Carbohydrate Digestion and Dietary Fibre

The Steps of Carbohydrate Digestion

Digestion is the process of breaking large, complex molecules into small monosaccharides that the body can absorb into the bloodstream. This occurs in several distinct stages:

1. The Mouth: Digestion begins here. The enzyme salivary amylase starts breaking down large starch molecules into shorter polysaccharides.
2. The Stomach: Once food reaches the stomach, the highly acidic environment inactivates salivary amylase, meaning no significant carbohydrate digestion occurs in this stage.
3. The Small Intestine: This is the primary site of digestion. Pancreatic amylases are released into the small intestine to continue breaking starch into oligosaccharides, disaccharides, and monosaccharides.
4. The Brush Border: Enzymes attached to the lining (villi) of the small intestine complete the job by breaking down the remaining disaccharides and oligosaccharides into individual monosaccharides.
5. The Large Intestine: Any remaining indigestible carbohydrates, such as fibre, pass into the large intestine.

Dietary Fibre and Gastrointestinal Function

Dietary fibre refers to certain complex carbohydrates that human enzymes cannot digest. Because we cannot break them down, they are not absorbed as energy in the small intestine. Instead, they perform vital roles in the large intestine.

Types of Fibre

• Soluble Fibre: This type dissolves in water. It can be found in foods like oats, apples, and beans. Soluble fibre is known to slow the absorption of nutrients, which helps regulate the glycemic response (how quickly blood sugar rises).
• Insoluble Fibre: This type does not dissolve in water and is found in wheat bran and fruit or vegetable peels. It adds bulk to the stool and helps maintain regular bowel movements.

Fermentation and Health Benefits

In the large intestine, bacteria partially break down these indigestible carbohydrates through fermentation. This process produces short-chain fatty acids and gas. High-fibre diets are associated with several health benefits, including a reduced risk of constipation, diverticulosis, and even colon cancer.

Exam Tip: Remember that while starch is a polysaccharide that is digested for energy, fibre is a polysaccharide that is not digested by human enzymes.

Lactose Intolerance

Lactose is the specific disaccharide found in milk. To digest it, the body requires the enzyme lactase.

In individuals with lactose intolerance, there is an insufficient amount of lactase. Consequently, the lactose cannot be broken down in the small intestine and instead passes into the large intestine. This can lead to gastrointestinal distress, including:

• Abdominal distention (bloating)
• Cramping
• Diarrhea

To manage this while still meeting calcium needs, individuals can consume calcium-fortified foods, plant-based proteins like tofu or fish, or use milk treated with the lactase enzyme.

Summary Table: Nutrient Comparison

To better understand the impact of food processing, compare unrefined (whole) grains to refined grains:

Source: Adapted from Canadian Nutrient File.

As shown above, refining processes often remove the bran and germ, significantly reducing the fibre and mineral content of the food.