weight maintenance will be summarized. Third, intervention studies on dietary fiber and weight loss will be presented. Fourth, potential mechanisms for how dietary fiber may aid weight loss will be described. The current obesity epidemic in developed countries demands that we design diets to improve weight loss and maintenance. In addition, the popularity of low-carbohydrate diets that are low in dietary fiber must be considered in this discussion.
Section snippets
What is dietary fiber?
Examining the relation between dietary fiber intake and body weight is difficult because we struggle to define dietary fiber and agree on recommended intake levels. New definitions for dietary fiber and recommendations for fiber intake were published as dietary reference intakes [1]. Dietary fiber consists of non-digestible carbohydrates and lignin that are intrinsic and intact in plants. Functional fiber consists of isolated, non-digestible carbohydrates that have beneficial physiologic
Fiber levels of popular weight-loss diets
In general, dietary fiber intakes are linked to total food intake and thus calorie intake. Fiber recommendations published in the dietary reference intakes are related to calorie intake and explain why the recommendation for men, 38 g/d, is higher than the recommendation for women, 25 g/d. Previous recommendations suggested that Americans consume 10 to 13 g of fiber per 1000 kcal consumed [7].
The fiber levels of popular diets have been reported by Anderson et al. [8]. When the dietary fiber
Epidemiologic studies on dietary fiber and weight
The “fiber hypothesis” suggests that consumption of unrefined, high-fiber carbohydrate-based foods protects against industrialized diseases such as diabetes, cancer, heart disease, and obesity. These traditional diets are high in dietary fiber and low in dietary fat, making it difficult to separate out these effects. Roberts and Heyman [9] suggested that that too often there is an overemphasis on dietary fat. Dietary factors less frequently examined for their role in obesity include fiber,
Fiber and body mass index
In cross-sectional observational studies, fiber intake is inversely associated with body weight [12], [13] and body fat [14]. In the Seven Countries Study, dietary fiber was significantly inversely associated with subscapular skinfold thickness [15]. Few studies have compared different fiber types and their association with body weight. In a longitudinal study, fiber intake was inversely associated with body mass index at all levels of fat intake after adjustment for lifestyle factors and other
Fiber and weight loss
Many intervention studies have investigated whether dietary fiber intake assists in weight loss and results have been inconsistent. Birketvedt et al. [19] found that the addition of dietary fiber to a low-calorie diet significantly improved weight loss, with the placebo group losing 5.8 kg and the fiber-supplemented group losing 8.0 kg. When postmenopausal women consumed higher-fiber diets as part of a study of very low-fat diets and weight loss, the higher-fiber diets were associated with
Does dietary fiber promote satiation?
Although emphasis has been placed on specific effects that can be detected as statistically significant when a particular fiber source is consumed, dietary fiber has many subtle, less easily quantifiable effects that are beneficial. A fiber-rich meal is processed more slowly and nutrient absorption occurs over a longer period [29]. Further, a diet of foods that provide adequate fiber is usually less energy dense and larger in volume than a low-fiber diet that may limit spontaneous intake of
Does dietary fiber alter glycemic or insulin response?
When viscous fibers are isolated and thereby concentrated, their effects on digestion are frequently easier to detect; when these types of fibers are added to a diet, the rate of glucose appearance in the blood is slowed, and insulin secretion is subsequently decreased. These beneficial effects on blood glucose and insulin concentrations are most evident in individuals who have diabetes mellitus. In healthy individuals, the rapid insulin secretion that causes rapid removal of glucose from the
Does dietary fiber decrease absorption of macronutrients and thus energy?
Theoretically, if dietary fiber could block or limit the absorption of macronutrients, it could aid in weight control. Even a small change in absorption could have long-term significance in weight maintenance. Few studies have been conducted in this area because of the need to collect fecal samples and measure fecal energy, fat, protein, or carbohydrate. Gades and Stern [37] measured the ability of a commercially available chitosan supplement on fecal fat excretion in men. Chitosan-based
Does dietary fiber affect gastric emptying?
Dietary fiber affects gastrointestinal physiology and functions including delaying gastric emptying [38]. Consumption of viscous fibers delays gastric emptying, which may cause an extended feeling of fullness [39] and may delay absorption of glucose and other nutrients. Yao and Roberts [40] summarized the effect of an increase in energy density on the rate of gastric emptying in humans and consistently found that high-fiber diets slowed gastric emptying. Although the exact mechanism whereby
Does dietary fiber affect secretion of gut hormones, including cholecystokinin, independently of glycemic response?
Beyond the effects of dietary fiber on postprandial glucose, insulin, and satiety discussed above, dietary fibers alter responses and actions of the gut hormones gastric inhibitory peptide [41], glucagon-like peptide-1 [42], and cholecystokinin (CCK). CCK is a peptide hormone and a neurotransmitter secreted by cells in the upper part of the small intestine after ingestion of food. CCK regulates gut motility, gallbladder contraction, and pancreatic enzyme secretion. CCK may mediate postprandial
Does dietary fiber decrease food intake at a later eating occasion?
Fiber may have the added benefit of helping consumers decrease food intake throughout the day. However, results of trials examining this possibility have been conflicting. In general, large intakes of dietary fiber at breakfast are associated with less food intake at a lunch. Burley et al. [51] found that 29 g of sugar beet fiber resulted in 14% less energy consumption at lunch. Levine et al. [52] also found less food intake at lunch when a high-fiber breakfast cereal was consumed earlier in
Summary
Long-term insufficient intake of dietary fiber represents a challenge for the dietetics professional that can be met with enthusiastic recommendations for a healthy dietary pattern. Modest increases in intakes of fruits, vegetables, legumes, and whole and high-fiber grain products would bring the majority of the North American adult population close to the recommended ranges of dietary fiber intake of 25 g/d for women and 38 g/d for men. In addition, a higher-fiber intake provided by foods is
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Novartis Consumer Health supported the preparation of this review with an educational grant.