Dietary fiber, generally referring to the indigestible carbohydrates naturally present in fruits, vegetables, nuts, and grains, plays a crucial role in maintaining overall health. A high-fiber diet is associated with a reduced risk of various diseases and is vital for a healthy digestive system and lowering cholesterol. It is recommended to consume at least 14 grams of fiber per 1,000 calories daily to promote heart health, aligning with the American Heart Association's guidelines.
The Role of Fiber in Weight Management
Fiber's benefits extend to weight management, primarily through appetite suppression and increased satiety. A recent systematic review and meta-analysis of 62 human studies indicated that viscous fiber intake affects body weight, body mass index (BMI), and waist circumference, independent of an energy-restricted diet.
To lose weight, one needs to be in a calorie deficit, meaning burning more calories than consumed. Fiber is often believed to suppress appetite, but evidence suggests that only certain types of fiber have this effect. The more viscous the fiber, the better it is at reducing appetite and food intake. Viscous soluble fibers, such as pectins, beta-glucans, psyllium, glucomannan, and guar gum, thicken in water, forming a gel-like substance in the gut. This gel slows the emptying of the stomach, increasing digestion and absorption times, resulting in a prolonged feeling of fullness and reduced appetite. The weight loss effects of fiber may specifically target belly fat, the harmful fat in the abdominal cavity associated with metabolic disease.
Soluble vs. Insoluble Fiber
Dietary fibers are categorized as soluble and insoluble. Insoluble fibers are nonviscous and aren’t degraded or fermented in the colon. Insoluble fiber forms a gel in the digestive system and is passed in the stool, so it can be helpful for treating constipation. Soluble fibers are water soluble, but viscosity and fermentability differ among soluble fibers depending on their structural characteristics. Soluble fibers composed of highly branched polymers (e.g., inulin, fructooligosaccharide [FOS], other fructans, wheat dextrin) are nonviscous, while those composed of straight-chain linear polymers are viscous. Gel-forming soluble fibers (e.g., b-glucan, raw guar gum, pectin, psyllium) produce cross-links between linear polymers that increase viscosity and water-holding capacity. Most soluble fibers undergo fermentation by bacteria within the colon, providing a food source for the bacteria and supplying small amounts of absorbable calories.
Fiber Supplements: Types and Effectiveness
Given the challenges some individuals face in obtaining sufficient fiber from food, dietary fiber supplements offer a practical alternative. Common nondigestible carbohydrates (NDCs) in dietary supplements are glucomannan, inulin, psyllium, and β-glucans. In 2016, the FDA identified seven isolated or synthetic nondigestible carbohydrates determined to have health benefits and meet the regulatory definition of dietary fiber. Since then, the FDA has indicated that manufacturers can include 11 additional nondigestible carbohydrates on nutrition and supplement facts labels, including acacia (gum arabic), alginate, arabinoxylan, cross linked phosphorylated RS4, galactooligosaccharide, glucomannan, high-amylose starch (resistant starch 2), inulin and inulin-type fructans, polydextrose, resistant maltodextrin and resistant dextrin, and mixed plant cell wall fibers such as sugar cane fiber and apple fiber.
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However, supplementing with isolated nutrients rarely makes much difference on its own. For the greatest effects, it’s best to combine fiber supplements with other weight loss strategies. Although glucomannan and other soluble fiber supplements are a good option, it’s best to focus your diet on whole plant foods.
Specific Fiber Supplements
- Glucomannan: A natural dietary fiber composed of β 1,4-linked D-mannose and D-glucose monomers derived from a tuber called Amorphophallus konjac. Its mechanism of action for weight loss may be based on its ability to absorb 50 times its weight in water volume, causing delayed gastric emptying along with a feeling of satiety.
- Inulin: A soluble, nonviscous, fermentable dietary fiber. Some studies suggest that inulin may promote weight loss in obese, dyslipidaemic individuals with cardiovascular risk factors. However, the most consistent finding in the literature is the ability of inulin to lower the concentration of triacylglycerols (TAGs).
- β-glucans: Soluble, viscous, gel-forming dietary fibers, which increase the viscosity of the dietary pulp to slow nutrient absorption and improve glycaemic control and the lipid profile. In addition, beta-glucans are readily fermentable; fermentation results in the loss of the gel- and water-binding capacity.
- Psyllium: A viscous, soluble, gel-forming, nonfermented fiber supplement. The advantage of taking psyllium compared to other soluble dietary fibers is that it is less easily fermented and therefore the prevalence of flatulence and abdominal bloating is fairly low. Psyllium can affect body composition through several mechanisms, including gastric emptying, a feeling of satiety, the secretion of intestinal hormones, such as cholecystokinin, and by altering the glycaemic index or insulin response.
Research on Fiber Supplement Combinations
A study investigated the additive effects of a combination of energy restriction (ER) and four dietary supplements on changes in the anthropometric and biochemical parameters in 100 overweight or obese participants who were randomly assigned one of the dietary fibre supplements containing different dietary fibres or a placebo for 8 weeks. The results confirmed that fibre supplements plus ER significantly (p < 0.01) reduced the body weight, body mass index (BMI), fat mass, and visceral fat and ameliorated the lipid profile and inflammation at 4 and 8 weeks after the start of the study, while in the placebo group, significant differences in some parameters were observed only after 8 weeks of ER. A fibre supplement containing glucomannan, inulin, psyllium, and apple fibre was the most effective at reducing the BMI, body weight, and CRP (p = 0.018 for BMI and body weight and p = 0.034 for CRP compared to placebo at the end of the intervention). Overall, the results suggest that dietary fibre supplements in combination with ER may have additional effects on weight loss and the metabolic profile.
Individual Responses to Fiber
Research indicates that individuals may respond differently to various types of fiber. A study by Snyder's team focused on arabinoxylan, a grain fiber and active ingredient in Metamucil and psyllium husk, and inulin, a fiber found in some fruits and vegetables, such as bananas and asparagus. The study showed that while arabinoxylan was overall a boon for reducing "bad" cholesterol, high doses of inulin caused a spike in inflammation in some people. There was, however, one volunteer whose body reacted well to high doses of inulin but not arabinoxylan.
Practical Considerations for Fiber Supplementation
When considering fiber supplements, it's important to:
- Start slowly: Significantly increasing dietary fiber may initially lead to erratic bowel movement frequency, flatulence, and abdominal discomfort, but gradually increasing fiber intake over a period of 1 to 2 weeks can improve tolerance.
- Stay hydrated: When taking fiber supplements, drink eight 8-ounce glasses of fluids, preferably water, per day. Fiber needs more water to digest properly.
- Consult a healthcare professional: Fiber supplements can lower blood sugar levels and may affect how well medicines work. Take fiber supplements at least 2 hours before or after taking medicines.
Dietary Sources of Viscous Fiber
Viscous fibers occur exclusively in plant foods. Rich sources include:
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- Beans and legumes
- Flaxseeds
- Asparagus
- Brussels sprouts
- Oats
The Gut Microbiome and Fiber
An estimated 100 trillion bacteria live in the gut, primarily in the large intestine. Along with other microbes found in the digestive system, these bacteria are often called the gut flora or gut microbiome. Different species of bacteria play important roles in various aspects of health, including weight management, blood sugar regulation and management, immunity, and brain function.
Soluble fiber passes through the digestive system mostly unchanged, eventually reaching the friendly gut bacteria, which digest it and turn it into usable energy. The fiber that benefits gut bacteria is known as prebiotic fiber or fermentable fiber. It’s considered very beneficial for health and body weight.
Fiber and Inflammation
Gut bacteria are well known for their effect on chronic inflammation. While acute (short-term) inflammation is beneficial because it helps the body fight foreign invaders and repair damaged cells, chronic (long-term) inflammation is a serious concern because it may begin to attack the body’s own tissues. Chronic low level inflammation plays a major role in many chronic diseases, including cancer, Alzheimer’s disease, and metabolic syndrome. Observational studies also demonstrate that a high fiber intake is linked to lower levels of inflammatory markers in the bloodstream.
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