Dietary fibre is the carbohydrate component of plants that cannot be digested by human enzymes. As a result, dietary fibre passes straight through the small intestine undigested and unabsorbed. Fibre was originally thought of as roughage to provide bulk to the food and nothing more. However, more recent research is starting to uncover the importance of fibre in the human diet and showing that fibre is far more complex than once considered.
Soluble Versus Insoluble Fibre
The two main divisions of fibre are soluble and insoluble fibre. This is an important distinction because these fibre types have very different effects in the body. For example, soluble fibre such as gums, mucilages and pectins are able to create a layer inside the gut and slow the absorption of glucose and thus favourably affect blood sugar levels. In contrast insoluble fibre such as cellulose are an important source of food for gut bacteria.
Short-Chain Fatty Acids
The insoluble fibre cellulose is an important source of food for gut bacteria in the colon. In exchange for supplying these bacteria with cellulose, gut bacteria provide their host with short-chain fatty acids. This process is the same fermentation seen in ruminant animals such as sheep and goats. Most people consider fibre to provide no energy, but the short-chain fatty acids such as propionate, butyrate and acetate are absorbed and used by humans as a source of energy.
Evidence suggest that when fibre is removed from its original plant material it loses some or all of its special properties. This is because normally fibre forms the cells walls of plants and this surrounds the starchy or sugar content of the cell. Eating the food in its whole form means that some or all of these cell walls are intact, and this inhibits the digestion rate of the contents, slowing its absorption. Processing the fibre into breakfast cereal for example destroys this property.
Legumes and Nuts
Nuts are a great source of fibre, but are also high in fat and energy. However, the energy in nuts is not fully absorbed due to the cell walls that encase the contents. Some of the cell walls can be broken open with chewing, but some slip through and the contents remain undigested. The parenchymal cells walls that surround nuts are also present in legumes and this process may also occur in beans and peas. This explains the weight loss effects of nuts and legumes.
What Is The Best Source of Fibre?
The truth it, the best source of fibre is a mixture of all fibre types. There are many fibres including hemicellulose, cellulose, pectin, gum, mucilage and lignan. All of these fibres have different roles to play in health, and health is therefore best supported by a mixture of all fibre types. Studies also show that supplemental fibre does not possess the same health effect as fibre in its food form, so try to get all the fibre you need from your diet.