Dietary Fiber: Types, Benefits, and Metabolism
Characteristics of Dietary Fiber
Dietary fibers are substances of plant origin, forming a complex network of molecules that cannot be digested by enzymes in the digestive tract. They may be partially fermented by bacteria in the colon and possess osmotic power.
Fiber: Key Components
Fiber is plant-derived material that resists digestion and reaches the colon undigested. Main components include:
- Mobile: Grains, seeds, vegetables, and fruits.
- Hemicellulose: Creates the entangled matrix of cellulose fibers.
- Lignin: Woody plant material.
- Pectin and gums: Soluble polysaccharide complexes found in fruit pulp.
Types of Fiber
True or Vegetable Fiber
Composed of the indigestible components of the plant cell wall. Does not include storage components of non-digestible plant (vegetable gum) or animal origin such as chitin or chitosan.
Crude Fiber
The ash-free residue resulting from treatment by heating with strong acids and bases. Constitutes 20-50% of total dietary fiber.
Total Dietary Fiber
Includes all compounds not digestible by human intestinal enzymes. May contain polysaccharides and other compounds, including non-fiber food components.
Differences Between Crude Fiber and Dietary Fiber
Foods: Whole wheat flour, banana, orange
- Crude fiber (g/100g): 2, 0.6, 0.5
- Dietary fiber (g/100g): 10, 2.8, 1.1
Crude fiber treatment hydrolyzes carbohydrates that would not be digested in the intestine, resulting in the loss of:
- 80% of hemicelluloses
- 50-60% of lignin
- 10-50% of cellulose
Insoluble Fiber
Cellulose, hemicellulose, and lignin make up 95% of the fiber found in plant cell walls. Insoluble in water, it absorbs large amounts of water (up to 15 times its weight), increasing fecal mass, creating bulkier and softer stools, facilitating intestinal transit, and helping prevent constipation. It is minimally fermentable by intestinal flora.
Food Sources: Cereal and whole grains, sprouts, beans, legumes.
Soluble Fiber
Pectin, gums, mucilages, some hemicelluloses, fructooligosaccharides (Inulin), fructo-oligosaccharides are soluble in water and have a great capacity to retain water. They are largely fermentable by gut bacteria. Absorption of sugars and fats is slowed in their presence, helping regulate blood sugar and cholesterol.
Food Sources: Oats, barley, strawberries, raspberries, dates, apples, oranges, and carrots.
Metabolism of Fiber by Microflora
This metabolic process releases gases such as CO2, H2, and CH4, and volatile fatty acids (short-chain) such as acetic, propionic, and butyric acid. These are absorbed in the colon (85%) and provide energy in the Krebs cycle, contributing 3% of total energy.
Beneficial Effects of Fiber
Obesity
- Fullness: Reduced food consumption
- Reduction in heat content of the diet (1 to 3 kcal / g)
Constipation
- Decreased intestinal transit time
- Increase in fecal mass
- Increased frequency and ease of evacuation
Diabetes
- Lower and slower absorption of glucose
Hypercholesterolemia
- Low cholesterol absorption
Intestinal Cancer
- Maintenance and development of the intestinal flora