February 2015 Issue

Fiber: Fiber's Link With Satiety and Weight Control
By Jill Weisenberger, MS, RDN, CDE, FAND
Today's Dietitian
Vol. 17 No. 2 P. 14

Epidemiologic studies suggest that dietary fiber intake is linked with the prevention of weight gain and is inversely associated with BMI.1 Researchers offer many possibilities for these associations. This article reviews fiber's possible roles in satiety and weight control.

Fiber Defined
Dietary fiber consists of nondigestible carbohydrates and lignin that are intrinsic and intact in plants. Functional fibers are isolated, nondigestible carbohydrates that have beneficial effects. Total fiber is the sum of dietary and functional fibers.1 Fibers have many roles such as increasing laxation, delaying gastric emptying, and reducing blood cholesterol concentrations. The Adequate Intake for total fiber is 38 g and 25 g per day for young men and women, respectively, and was set based on the amount of fiber observed to protect against coronary heart disease.1 The average fiber intake among American adults is 16 g.2

Fiber, Satiety, and Weight Relationships
The difference in body weight between individuals with high fiber intakes and low fiber intakes is small, but significant, says Raylene Reimer, PhD, RD, a professor of nutrition and metabolism at the University of Calgary. Typically, people with high fiber intakes weigh 0.5 kg less than those with low fiber intakes, Reimer explains. A 2011 review article in Obesity Reviews concluded that greater intakes of dietary fiber reduced appetite by 5%, decreased long-term energy intake by 2.6%, and lowered body weight by 1.3%.3 "As you can see, the effects are not huge, but are meaningful in an environment where weight gain is the norm and the obesity epidemic continues worldwide," she says. An earlier review concluded that with ad libitum intake, consumption of an additional 14 g of fiber daily is associated with a 10% decrease in energy intake and weight loss of 1.9 kg over almost four months.
Obese subjects experienced even greater weight loss.4 In particular, epidemiologic studies show that fibers from grains have a greater effect on weight management than fruit or vegetable fibers, says Joanne Slavin, PhD, RD, a professor of food science and nutrition at the University of Minnesota.

Not all studies, however, find a fiber effect on satiety or subsequent food intake, Slavin explains. Slavin is coauthor of a 2013 systematic review designed to determine the short-term effects of fiber consumption on satiety and food intake in healthy adults.5 This review consisted of 107 individual treatments (or individual studies within studies) measuring satiety and 55 individual treatments measuring food intake within 44 published articles. Most treatments showed no effect. Only 39% of the treatments reduced subjective satiety ratings, and 22% of the treatments reduced food or energy intake compared with the control groups.

Of the 38 fiber types studied for satiety effects, only whole grain rye, rye bran, beta-glucan from oats and barley, lupin-kernel fiber (ie, sweet Australian lupin, a legume used primarily in animal feed and as a functional fiber), and a mixed diet of specific fiber-containing foods demonstrated a benefit in most or all treatments. The remaining fibers demonstrated no benefit, provided inconclusive evidence, or were reported in only one publication. Of the 25 fiber types examined for effects on subsequent food or energy intakes, only the ready-to-eat cereal Fiber One, carrots, and resistant starch demonstrated a benefit in more than one treatment. Of note is that of the fibers showing an effect on satiety, beta-glucan from both oats and barley also reduced subsequent energy intake, but were reported in only one publication each. Lupin-kernel fiber showed mixed results.

Fiber's Mechanisms of Action
Satiety is the feeling of fullness that persists after eating. It affects the length of time between eating events and possibly the amount of energy consumed at the next. Satiation is the satisfaction of appetite that occurs during the eating occasion and causes one to stop eating.6 Satiation is important in controlling the amount of energy intake at each meal and snack. Both satiety and satiation affect total energy intake, and both may be influenced by fiber intake.

Fiber-rich foods tend to require more chewing than low-fiber foods; this may decrease the rate of ingestion and increase satiation, thus reducing the total amount of food consumed. More chewing also may limit food intake by increasing saliva and gastric juice, resulting in the expansion of the stomach.7

Moreover, foods rich in fiber may displace foods of greater energy density, aiding in weight control. "If we lower the calorie density of food, we get more weight and volume of food for a given number of calories," explains Barbara Rolls, PhD, a professor and Guthrie Chair of Nutritional Sciences at the Pennsylvania State University, and author of The Ultimate Volumetrics Diet. "Fiber hangs out in foods that enhance satiety," Rolls says, adding that many high-fiber foods have lower energy density than high-fat or low-fiber foods.

"It's not all about one thing," Rolls continues. Simply seeing a greater volume of food on the plate may help some people feel full. Some fibers slow gastric emptying. Satiety hormones are affected, as is the glucose-insulin response, she says. In addition, the fermentation of some fibers and the resultant short chain fatty acids also may facilitate satiety. Finally, Slavin notes that decreased digestion and absorption of macronutrients is another possible way high-fiber diets may lead to lower BMI.

Why Conflicting Research?
Just as there are many ways in which fiber and fiber-rich foods may affect satiety and weight management, there are many potential reasons for conflicting research. "The chief reasons relate to the fact that not all fibers are the same, even though we often talk about them as one nutrient," Reimer says. Fibers differ in their physical and chemical properties, she adds. Some are viscous and have the ability to thicken the contents of our intestines; this slows the entry of glucose into the bloodstream. "Some ferment in the colon, and some are very resistant to fermentation. Some are insoluble, while others are soluble." These and other factors will influence a study's outcome, she explains. Furthermore, the scientific literature includes studies of various fiber amounts. A greater dose of fiber may be expected to produce a greater response, but this is still unclear, Slavin says.

Guiding Clients and Patients
Research doesn't clearly support an ideal amount or type of fiber to manage weight or enhance satiety. Given that the average intake of fiber is only about one-half of the recommended amounts and that fiber has a recognized role in coronary heart disease prevention, it's reasonable for dietitians to encourage clients to consume the Dietary Reference Intake levels of fiber and enjoy a variety of fiber types such as fibers found naturally in foods and functional fibers.

Using MyPlate as a guide, clients can create meals with filling foods, says Joan Salge Blake, MS, RDN, LDN, FAND, a clinical associate professor at Boston University and spokesperson for the Academy of Nutrition and Dietetics. Each food group on the plate consists of high-fiber foods. RDNs can encourage fruits and vegetables at each meal, beans several times per week, and more whole grains than fiber-poor, heavily processed grains, she adds. Rolls suggests that by using the concept of energy density, clients can further personalize their diets. Some will add fruits and vegetables to recipes to increase the volume of food, and others will switch from high-fat to low-fat or nonfat dairy foods, she says.

Though the fiber-satiety research is conflicting, RDs should continue to teach that consuming a variety of fiber-rich foods is important for overall health and is associated with lower BMIs.

— Jill Weisenberger, MS, RDN, CDE, FAND, is a freelance writer and a nutrition and diabetes consultant to the food industry. She specializes in weight management, diabetes, and heart health, and has a private practice in Newport News, Virginia. She's the author of The Overworked Person's Guide to Better Nutrition and Diabetes Weight Loss — Week by Week.

References
1. National Academy of Sciences, Institute of Medicine, Food and Nutrition Board. Dietary, Functional and Total Fiber. In: Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, D.C.: The National Academies Press; 2005:339-421.

2. King DE, Mainous AG 3rd, Lambourne CA. Trends in dietary fiber intake in the United States, 1999-2008. J Acad Nutr Diet. 2012;112(5):642-648.

3. Wanders AJ, van den Borne JJ, de Graaf C, et al. Effects of dietary fibre on subjective appetite, energy intake and body weight: a systematic review of randomized controlled trials. Obes Rev. 2011;12(9):724-739.

4. Howarth NC, Saltzman E, Roberts SB. Dietary fiber and weight regulation. Nutr Rev. 2001;59(5):129-139.

5. Clark MJ, Slavin JL. The effect of fiber on satiety and food intake: a systematic review. J Am Coll Nutr. 2013;32(3):200-211.

6. Slavin JL. Position of the American Dietetic Association: health implications of dietary fiber. J Am Diet Assoc. 2008;108(10):1716-1731.

7. Slavin JL. Dietary fiber and body weight. Nutrition. 2005;21(3):411-418.