Dietary Fibre: Types and Nutritional Functions

DIETARY FIBRE

By Tumwebaze Joel

Graduate researcher

School of FTB&N Makerere University Kampala, 2011

Dietary fibre

Definition

Dietary fibre is a complex of substances of cell walls of plants (mainly cereals, fruits and vegetables) which are not digested and not absorbed in a human’s digestive tract. Dietary fibre is also called roughage, bulk though these two terms may be misleading since some forms of fibre are water soluble and aren’t bulky or rough at all. It’s a mixture of substances of polysaccharide character (cellulose, hemicellulose, pectins, gums, mucilages) and non polysaccharide one (lignin).

They are two categories of fibre and our daily diet should consist both of them and these include:

 Soluble fibre- forms a gel-like material in water and includes pectins, gums and mucilage, which are found mainly in plant cells. One of its major roles is to lower blood cholesterol levels. Good sources of soluble fibre include fruits, vegetables, oat bran, bran, barley, seed husks, flaxseed, psyllium, dried beans, lentils, peas, soymilk and soy products. Soluble fibre can also help with constipation.

Insoluble fibre- doesn’t dissolve in water and includes cellulose, hemicellulose and lignin, which make up the structural parts of plant cell walls. A major role of insoluble fibre is to add bulk to feces and to prevent constipation and associated problems such as haemorrhoids, irritable bowel syndrome (IBS). Good sources include wheat bran, corn bran, rice bran, and skins of fruits and vegetables, nuts, seeds, dried beans and whole grain foods.

Resistant starch isn’t traditionally thought of a fibre but acts in a similar way. Is part of the starchy food (approximately 10%) that resists normal digestion. Its found in many unprocessed cereals and grains, firm bananas, potatoes and lentils and its added to bread and breakfast cereals as Hi-maize. It can also be formed by cooking and manufacturing processes such as snap freezing. Bacteria in the large bowel ferment and change the resistant starch into short chain fatty acids, which are important to bowel health and may protect against cancer. These fatty acids are also absorbed into the blood stream and may play a role in lowering blood cholesterol levels.

Benefits of dietary fibre 

•  It helps to prevent constipation, diventiculosis, diarrhea and aches assisting hemorrhoids, prevents colon cancer and rectal cancer. Soluble fibre soaks up water like a sponge, which helps to plump out faeces and allows it to pass through the gut more easily. It acts to slow down the rate of digestion. This slowing down is often overridden by insoluble fibre, which doesn’t absorb water and speeds up the time that food passes through the gut. Dietary fibre travels through the whole digestive tract in practically unchanged state. It makes regular defecation easier. Food containing much dietary fibre helps to prevent constipation and aches assisting hemorrhoids because it increases the water content in feces by what it is easier to expel it from organisation.

• It helps in fight against obesity, in weight loss and weight reduction. Fibrous foods are often bulky and therefore, filling. Soluble fibre forms a gel that slows down the emptying of the stomach and the transist time of food through the digestive system. This extends the time a person feels satisfied or full.

• It also delays the absorption of sugars from the intestines. This helps to maintain lower blood sugar levels and prevent a rapid rise in blood insulin levels, which has been linked with obesity and an increased risk of diabetes. Dietary fibre helps to get rid of redundant kilograms because it absorbs water, by what it expands and fulfills stomach decreasing hunger and also delaying the moment in which food leaves stomach.

• The extra chewing time often required of high fibre foods also helps contribute to feeling satisfied. As, a result, a person on a high fibre diet is likely to eat less food and so consume less calories.

•  It decreases level of cholesterol and triglycerides.  Soluble dietary fibre (pectins) is decomposed to tricarboxylic fatty acids which are responsible for inhibition of cholesterol biosynthesis in liver. Because of its consistency it catches and removes gall acids from intestines. Because liver uses them in cholesterol production, the cholesterol level drops down when they have been expelled. Expelling of gall acids is a main way of cholesterol excess removing by organism, because it can’t be metabolized as other fats. Insoluble dietary fibre decreases level of triglycerides in blood by what it reduces the hazard of arteriosclerosis.

•   It stabilizes glucose concentration in blood. Dietary fibre postpones carbohydrate transformation by partial blocking of access of glucose to blood, what next increases insulin secretion is partially activated and regulated by hormonal system (stomach peptide inhibiting GIP and enteroglucagon). Increment of dietary fibre consumption leads to decrease of hormone level.

·         It purifies organism from toxins and heavy metals. Soluble and insoluble dietary fibre helps to eliminate from organisms toxins, gall acids, heavy metals and even carcinogenic substances i.e. level of nitrites in organisms is efficiently decreased by pectins found in apple, currants and other fruit if they have not been excessively sprayed). With the help of fibre waste material passes through the body quickly and doesn’t stay in the intestine for very long hence toxins are not able to build up and accumulate.

•  High dietary fibre dose may lower efficiency of other medicines e.g. oral contraceptives and medicines decreasing cholesterol concentration. One should keep a 2-hour interval between a meal and such medicine taking.

o     Swallowing of big quantity of pills or capsules containing dietary fibre may be dangerous. After too low water saturation they increase their volume and may stick in throat or intestines.

o   Dietary fibre intake of more than 40g daily may restrict absorption of nutrients such as iron, magnesium, phosphorus, zinc and calcium by forming complexes with these minerals forming insoluble complexes which are then excreted. Fibre users are advised to avoid taking supplements at the same time as or soon before or after, taking vitamin or mineral supplements.

o      Sudden witch from low to high food diet may cause flatulence and abdominal pain. It concerns especially pod vegetables. In some people products containing much dietary fibre, especially wheat ones, may irritate stomach.

Ways of increasing ones fibre intake

   Eat breakfast cereals that contain high fibre like barley, wheat or oats. Start your day with porridge or wholemeal bread.

      Switch to whole meal or multigrain breads and brown rice, whole meal pasta.

      Add an extra vegetable to every evening meal

      Snack on fruit, dried fruit, nuts or whole meal crackers

      Eat potatoes with their skins intact

      Buy foods containing whole grains.

      Add barley to home made soups

Dietary fibre RDA

Who recommends consumption of 20-40g of dietary fibre daily.

DRI for dietary fibre

Age	g/day
Children
1-3 years	19
4-8 years	25
Males
9-13 years	31
14-18 years	38
19-30 years	38
31-50 years	38
51-70 years	30
70+ years	30
Females
9-13 years	26
14-18 years	26
19-30 years	25
31-50 years	21
51-70 years	21
70+ years	21
Pregnancy
Less than 18 years	28
19-30 years	28
31-50 years	28
Lactation
Less than 18 years	29
19-30 years	29
31-50 years	29

Source: National academy of sciences, Food and Nutrition Board, USA.

Dietary fibre supplements

Some people don’t tolerate fibrous foods well. If you can’t consume enough fibre in your diet alone, certain stool softening and bulking agents are available. These products absorb water and produce the necessary for the digestive tract to perform naturally. They help create a soft and well-formed stool. For this reason, they can be very useful in preventing and treating digestive tract disorders.

Products of animal origin, alcohol, butter, oils and other fat products do not contain dietary fibre.