Obesity has become a significant worldwide health problem, more than doubling in incidence since 1980. The World Health Organization projects that obesity will affect one in five people worldwide by 2025. Obesity is a critical risk factor in the development of type 2 diabetes and cancer, likely tied to obesity-associated chronic inflammation, oxidative stress, and hyperglycemia. Interventions that mitigate the body's ability to utilize dietary fat might forestall obesity. Dietary fibers have been proposed as an intervention against obesity, with a demonstrated ability in humans to reduce appetite, energy intake, and body weight. Both soluble and insoluble fibers can promote weight loss, but there is an inconsistent relationship between fiber solubility and its ability to reduce appetite and body weight.
Understanding Dietary Fiber
Dietary fiber encompasses a heterogeneous group of natural food sources, processed grains, and commercial supplements. It's generally carbohydrates from plants that resist digestion and absorption in the human small intestines. Dietary fiber is defined as “the edible parts of plants or analogous carbohydrates that resist digestion and absorption in the human small intestine, with complete or partial fermentation in the human large intestine. It includes polysaccharides, oligosaccharides, lignin, and associated plant substances. A simpler classification divides DF into soluble (pectins, gums, mucilages and storage polysaccharides) and insoluble fiber (cellulose, hemicelluloses, lignin) on the basis of water solubility. Soluble fibers dissolve in water and are easily fermented by the microbiota of the large intestine, while insoluble fibers do not dissolve in water and resist fermentation. Cellulose is the main structural component in plants, generally the most consumed of the dietary fibers.
Mechanisms of Action
Diverse factors and mechanisms have been reported as mediators of the effects of DF on the metabolic syndrome and obesity. Among this array of mechanisms, the modulation of gastric sensorimotor influences appears to be crucial for the effects of DF, but also quite variable. The potential beneficial effects attributed to DF were based on earlier epidemiological, indirect evidence, claims of efficacy in a predominantly over-the-counter, unregulated domain, and the public’s perception that if a product is natural, it is safe and efficacious.
Scientific Evidence on Cellulose and Weight Loss
The scientific literature documents several favorable effects of DF on glucose homeostasis, lipid metabolism, and calorie intake. The gastrointestinal tract plays a role in these functions. The stomach signals satiation in response to a meal and affects the rate of delivery of macronutrients to the small intestine, which is the site for most nutrient and energy absorption. Gastric and small intestinal functions are integrated with glucose-regulatory mechanisms originating in the pancreas.
Human Studies
A number of interventional human trials have shown weight reduction with diets rich in DF or DF supplements, however other studies failed to demonstrate any effect. Recent meta-analyses of randomized controlled studies (RCTs) suggest only minor effects on weight loss for commonly used DF supplements. In many studies, DF induced greater satiety compared with digestible polysaccharides and simple sugars. Greater satiety may result from several factors: the intrinsic physical properties of DF (bulking, gel formation and viscosity change of gastric contents), modulation of gastric motor function and blunting of postprandial glucose and insulin responses.
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Animal Studies
Animal studies have provided valuable insights into the potential benefits of cellulose and other fibers in managing weight and related metabolic issues. One study examined the effects of different types of fiber and fat intake on diet-induced obesity in mice. Mice were fed an obesogenic diet containing 60% fat + 10% cellulose (HFC) or a control diet containing 10% fat + 10% cellulose (LFC) for 12 weeks. Subgroups of mice were then switched from HFC to each of the following diets for an additional 5 weeks: 1) 60% fat + 10% pectin (HFP), 2) LFC or 3) 10% fat + 10% pectin (LFP).
The results indicated that, in comparison to HFC, HFP prevented additional weight gain, while LFC and LFP triggered weight loss of 22.2 and 25.4%, respectively. Mice continued on HFC experienced a weight increase of 26% during the same 5-week interval. After 12 weeks, HFC decreased mouse locomotion by 18% when compared to the control diet, but a diet switch to LFC or LFP restored mouse movement. Importantly, HFP, LFC, and LFP reduced fasting blood glucose when compared to HFC.
Soluble vs. Insoluble Fiber
Soluble DF is associated with lower postprandial glucose levels and increased insulin sensitivity in diabetics and healthy subjects, effects that are generally attributed to the viscous and/or gelling properties of soluble fiber. Insoluble DF exerts negligible effects in postprandial glycemia. Soluble DF exerts physiological effects on the stomach and small intestine that modulate postprandial glycemic responses. Additionally, intestinal absorption of carbohydrates may be prolonged by soluble DF, in part by altering incretin levels. The main effect of insoluble fiber in diabetes risk or glycemia involves enhancement of insulin sensitivity. The exact underlying mechanism is still unclear. Grains rich in soluble β-glucans (oats, rye, barley) improve glucose tolerance more than wheat. Additional factors may also favor the hypoglycemic effects of grains: greater fiber particle size, lower level of processing and refinement, which results in slower GE rate; and high ratio of amylose:amylopectin.
Cellulose Supplements and Satiety
In many studies, DF induced greater satiety compared with digestible polysaccharides and simple sugars. Greater satiety may result from several factors: the intrinsic physical properties of DF (bulking, gel formation and viscosity change of gastric contents), modulation of gastric motor function and blunting of postprandial glucose and insulin responses.
Additional Benefits of Dietary Fiber
Recent clinical trials and meta-analyses support the cholesterol-lowering properties of soluble DF (pectin, guar gum, psyllium, and oat β-glucan). LDL reductions of 6-15% but no alterations in HDL or triglyceride levels have been consistently reported. Several trials and observational studies have demonstrated a beneficial effect of increased fiber intake (both soluble and insoluble) on the control and possibly prevention of hypertension. Three large-scale population studies reported an inverse association of high fiber intake or whole grain consumption with risk for cardiovascular disease (CVD). The first study did not examine specific effects of different DF sources; thus its effects may be attributable in part to other biologically active compounds present in high-fiber diets (antioxidants, phytochemicals). In the two other studies, the lower CVD risk was not fully explained by the intake of whole grain fiber and antioxidants, suggesting that other constituents of a natural fiber diet contribute to the effect. In summary, large observational studies support an inverse association of DF intake from natural food sources and CVD risk. The association persisted after adjustment for confounders (BMI, age, smoking, and vitamin supplementation).
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Open-Label Placebos and Weight Loss
Recent research has explored the potential of placebo effects in obesity treatments. Studies have shown that patients in sham bariatric surgery exhibit a significant percentage of the weight loss seen in active surgery groups, pointing to strong nonspecific or placebo effects. Furthermore, placebo effects for feelings of hunger have been demonstrated.
The view that deception is essential for seeing placebo responses has been challenged by research in the last decade. Several studies reported that placebos given without deception (open-label placebos, OLPs) might also have beneficial effects, e.g., for (chronic) pain, emotional distress, and anxiety. Since the application of OLPs is not involved in ethical problems (even though conventional placebos may be preferred), OLPs may offer important new treatment possibilities.
One study examined whether an OLP treatment affected weight and hunger feelings in obese and overweight patients who were trying to lose weight by reducing their caloric intake and following a sports strategy. The study included fifty-seven overweight and obese patients who aimed to lose weight using a combination of diet and sports. Patients were randomly divided into two groups. Participants in the open-label placebo group received two placebos each day. A treatment-as-usual group received no pills. Primary outcome included changes of body weight. Secondary outcomes were change of eating behavior and self-management abilities. After 4 weeks, it was found that participants in the open-label placebo condition lost more weight than the treatment-as-usual group. Furthermore, OLP treatment affected eating behavior. No effects for self-management abilities were found.
Plenity: A Cellulose-Based Medical Device for Weight Management
Plenity is a unique nonsurgical device for weight management in overweight and obese adults (tested on participants with a body mass index of 27 to 40 kg per m2) in conjunction with diet and exercise. Plenity is available in capsule form, but it is not considered a drug because it is not absorbed by the body. Rather, the hydro-gel capsule releases gel particles containing cellulose and citric acid that absorb water in the stomach and small intestine, expanding significantly in size. This action works to create increased bulk, thus signaling satiety. The particles break down in the colon and are excreted in the stool without being absorbed.
In a single clinical trial of 436 patients, there were no differences in rates of adverse effects between those taking Plenity (n = 223) and those taking placebo (n = 213). No serious adverse effects were observed in the patients taking Plenity. About 40% of patients taking Plenity will experience abdominal distention, abdominal pain, changes in bowel movements, and bloating. These rates are similar to those reported with a placebo.
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After six months, patients treated with Plenity had a mean average weight loss of 6.4% compared with 4.4% in those receiving placebo. Significantly more patients treated with Plenity (59% vs. 42% with placebo) achieved a weight loss of at least 5%, and 27% (vs. 15% with placebo) lost at least 10% of their body weight.
CM3: Highly Cross-Linked Cellulose
CM3, a highly cross-linked cellulose in capsule form, expands in the stomach to a size several fold of its original volume. It is purported to induce a prolonged feeling of satiation and a delay in gastric emptying, thus promoting weight loss. One study examined whether CM3 delays gastric emptying (using the stable isotope (13)C-octanoic breath test) and whether it influences subjective feelings of appetite sensations (using visual analog scales, VASs).
The study was a double-blind randomized placebo-controlled crossover trial in 19 moderately obese but otherwise healthy subjects (mean age 55 +/- 9 years, BMI 31.1 +/- 4.6 kg/m(2)). The subjects were treated with six capsules of CM3 or matching placebo 30 min before a standardized solid meal. Breath collection and VASs were performed over 4 h every 15 min and 30 min, respectively. The results indicated that appetite sensations (hunger, satiation, fullness, prospective food consumption, desire to eat something sweet, salty, savory, or fatty) changed over time during the course of the postprandial phase but were not influenced by CM3
Dietary Fiber Intake Recommendations
There are several studies showing that the general population and diabetics in the United States do not meet adequate mean daily fiber intake in their diets. On the other hand, there are clear and multiple benefits from the dietary incorporation of fiber supplements and natural foods and grains on metabolic syndrome, CVD risk and, possibly, on their prevention.
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