5. DIETARY FIBRE
While not considered an essential nutrient, adequate daily consumption of dietary fibre is essential to long-term good health. Dietary fibre has demonstrated benefits for health maintenance, disease prevention and as a component of nutrition therapy.
What is dietary fibre?
Dietary fibre is primarily the storage and cell wall polysaccharides of plants that cannot be digested by the non-microbial enzymes of the human digestive tract. Fibre consists of a variety of substances, most of which are complex carbohydrates. These substances can be divided into two general categories: soluble fibre and insoluble fibre. This distinction is important because the two types of fibre have different effects on the human digestive system.
Soluble fibre is that fraction of the total fibre that is suspended in water during analysis. Soluble fibre can vary from 15% to 50% of the total fibre using different analytic methods. About one- quarter to one-third of the dietary fibre in typical mixed-food diets is water-soluble. Soluble fibres form a gel-like consistency in water and include gums, mucilages, some pectins and some hemicelluloses. Foods that contain substantial portions of soluble fibre include beans, corn, oats, barley, peas, lentils, dates, blackberries, cranberries, seeds, apples, bananas, citrus fruits and certain vegetables such as white potatoes and sweet potatoes. Most other grains, fruits and vegetables contain smaller proportions of soluble fibre. The enzymes produced by bacteria in the large intestine can break down (ferment) most soluble fibres and do not promote laxation. Two noted exceptions to these general guidelines are oats, which contain up to 50% soluble fibre and phylum seed husks that analyze as soluble fibres. Both of these fibre sources promote laxation and modulate gastric and small intestine physiology.
Some soluble fibres slow the appearance of glucose in the blood. For a soluble fibre to be effective, it
must also be viscous. Viscosity slows transit time of chyme in the upper gastrointestinal tract, resulting in slower absorption rates, lower blood concentrations of nutrients and altered hormonal responses to these absorbed nutrients. Viscosity also appears to be a requirement for fibre to lower blood cholesterol.
Insoluble fibres include lignin, cellulose and some hemicelluloses and pectins. Insoluble fibre makes up from two-thirds to three-quarters of the dietary fibre in a mixed food diet. Foods particularly rich in insoluble fibre include bran (the outer covering of corn, oats, rice, wheat), whole grains (corn, barley, rice, wheat, oats), cereals, edible skins of fruits and vegetables, celery, brown rice and some vegetables.
Insoluble fibres are resistant to breakdown by the bacterial enzymes of the large intestine. Insoluble fibres speed up intestinal transit time and increase stool weight and promote laxation.
Recommended daily fibre intake
Recommendations for dietary fibre intake for adults are in the range of 25 to 30g/day or 12 to 15 g of
dietary fibre per 1,000 kcal. The average American diet barely provides half of this amount with an intake of 10 to 15 grams daily. Increasing the consumption of complex carbohydrates is the best way to increase fibre intake. Following are some suggestions to increase fibre intake:
• Choose fresh fruit of vegetables rather than juice
• Eat the skin and membranes of cleaned fruits and vegetables
• Choose bran and whole grain breads and cereals daily
• An increase in fibre should be accompanied by and increase in water
• Eat less processed foods and more fresh ones
• Fibre from fresh foods is preferable over fibre supplements
Therapeutic uses of fibre
• Adequate daily fibre intake may be protective against colon and rectal cancer.
• It is estimated that the risk of colorectal cancer in the US population could be reduced by about
31% if fibre intake from food sources were increased by an average of 13 grams per day.
• High fibre intake has been associated with a reduction in breast cancer rates. International
comparisons show an inverse correlation between breast cancer death rates and the consumption
of fibre rich foods.
• High fibre foods such as vegetables, fruits and whole grains may protect against prostate cancer.
• High fibre diets may reduce the risk of diabetes. Fibre has a role in the treatment of diabetes
because it regulates blood sugar by slowing the absorption of glucose from the small intestine. By
coating the gut’s lining and delaying stomach emptying, sugar absorption after a meal is slowed,
reducing the amount of insulin needed.
• Dietary fibre has a role in the prevention and treatment of cardiovascular disease due to its
cholesterol lowering ability. Viscous, soluble fibres can lower blood cholesterol levels by 5% or
more due to their interference in the absorption of bile acids. By causing more cholesterol to be
used for bile acid production, soluble fibre helps take cholesterol out of circulation in the
intestines and the bloodstream. Fatal and non-fatal myocardial infarctions have been inversely
associated with total fibre intakes in some studies.
• Adequate daily fibre intake promotes laxation and helps to prevent constipation and
haemorrhoids. The gastrointestinal tract is remarkably sensitive to dietary fibre. A high fibre diet
eliminates exacerbation of diverticulitis reducing the long-term risk of developing bowel disease.
• Soluble fibres called cellulose and hemicelluloses expand in the stomach giving a sense of
fullness. This reduces food intake and helps in weight control.
Normal bowel habits
It is normal for one to have one or two soft formed, easily passed bowel movements a day without effort or straining. It is not normal to miss at least one bowel movement per day. If your bowels move daily, but with difficulty or straining, if your stool is dry or hard or you don’t move your bowels at least once a day, you need to adjust your diet for the right amount of fibre and water intake.
When there is adequate fibre and water in the diet, the fibre (viewed as millions of tiny water-attracting particles) mixes with the stool. Each particle soaks up available liquid, and enlarges into a minute gel bead. These particles give the stool size, shape and moisture, allowing the colon to move the stool along easily. To facilitate this process, the colon manufactures mucous to lubricate the passage. If the colon is dry because of too little mucous or too little water, the stool becomes dry and will stick to the colon, requiring one to strain to eliminate, giving rise to constipation, haemorrhoids and to bowel disease.