Dietary Fibre: Types and Physiological Effects

Dietary fibre is a type of carbohydrate that the human digestive system cannot fully break down and absorb. Despite not being absorbed as a nutrient, fibre plays crucial roles in digestive health, metabolic function, and overall wellbeing. Understanding the different types of fibre and how they function in the digestive system provides valuable nutritional knowledge.

What is Dietary Fibre?

Fibre is a carbohydrate found in plant foods including vegetables, fruits, whole grains, legumes, nuts, and seeds. Unlike other carbohydrates, fibre is not broken down into glucose and absorbed for energy. Instead, it passes largely unchanged through the digestive system or is fermented by gut bacteria.

The human digestive system lacks the enzymes needed to break the bonds in fibre molecules, which is why fibre cannot be absorbed. However, this does not mean fibre is non-functional—quite the opposite.

Soluble Fibre

Soluble fibre dissolves in water to form a thick, gel-like substance in the digestive tract. This affects how food moves through the digestive system and how nutrients are absorbed.

Sources of Soluble Fibre

• Oats and oat bran
• Beans and legumes
• Apples, berries, and citrus fruits
• Barley
• Psyllium seed husks

Physiological Effects of Soluble Fibre

• Slows digestion: The gel formed by soluble fibre slows the movement of food through the digestive tract, which can affect nutrient absorption timing and blood sugar response.
• Affects satiety: The slower movement and gel formation may influence feelings of fullness, though individual responses vary.
• Supports gut bacteria: Soluble fibre serves as a prebiotic, feeding beneficial bacteria in the large intestine.
• Blood sugar response: By slowing carbohydrate absorption, soluble fibre may moderate blood glucose spikes after meals.

Insoluble Fibre

Insoluble fibre does not dissolve in water and passes largely unchanged through the digestive system. It adds bulk to stool and stimulates the intestinal muscles to contract, moving food through the digestive tract.

Sources of Insoluble Fibre

• Wheat bran and whole wheat products
• Vegetables including broccoli, carrots, and spinach
• Legumes and beans
• Nuts and seeds
• Whole grains

Physiological Effects of Insoluble Fibre

• Promotes regularity: By adding bulk to stool, insoluble fibre stimulates bowel movements and supports regular digestive function.
• Supports digestive health: Regular bowel movements are considered important for digestive system health.
• Supports gut bacteria: Insoluble fibre also serves as a substrate for beneficial bacteria.
• Transit time: Insoluble fibre increases the speed at which food moves through the digestive tract.

Total Fibre and Digestion

Most plant foods contain both soluble and insoluble fibre, though in different proportions. For example, beans are rich in both types, while wheat bran contains predominantly insoluble fibre, and oats contain predominantly soluble fibre.

The combined effects of both types of fibre support overall digestive function. Adequate fibre intake supports the health of the digestive system and the trillions of bacteria that inhabit the human gut. These gut bacteria play roles in digestion, immune function, and even mood and cognition.

Fibre and Satiety

Foods high in fibre tend to provide greater satiety compared to highly processed, low-fibre foods. This is due to several factors: the physical bulk of fibre, the slower digestion of fibre-containing foods, and the effects on appetite-regulating hormones. However, individual responses vary substantially.

Fibre Intake and Adaptation

Increasing fibre intake significantly and rapidly can cause digestive discomfort as the digestive system adapts to processing more fibre. Gradual increases in fibre intake, combined with adequate fluid consumption, typically result in better tolerance.

Recommended Fibre Intake

Organisations such as the British Dietetic Association recommend fibre intake of approximately 30 grams per day for adults. However, average intake in developed countries is often significantly lower. Most dietary fibre should come from whole plant foods rather than supplements.

Key Takeaways

• Fibre is a carbohydrate that cannot be digested by human enzymes but has important physiological roles
• Soluble fibre forms a gel and slows digestion
• Insoluble fibre adds bulk and promotes bowel regularity
• Both types of fibre support gut bacteria and overall digestive health
• Fibre-containing foods tend to provide greater satiety
• Adequate fibre intake is associated with various health benefits