Dietary Fibre

Dietary fibre is a collective term for a variety of plant substances that are resistant to digestion by human gastrointestinal enzymes. Many forms of dietary fibre are however digested by bacteria in the gut flora. They can be classified into two groups depending on their solubility in water.

Increased intake of soluble dietary fibre such as oats, psyllium, pectin and guar gum have been promoted as a means of lowering total and low density lipoprotein (LDL) serum cholesterol (although this begs the question of whether or not lowering serum cholesterol levels is actually necessary or desirable). Meta-analysis of available data has indicated that 1g of soluble fibre (equivalent to the fibre content of an apple) can lower total cholesterol by about 0.045 mmol/L. The current UK recommendation is an average intake of total soluble/insoluble fibre of 9g, which would lower cholesterol by about 0.40 mmol/L. The ‘acceptable’ cholesterol level is up to 5.2 mmol/L for an average male, and so 0.40 mmol/L represents 7-8% of the total, or less in someone who exceeds the recommended maximum serum level.

Of greater importance is the fact that fibre may be beneficial in treating or preventing constipation, hemorrhoids and diverticulosis, and that there is a clear link between intake dietary fibre and reduced risk of colonic cancer. Insoluble and crude fibre absorb organic molecules and may also have a gentle scouring action on the gut lining. This therefore reduces the time for which food decomposition products may be in contact with the colonic lining and speeds ejection of such compounds in the faeces. This combined action has been proposed as the protective mechanism in connection with colonic cancer.

High-fiber diets may be useful for people who wish to lose weight. Fibre such as bran release few calories, yet provides a "full" feeling because of its water-absorbing ability. For example, an apple is more filling than a half cup of apple juice that contains about the same calories. Foods high in fibre often require more chewing, preventing ingestion of a large number of calories in a short amount of time.