Carbohydrates and Fibre

Definition

Fibre is largely a carbohydrate. The building blocks of all carbohydrates are different types of sugars and they can be classified according to how many sugar molecules are combined in the carbohydrate. Simple sugars consist of 1-2 sugar molecules such as glucose, fructose, sucrose, maltose, lactose. Oligosaccharides consist of 3-10 glucose molecules joined together. Starch polysaccharides have more than 10 glucose molecules joined together.Non-starch polysaccharides have more than 10 sugar molecules such as xylose, arabinose, mannose. Dietary fibre includes non-starch polysaccharides, oligosaccarides, liginin (not a carbohydrate) and associated plant substances.

In 1997 the World Health Organisation (WHO) and Food Agriculture Organisation (FAO) reviewed all the evidence regarding carbohydrates and fibre and reported that:
'dietary fibre is a nutritional concept, not an exact description of a component of diet and that the use of the term dietary fibre should always be qualified by a statement itemizing those carbohydrates and other substances intended for inclusion'.

The WHO/FAO recommended that carbohydrates be defined in several ways:
1. Chain length: this describes whether the carbohydrate consists of one-two molecules (simple sugar), a short chain (oligosaccharides) or a long chain of sugar molecules (polysaccharides or non-starch polysaccaride).
2. Glycaemic or non-glycaemic: this describes whether the carbohydrate can be digested to provide glucose for absorption from the small intestine; the corollary is, if it is undigestible, it passes onto the large intestine where it may be fermented by bacteria
3. Fermentable or non-fermentable: this describes whether the carbohydrate has survived digestion in the small intestine, if it has survived digestion, it is fermented (broken down) by bacteria in the large gut (colon).

This is a new way of thinking and has not yet been widely written about in text books or in the lay literature. This article is therefore based on the earlier definitions of carbohydrates.

Where is fibre found?

Dietary fibre is found in cereals, fruits and vegetables. Fibre is made up of the indigestible parts or compounds of the plant that pass relatively unchanged through our stomach and intestines, although bacteria in the large intestine can partly digest fibre through fermentation. Other terms for dietary fibre include 'bulk' and 'roughage' - which can be misleading, since some forms of fibre are water soluble and aren't bulky or rough at all.

There are broadly two categories of fibre and we need to eat both in our daily diets:
Soluble fibre - 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, oatbran, barley, seed husks, flaxseed, psyllium, dried beans, lentils, peas, soymilk and soy products. It can also help with constipation.
Insoluble fibre - includes cellulose, hemicelluloses and lignin, which make up the structural parts of plant cell walls. One of its major roles is to add bulk to faeces, and prevent constipation and associated problems such as haemorrhoids. Good sources include wheat bran, corn bran, rice bran, the skins of fruits and vegetables, nuts, seeds, dried beans and wholegrain foods.Both types of fibre are beneficial to the body and most plant foods contain a mixture of both types.
Resistant starch
Resistant starch - while not traditionally thought of as fibre - acts in a similar way. Resistant starch is a part of starchy food (approximately 10 per cent) that resists normal digestion. It is found in many unprocessed cereals and grains, firm bananas, potatoes and lentils, and is added to bread and breakfast cereals as Hi-Maize. It can also be formed by cooking and manufacturing processes such as snap freezing. Resistant starch is also important in bowel health. 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 bloodstream and may play a role in lowering blood cholesterol levels. Resistant starch has some of the nutritional characteristics of fibre but its effect on heart disease risk factors are equivocal.

How much fibre do we need?

Most Australians don't consume enough fibre. On average, most Australians consume 18 to 25g of fibre daily. The Australian Heart Foundation recommends that adults should consume approximately 30g daily. Australian and American experts have suggested that a child should eat 10 or 5g of fibre, respectively, plus an additional gram for every year of age. For instance, a 10 year old child should eat 15 to 20g of fibre per day.

Disorders which can arise from a low fibre diet include:
· Constipation - small, hard and dry faecal matter that is difficult to pass
· Haemorrhoids - varicose veins of the anus
· Diverticulitis - small hernias of the digestive tract caused by long term constipation
· Irritable bowel syndrome - pain, flatulence and bloating of the abdomen
· Being overweight and obesity - carrying too much body fat
· Coronary heart disease - a narrowing of the arteries due to fatty deposits
· Diabetes - a condition characterised by too much glucose in the blood
· Colon cancer - cancer of the large intestine.

Dietary fibre is mainly needed to keep the digestive system healthy. It also contributes to other processes, such as stabilising glucose and cholesterol levels. In countries with traditionally high fibre diets, diseases such as bowel cancer, diabetes and coronary heart disease are much less common than in the West.

The health of the digestive tract
The principle advantage of a diet high in fibre is the health of the digestive system. The digestive system is lined with muscles that massage food along the tract - from the moment a mouthful is swallowed until the eventual waste is passed out of the bowel (a process called peristalsis). Since fibre is relatively indigestible, it adds bulk to the faeces. Soluble fibre soaks up water like a sponge, which helps to plump out the faeces and allows it to pass through the gut more easily. It acts to slow down the rate of digestion. This slowing down effect is usually overridden by insoluble fibre, which doesn't absorb water, but which speeds up the time that food passes through the gut.

Fibre and ageing
Fibre is even more important for older people. The digestive system slows down with age, so a high fibre diet becomes even more important.

A method of weight control
In many cases, people who are overweight or obese have been shown to lose significant amounts of excess body fat simply by increasing the amount of dietary fibre, especially soluble fibre, in their daily diet. Fibrous foods are often bulky and, therefore, filling; they also tend to be low in fat. Soluble fibre forms a gel which slows down the emptying of the stomach and the transit 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. 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 and excessive kilojoules (calories) are less likely to be consumed.

Good for people with diabetes
Soluble fibre forms a gel when mixed with liquid, which slows the stomach emptying time and the absorption of carbohydrate from the intestines. This helps to control blood sugar levels. Glucose is a simple sugar produced from the breakdown of carbohydrates. In people who are sensitive to glucose, such as those with diabetes, eating a diet high in fibre slows the glucose absorption from the small intestine into the blood. This reduces the possibility of a surge of insulin, the hormone produced by the pancreas to stabilise blood glucose levels.

Good for the prevention of coronary heart disease (CHD)
The evidence that fibre protects you from heart disease is strong. A study on more than 43,000 US male health professionals (Rimm et al. JAMA 1996; 275: 447-451) revealed a strong link between consumption of fibre and protection against CHD. Fibre in vegetables, fruits and cereals was associated with a 55% reduction in risk of fatal CHD, but cereal fibre was most strongly protective. There was a 29% lower risk of CHD for each additional 10g cereal fibre eaten daily.

Lowering blood cholesterol
Recently, there has been a great deal of interest in oatbran, since some studies showed that regular intake of foods high in soluble fibre - such as oat bran, baked beans and soybeans - reduced blood cholesterol levels. The only heart disease risk factor that has been shown to be modifiable by dietary soluble fibre is the level of plasma cholesterol. A scientific review (Glore et al. J Am Diet Assoc 1994; 94: 425-436) concluded that comparable cholesterol lowering can be achieved with daily consumption of 6-40g pectin, 8-36g gums (e.g. guar gum), 100-150g dried beans/legumes, 25-100g oat bran and barley bran (contain b-glucan) or 10-30g psyllium. LDL cholesterol can be reduced by 5-10%. It is thought that soluble fibre lowers blood cholesterol by binding bile acids (which are made from cholesterol to digest dietary fats) and then excreting them (Zhang AJCN 1992; 56: 99-105). A study from Finland (Leinonen et al. J of Nutr 2000; 130: 164-70) also demonstrated that 8 slices of whole rye bread or whole rye crispbread daily may help lower cholesterol in men with slightly elevated blood cholesterol levels.

Not all of fibre’s protective powers come from soluble fibre reducing plasma cholesterol levels.It may also slow your rate of digestion, which lowers insulin levels. Extra insulin can raise your blood pressure, reduce HDL and increase your risk of diabetes, all of which can affect your heart. High fibre meals are usually more filling, so you eat fewer calories and this helps with weight loss. Some chemicals formed by bacteria when they break down soluble fibre may reduce blood clots, which would lower the risk of heart attacks and stroke.

Prevention of cancer?
A recent study (Alberts et al. NEJM2000; 342 (15)) suggested that a high fibre diet may not prevent colon cancer and the evidence for fibre and prevention of breast cancer is also inconclusive. Cereal fibre has been shown by several studies to protect against colon cancer. In the Australian Polyp Prevention Project a combination of low fat and wheat bran supplemented diets (providing 11.5g fibre/day) was associated with fewer new large adenomas over the 4 years of study (MacLennan et al. J Nat Cancer Inst 1995; 87: 1760-6). A study from the Arizona Cancer Centre involving 1429 men and women aged 40-80 who had recently removed an adenoma from the bowel was unable to show that taking a fibre supplement (13.5g fibre/day) would prevent the recurrence of an adenoma. In fact, multiple adenomas were more common in the high fibre group (Alberts et al. NEJM 2000; 342 (15)). The different results obtained in these two studies may be explained by the types of fibre supplements used. In the Australian study an unrefined wheat bran was used which was probably higher in protective phytochemicals (phytoestrogens), vitamin E, B6 and folate. The American study used a more processed/refined fibre supplement. 

Mechanisms by which fibre is protective?
Studies that show a reduced risk of some cancers and coronary heart disease have received much attention. How these apparent health benefits arise is not fully understood. It is very likely that these observed health benefits occur, indirectly, through the protective effects of 'phytochemicals' (such as antioxidants) that are closely associated with the fibre components of fruits, vegetables and cereal foods. It has also been suggested that dietary fibre dilutes harmful substances and possible carcinogens present in the diet, thus reducing the gut's exposure to such compounds.

Ways to increase your fibre intake
Simple suggestions on how to increase your daily fibre intake include:
· Eat breakfast cereals that contain barley, wheat or oats
· Switch to wholemeal or multigrain breads and brown rice
· Add an extra vegetable to every evening meal
· Snack on fruit, dried fruit, nuts or wholemeal crackers.

You don't need to eat alot more calories to increase your fibre intake; you can easily double your fibre intake without increasing your calorie intake by being more selective - see table below

Typical low fibre intake of less than 20g/day

Typical high fibre intake of more than 30g/day

>30g can be easily achieved if you cosume wholegrain cereal products and by eating more fruit, vegetables and legumes and instead of low fibre cakes and biscuits have nuts/seeds as a snack or use in meals

You must drink lots of fluid
A high fibre diet may not prevent or cure constipation unless you drink enough water every day. Some high fibre breakfast cereals may have around 10g of fibre per serve and
if this cereal is not accompanied by enough fluid - it may, in fact, cause constipation.

A sudden increase in fibre can have explosive effects
It should be noted that a sudden switch from a low fibre diet to a high fibre diet can create some abdominal pain and increased flatulence. Also, very high fibre diets (more than 40g daily) are linked with decreased absorption of some important minerals, such as iron, zinc and calcium. This occurs when fibre binds these minerals and forms insoluble salts, which are then excreted. This could increase the risk of developing deficiencies of these minerals in susceptible individuals. Adults should aim for a diet that contains 30 to 35 g of fibre per day, and to gradually introduce fibre into the diet to avoid any negative outcomes. Adding fibre to the diet from food sources is much more preferable to adding fibre supplements. Fibre supplements can aggravate constipation, especially if daily water consumption is not also increased.

Take home messages: 

1. Consumption of dietary fibre, especially cereal fibre (both soluble and insoluble), is associated with a lower risk of CHD 
2. Foods rich in soluble fibre reduce the level of plasma cholesterol 
3. Consume a variety of whole grain cereals to ensure an adequate intake of nutrients, especially phytochemicals such as phytoestrogens and phytic acid.

References

FAO/WHO Carbohydrates in human nutrition. Report of a joint FAO/WHO Expert Consultation, Rome 14-18 April 1997, Rome: FAO, 1998
http://www.fao.org/docrep/w8079e/w8079e00.htm

See also HEC article "Grains and cereals debate".

 

Article co-authored by Better Health Channel
(Australian -Victorian Government website)

Last Updated: October 2002.