Dietary Fiber


Dietary Fiber and Health

What is Dietary Fiber?

  • Definition: Non-digestible carbohydrates (CHO) that pass into the large intestine.

  • Role in Microbiota: May serve as a substrate for gut microbiota, fermented to short-chain fatty acids (SCFA) and volatile fatty acids (VFA).

  • Sources:   - Traditionally: plant cell wall CHOs, including celluloses, hemicelluloses, and pectins.   - Other sources: oligosaccharides in legumes and milk, modified starches, plant gums, mucilages, inulin, algal- and bacterial polysaccharides, and chitin.   - Also includes manufactured oligosaccharides.

Types of Dietary Fibers

  • Psyllium or Ispaghula:   - Source: Derived from the husk of seeds of the plant genus Plantago (e.g., Greater Plantain Plantago major).   - Commercial Use: Used to produce mucilage (a thick, gooey substance).

  • Fructo- and Galacto-Oligosaccharides:   - FOS: Derived from inulin (e.g., from Jerusalem artichoke and chicory).   - GOS: Formed through enzymatic conversion of lactose.   - Properties: Have prebiotic effects, promote calcium absorption, relieve constipation, and are highly fermentable.

  • Resistant Starch:   - Formation: Created by cooking and cooling starch to stimulate recrystallization (retrogradation). Repeating this cycle maximizes resistant starch content.   - Process for Wheat Products: Boiling or baking followed by refrigeration (or freezing) for 12-24 hours converts digestible starch into resistant starch, acting as prebiotic fiber.

Fermentability of Fiber

  • Variation in Fermentability: There is a wide range; cellulose has low fermentability, while inulin is highly fermentable.

  • Effect on Fecal Bulk: Regardless of fermentability, fiber increases fecal bulk.

  • SCFA Production: Provides energy for microbes and hosts, with common ratios of acetate, propionate, and butyrate approximately as follows:   - Acetate: 60%, Propionate: 20%, Butyrate: 20% (or variations such as 60:25:15).

  • Butyrate Utilization: Preferred energy source for colonocytes.

  • Energy Value: Fiber provides between 1.5 to 2.6 kcal/g.

Definitions of Dietary Fiber (Institute of Medicine & FDA)

  • Dietary Fiber: Non-digestible plant CHO and lignin that are intrinsic and intact in plants.

  • Functional Fiber: Isolated non-digestible CHOs with beneficial physiological effects in humans.

  • Total Fiber: The sum of dietary and functional fiber.

  • FDA Definition:   - a) Non-digestible soluble and insoluble CHOs and lignin that are intrinsic and intact in plants.   - b) Isolated or synthetic non-digestible CHOs that have beneficial physiological effects.

Sources of Soluble and Insoluble Fiber

  • Soluble Fiber Sources: Found in foods such as:   - Oat bran, barley, nuts, seeds, beans, lentils, fruits (including citrus and apples), strawberries, and a variety of vegetables.

  • Insoluble Fiber Sources: Present in:   - Whole wheat and whole grain products, vegetables, and wheat bran.

Solubility, Viscosity, and Fermentability

  • Viscosity: Refers to the thickness or stickiness of a fiber solution, which can affect how fiber interacts with food in the digestive system.

  • What does it mean if something is more viscous?

    • A more viscous fiber solution can slow down gastric emptying, leading to increased satiety and potentially lower blood sugar levels after meals.

  • Water Interaction: All fibers hold water; soluble fibers tend to hold more water.

  • Viscosity & Fermentability:   - Soluble fibers (e.g., gums, mucilages, beta glucans, algal polysaccharides, most pectins) are typically more viscous and fermentable.   - Insoluble fibers (e.g., cellulose, lignin, some hemicelluloses) are non-viscous and poorly fermentable.   - Psyllium: An example of very viscous fiber with relatively low fermentability.

Physiological Functions/Effects of Dietary Fiber

  • Appetite Regulation: Contributes to managing appetite and intake.

  • Blood Glucose: Influences postprandial glucose levels and Type 2 Diabetes (T2D).

  • Cholesterol Levels: Impacts serum cholesterol and cardiovascular disease (CVD) risk.

  • Large Bowel Function: Affects bowel function and health.

  • Microbiota Effects: Influences gut lining health through microbiota interactions.

  • Inflammation: Potential role in moderating inflammation.

  • Cancer Links: Discussed connection to cancer prevention, particularly colon cancer.

Irritable Bowel Syndrome (IBS)

  • Nature of IBS: Not a distinct disease, but a collection of symptoms.

  • Effectiveness of Fiber: Fiber may not always improve outcomes; soluble fiber may be preferred over insoluble fiber.

Studies and Findings on Fiber and Health

  • Fiber Intake and Diabetes:   - Reference study by Brenelli et al., assessing intake of 10 g guar gum, 40 g cellulose, 11.7 g pectin, as related to diabetes management.

  • Review of Isolated Soluble Fiber Effects:   - Systematic review highlighting isolated soluble fiber's role in body weight, glycemia, and insulinemia in adults with overweight and obesity.   - Key publication: Am J Clin Nutr. 2017;106(6):1514-1528.

Fiber and Cardiovascular Disease (CVD)

  • Types of Fiber for CVD: Emphasis on mostly viscous (soluble) fiber.

  • Mechanism:   - Interferes with micelle formation, reducing cholesterol uptake.   - Binds bile acids, enhancing excretion.   - Propionate may inhibit HMG-CoA reductase, contributing to changes in lipid metabolism:     - Decreased secretion rate of apo B, up-regulation of LDL receptors promote faster LDL apo B turnover, leading to lower plasma cholesterol concentrations.

Sodium Propionate Effects

  • Study Findings: Affects lipid metabolism in finishing pigs, showing significant reductions in total cholesterol and LDL cholesterol at different sodium propionate concentrations.   - Reference: YouJin et al., 2004, Chinese Journal of Animal Science.

Effects of Propionate Infusion

  • Serum Lipid Concentrations following propionate vs. saline infusion studied, yielding notable differences in triglyceride and cholesterol levels.   - Reference: K. E. Beaulieu and M. I. McBurney, J. Nutr. 122:241-245, 1992.

Cholesterol Biosynthesis Pathway

  • Chemical Pathway Overview:   - Involves multiple steps leading to the production of cholesterol and other vital compounds (Dihydrolipoamide, Mevalonate, Isoprene derivatives, etc.) and regulated by HMG-CoA reductase.

High-Fiber Diet Studies

  • Study Findings:   - Assessment of high-fiber diet effects on gut microbiota and blood glucose regulation in Type 2 Diabetics.   - Control group compared with a treatment group maintaining higher dietary fiber intake, both supplemented similarly (e.g., acarbose).   - Key studies published in Science 2018.

Activation of Short-Chain Fatty Acid (SCFA) Receptors

  • Receptor Types: SCFA recognized by FFAR2 and FFAR3.

  • Location: Receptors found on L cells (enteroendocrine cells, EECs) that produce GLP-1 (an incretin) and peptide YY (anorexigenic hormone).

  • Functions:   - Regulation of EECs, immune cells (white blood cells), pancreatic α and β cells, white adipose tissue, and enteric nervous system cells in the intestine.

Western Diet Effects on Mucosal Health

  • Study Insights:   - Diet impacts the mucus layer of the colon negatively, leading to microbiota alterations.   - Transplants of microbiota from chow-fed mice help mitigate mucus aberrations observed in rodents on a western-style diet.

Inulin's Preventative Effects

  • Benefits:   - Protects against high-fat diet-induced metabolic syndrome by increasing gut epithelium proliferation and preventing colon atrophy.   - Effects tied to microbiota and IL-22, independent of SCFA signaling.

Dietary Fiber's Role in Gut Health

  • Gnotobiotic Mice Studies:   - Study of dietary fiber deprivation vs. fiber-rich diets on microbial health and barrier function of the colonic mucus layer.   - Findings indicate fiber-rich diets maintain intact barrier function while fiber deprivation leads to compromised mucosal health.

Fiber and Cancer Links

  • Cancer Prevention Hypothesis:   - Potential protective role against colon cancer and breast cancer by:     - Adding bulk, which may reduce concentrations of harmful agents (dilution effect).     - Decreasing transit time, leading to less contact with harmful agents.     - SCFAs and lower pH promoting healthier gut flora, and possible effects on the immune system.     - Potential for fiber to bind to estrogen, potentially lowering circulating estrogen levels.

Lactulose as Dietary Fiber

  • Composition: Contains galactose and fructose, exhibiting sweet taste and properties of soluble fiber.

  • Medical Uses: Prescribed as a laxative and ammonia-detoxifying agent.