Currently, an estimated 68% of the U.S. adult population is overweight or obese (6). Obesity can increase the risk of chronic diseases, such as metabolic syndrome, hypertension, cardiovascular disease and type 2 diabetes (12). Improvements in physical activity and diet can help manage these conditions. One dietary behavior, increased fiber intake, is associated with a reduced risk of inflammation, cardiovascular disease, diabetes, and obesity. Increased fiber also is associated with gastrointestinal health and enhanced immune function (3,11,23). These health and weight benefits are either directly or indirectly related to the effect fiber has on the maintenance of beneficial intestinal bacteria, stool bulk, total energy intake, dietary energy density, appetite and satiety, and overall dietary patterns (11,23).
HOW MUCH FIBER DO I NEED?
The current dietary fiber recommendations are based on a level of dietary fiber observed to protect against coronary heart disease (11). For adults, the adequate intake (AI) for fiber set in the Dietary Reference Intakes (DRIs) is 14 g/1,000 kcal per day or 25 g/day for women and 38 g/day for men (9). Unfortunately, the consumption of fiber is well below this recommendation, with usual intakes averaging from 15 to 16 g/day (10). Thus, inadequate intakes of fiber may be a contributing factor to obesity and many disease states.
WHAT IS FIBER? WHAT FOODS ARE GOOD SOURCES OF FIBER?
Fiber is a general term used to describe edible nondigestible carbohydrates. Examples include cellulose (the main structural component of plant cell walls), hemicellulose (surrounds cellulose in plant cell walls), beta-glucans (components of fungi, algae, barley, and oats), pectins (a viscous fiber found in many fruits such as berries), psyllium (highly viscous fiber derived from psyllium husks), and resistant starch (incompletely digested starch found in plant cell walls). Dietary fiber is defined as plant carbohydrates and lignin (component of “woody” plant cell walls such as flaxseeds) that cannot be digested by human digestive enzymes. Functional fibers are those nondigestible carbohydrates that have been isolated, extracted, or manufactured and shown to have some beneficial physiological effect (e.g., fiber supplements such as Metamucil®). Total dietary fiber is the sum of both dietary and functional fibers. These fibers, whether derived from food or a supplement, pass through the small intestine relatively unchanged into the colon, where much of their beneficial action occurs (9).
Fibers frequently are categorized by their characteristics or physiological function within the body (Table 1). Historically, fibers have been classified by the amount of soluble or insoluble fibers they contain. Soluble fibers attract water and become viscous (e.g., more gelatinous) during cooking or digestion. Foods high in soluble fibers include oats, peas, beans, apples, citrus fruits, carrots, barley, and psyllium (a common fiber supplement). Insoluble fibers do not dissolve in water and are found primarily in whole-wheat flours, wheat bran, nuts, and vegetables. Some soluble fibers absorb water and become viscous in the intestine, which slows stomach emptying and, thus, reducing carbohydrate absorption and blunting the postmeal glycemic response (9). These viscous soluble fibers also can physically trap cholesterol and bile acids in the gut and decrease their absorption, thus, contributing to the lowering of blood cholesterol. Insoluble fibers add bulk to stool and promote improved bowel function (24).
Second, fibers also can be classified based on their metabolic effects. The undigested carbohydrates (e.g., fibers) that reach the colon undergo various degrees of degradation or fermentation by gut bacteria. Fibers with a higher degree of fermentation produce energy and other substances such as short-chain fatty acids (SCFA), which enhance microbial growth and gut health and improve immune function. Some SCFAs can be absorbed and transported to the liver, potentially lowering the synthesis of cholesterol by the liver. Conversely, nonfermentable fibers pass through the gut virtually unchanged and increase fecal volume (bulk), thus, improving bowel function and laxation, while also trapping toxins and eliminating them from the system (23).
Prebiotics, a specific fiber classification, have received attention for their reported health benefits. Although all prebiotics are fiber, not all fibers are prebiotics. To be considered a prebiotic, the fiber must resist absorption, be fermented by intestinal bacteria, and stimulate the growth of gut bacteria, which may improve overall health (23).
WHAT IS THE LINK BETWEEN FIBER AND WEIGHT MANAGEMENT?
Current research shows an inverse relationship between body weight and composition (% body fat) and dietary fiber intake (3). For example, Davis et al. (4) found that overweight/obese adults consumed less fiber than their normal-weight counterparts by an average of 4 g/1,000 kcal per day. Normal-weight individuals also had fiber intakes closer to the DRI, averaging 12 g of fiber per 1,000 kcal per day. A 4-year longitudinal study found that adults with fiber intakes more than 15.5 g/1,000 kcal per day lost more weight (2.5 kg; 5.5 lbs) and had greater reductions in waist circumference (WC) (2 cm; 0.75 inches) than individuals consuming less fiber (<10.9 g/1,000 kcal per day) (15). Similarly, Du et al. (5) followed healthy adults for 6.5 years and found an inverse relationship between fiber intake and body weight and WC (e.g., higher fiber eaters had lower weights and smaller waists). They also reported that a 10 g/day increase in fiber, especially from cereal fibers, was associated with a decrease in body weight of 0.5 kg (1.1 lbs) and a 0.65 cm reduction in WC.
Thus, improved fiber intake may help clients achieve a healthy body weight. Unfortunately, the research evidence examining the role of increased fiber intake on promoting weight loss is less strong because few studies have been specifically designed to examine this outcome. However, one weight loss regimen that is high in dietary fiber is the Dietary Approaches to Stop Hypertension (DASH) eating plan (Table 2). Azadbakht et al. (1) compared those participants who were part of an energy-restricted eating plan (e.g., focused on reduction of kilocalories per day) to participants on the DASH eating plan (e.g., focused on better food selection and reducing kilocalories per day). Both groups decreased energy intakes by 500 kcals. They found that those on the DASH diet consumed more fiber (28 g/day vs. 21 g/day) and lost 2 to 3 kg more weight after 6 months.
Studies also have examined the effect of supplemental fiber on weight loss. For example, when supplemental fiber was added to a meal (33–41 g/day), individuals reported feeling more full (21) and consumed less energy (kcal) at the subsequent meal and throughout the day (7). As reviewed by Birketvedt et al. (2), individuals consuming additional fiber during a 1,200-kcal diet lost more weight than those on an energy-restricted diet only (3.8–4.4 kg vs. 2.1–2.7 kg). However, the overall results from supplemental fiber studies for weight loss are mixed, with some indicating significant weight loss compared with controls (3–4 kg), with others showing minimal to no effect (18). Thus, the type of fiber supplement may be important in weight loss. Typically, studies with highly viscous liquid fiber supplements result in improved satiety, which may impact energy intake and weight management. Only limited research examines the role of nonviscous fibers, such as resistant starch, in weight management (24).
HOW DOES FIBER CONTRIBUTE TO WEIGHT LOSS OR LESS WEIGHT GAIN?
Dietary fiber can lower body weight and promote weight maintenance through a number of different potential mechanisms (Figure). Three of these mechanisms are discussed below.
- Greater satiety. A high-fiber diet improves satiety (e.g., feelings of satisfaction and fullness that cause us to stop eating) through a variety of ways, including those bulleted below (22,23,25):
Altered fat oxidation and storage. Fiber also may alter how the body oxidizes and uses fat. As a result of delaying gastric emptying, insulin secretion in response to a carbohydrate-containing meal is reduced and blood glucose is maintained within normal concentrations. This approach lessens the potential for glucose spikes and valleys and the need for the liver to remove glucose from the blood for fat synthesis or to release glucose to maintain blood concentrations. High insulin concentrations also inhibit hormone-sensitive lipase, the enzyme responsible for breaking down stored triglycerides. Thus, reducing blood glucose and insulin concentration could potentially reduce fat storage and favor fat oxidation (22).
Impact of fiber on beneficial gut bacteria. Emerging research suggests that gut microbes may be involved in regulating energy balance. Preliminary studies in mice show that lean mice have different gut bacteria than obese mice, suggesting that bacteria population may influence how efficiently energy is absorbed, used, and stored. Obese mice had higher levels of Firmicutes in comparison with lean mice, which had higher levels of Bacteroidetes (13). Furthermore, when obese mice were supplemented with different levels of prebiotics, their microbial populations were altered such that the levels of Bacteroidetes increased (19). Lastly, Ley et al. (14) showed that obese individuals had higher amounts of gut bacteria (Firmicutes) and less of others (Bacteroidetes) compared with lean controls yet, after the obese individuals participated in a 1-year diet-induced weight loss program, these differences disappeared. Greater increases in Bacteroidetes also were correlated with greater losses in body weight. Taken together, research suggests that fiber intake, specifically prebiotics, could favorably alter gut bacteria in a manner that improves energy balance.
- Lowering energy density of the diet (e.g., food with lower kilocalories per gram).
- Providing a larger volume of food without contributing additional calories (e.g., high-fiber foods add bulk to a meal and increase the sense of fullness).
- Displacing or substituting a high–energy-dense food (e.g., candy bar) for a lower–energy-dense food (e.g., apple) as a snack.
- Expansion of fiber in the stomach and gut, sending signals to the brain that satiety has been achieved.
- Greater release of satiety signals from the gut to the brain.
- Increased chewing time (e.g., eating an apple vs. drinking apple juice).
- Delayed gastric emptying and prolonged digestion time caused by the viscosity of the fiber.
- Fermentation of specific fibers (such as pectin and resistant starches) by gut bacteria to SCFAs (butyrate, acetate, and propionate) has been shown to activate gut satiety hormones, potentially reducing overall food intake.
Although this research suggests that increasing beneficial gut bacteria may promote weight loss, more data are needed showing a direct causal relationship between fiber intake, gut microbes, and body weight in humans (17).
HOW MUCH FIBER IS REQUIRED FOR WEIGHT MANAGEMENT?
The amount of fiber needed for weight loss or weight maintenance has not been established clearly. A position paper of the Academy of Nutrition and Dietetics suggests that health and weight loss benefits occur with fiber intakes of 20 to 27 g/day from whole foods or up to 20 g of fiber per day from supplements (24). A review by Howarth et al. (8) examined more than 50 fiber and weight loss intervention studies. They found that a 14 g/day increase in fiber was associated with a 10% decrease in energy intake and a 2 kg weight loss during a 4-month period. The results were the same regardless of the form of fiber (e.g., diet or supplement) consumed.
The type of dietary fiber most beneficial for weight loss or prevention of weight gain also has not been established clearly. Because high-fiber foods, such as whole fruit and vegetables, whole grains, and legumes, contain other beneficial compounds (e.g., macronutrients and micronutrients, phytonutrients, water), it is difficult to separate out the specific effects of fiber. Fiber may have a synergistic effect with other compounds found in foods because high-fiber foods have been shown to affect weight management positively.
Further work is also needed to determine how liquid and solid forms of fiber-containing foods differ in impacting body weight and energy intake in the long-term. However, foods in their whole less-processed form (whole vs. pureed carrots, a whole apple compared with apple sauce or apple juice) appear to result in greater satiety (24). Thus, current research suggests that consuming high-fiber foods in their more intact forms may promote greater feelings of fullness, assisting in long-term weight control.
HOW CAN I INCREASE MY DIETARY FIBER INTAKE?
Whole fruits and vegetables, legumes, nuts, seeds, and whole grains are all high-fiber foods that can increase total dietary fiber intake (Table 3). Two dietary strategies can assist with increasing the intake of these foods. One approach is the DASH eating plan (Table 2), which encourages high intakes of fruits and vegetables, whole-grain consumption, and incorporating legumes and healthy sources of fats, such as nuts. A second approach is to increase fiber intake by following a low-energy density eating plan. Energy density refers to the energy (kilocalories) in a specific amount (grams) of food. This approach emphasizes incorporating whole fruits and vegetables into meals and snacks, increasing high-fiber whole grains, and reducing saturated fat intake by selecting lean meats and dairy. Low–energy-dense foods are high in water and fiber content and lower in fat; thus, the feeling of satiety is maintained because of the bulk of the food consumed (20). More recently, fiber is being added to traditionally non–fiber-containing foods, such as yogurt and refined grain products. Fiber supplements also can be used as a method to increase fiber intake. However, increasing fiber-containing foods may offer other health benefits, such as decreasing energy intake while increasing intakes of vitamins, minerals, antioxidants, and phytochemicals that are found in whole fruits and vegetables, whole grains, and legumes that supplemental fiber alone cannot provide.
INCREASING FIBER AS PART OF A LIFESTYLE CHANGE
Although research examining the effects of fiber on health and weight is ongoing, dietary changes can be made based on existing research. Increasing dietary fiber intake from nutritious fiber-containing foods will offer multiple health benefits, including weight management. Regardless of how an individual chooses to increase fiber consumption, it must be done in a manner that can be sustained and part of an overall lifestyle change. Fiber intake should be increased slowly to lessen some potential negative effects of an increased fiber intake, such as cramps, bloating, and gas. In addition, it is important to increase fluid intake along with dietary fiber (9). Table 4 provides strategies to increase fiber, and Table 5 compares a lower-fiber diet to a higher-fiber diet that follows the DASH eating plan (16). This diet provides approximately 2,000 kcal/day and 42 g of fiber per day, along with 4 servings of fruit, 4½ servings of vegetables, and 6 servings of whole grains. With this variety of high-fiber foods, one would obtain a mix of soluble and insoluble fibers.
Increasing fiber intake can be an easy and effective way to reduce energy intake, manage appetite, and maintain weight or promote weight loss. These results may be caused by changing food habits and replacing higher-calorie (i.e., potato chips) for lower-calorie high-fiber foods (i.e., carrot sticks or apple) that are more filling. Other factors may contribute to the effect of increased fiber intake on weight, such as changes in beneficial gut bacteria, increased release of satiety hormones, and expansion of fiber-containing foods in the gut, promoting a greater feeling of fullness. Increasing fiber-containing foods also is a method to increase nutrient intake from beneficial plant foods and improve habitual eating patterns, which may offer additional health benefits.
BRIDGING THE GAP
Many individuals struggle to maintain a healthy weight. Increasing fiber intake from whole fruits and vegetables, whole grains, and legumes while selecting low-fat dairy and meat is an effective method to reduce energy intake for weight loss or maintenance. This nutrient-dense dietary approach, when combined with increased physical activity, promotes successful weight management.
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