Diet Posy

Dietary fiber is a plant compound classified under carbohydrates that is not converted into energy in the body and passes through the digestive system unchanged. It is divided into soluble and insoluble fiber, each with distinct functions in the body. Most foods contain both types of fiber.

Soluble and Insoluble Fiber

Soluble Fiber

Soluble fibers delay gastric emptying, slow down carbohydrate digestion to prevent rapid spikes in blood sugar levels, and support control of cholesterol and triglyceride levels. Sources of soluble fiber include oats, barley, legumes, peas, apples, oranges, carrots, and psyllium seeds.

Subcomponents

• Pectin: Found in citrus fruits and some vegetables. It is fermented in the colon to produce short-chain fatty acids, particularly butyrate. It reduces the activity of β-glucuronidase, β-glucosidase, and tryptophanase, and suppresses cellular levels of β-catenin, cyclin D1, and c-myc.
• Guar gum: A soluble fiber component found in guar beans. It increases colon volume and viscosity, enhances glutathione-S-transferase activity, and exhibits antioxidant and anti-inflammatory effects.
• Resistant starch: A type of starch that is not digested. It has four subtypes (I–IV) and supports the gut microbiota’s production of short-chain fatty acids, increasing butyrate production.
• Inulin and fructooligosaccharides: Found in chicory, bananas, onions, wheat, and garlic. They balance the gut microbiota, increase the number of lactobacilli, reduce coliform bacteria, and lower the activity of COX-2 and NF-κB.

Insoluble Fiber

Insoluble fibers contain cellulose, hemicellulose, and lignin, and constitute the majority of total dietary fiber. By absorbing water, they increase bowel motility without being digested, add bulk and softness to stool, prevent constipation, and reduce the transit time of substances that could damage the intestinal wall.

Subcomponents

• Cellulose: Found in the cell walls of green plants and vegetables. It shortens fecal transit time, reduces absorption of carcinogenic substances, and enhances mineral absorption.
• Hemicellulose: Makes up approximately one-third of the fiber content in vegetables, fruits, and legumes. It is particularly abundant in cereal grains and whole grains.
• Lignin: Diphenolic compounds found in cereals and woody plants. Present in foods such as wheat bran and flaxseed. It is metabolized by the gut microbiota into enterodiol and enterolactone, exhibiting antioxidant and free radical-scavenging effects.

Dietary Fiber Sources and Consumption Recommendations

Sources of soluble and insoluble fiber include whole grains, oats, barley, legumes, peas, fruits (especially those eaten with their peels), vegetables, edible seeds, and psyllium seeds. Raw and unpeeled foods contain higher fiber content than cooked or peeled alternatives.

Examples of Fiber-Rich Foods (Pexels)

Methods to Increase Fiber Intake

• Choose whole grain, oat, or rye bread instead of white bread.
• Consume bulgur instead of rice and pasta.
• Eat whole fruits instead of fruit juice, and consume fruits with edible peels without peeling them.
• Include vegetables and salads in every meal.
• Consume legumes 2 to 3 times per week.
• Check food labels and choose products high in dietary fiber.

Physiological Effects of Dietary Fiber

Dietary fiber delays gastric emptying, reduces appetite, and decreases the absorption of simple carbohydrates by increasing viscosity in the small intestine. It provides essential prebiotics for the gut microbiota, increases stool volume, prevents constipation, and supports colon health. It plays a role in regulating cholesterol, triglyceride, postprandial glucose, and insulin levels.

Dietary Fiber and Cancer

Both soluble and insoluble dietary fibers can reduce the risk of cancer development through various mechanisms. Soluble fibers promote the production of short-chain fatty acids and exhibit antioxidant and anti-inflammatory effects. Insoluble fibers shorten colonic transit time, reduce the absorption of carcinogens, and prevent bile acids from causing DNA damage.

Daily Fiber Requirement

The daily intake of dietary fiber for adults should be at least 25 g. According to the Turkish Nutrition Guide, recommended daily dietary fiber amounts vary by age group: 16 g/day for children aged 7–10 years, 19 g/day for those aged 11–14 years, 21 g/day for those aged 15–17 years, and 25 g/day for adults.