Dietary Fibre

Functions

Dietary fibre is defined as the fraction of the edible parts of plants that our bodies cannot digest or absorb and which promotes either laxation, reduction in blood cholesterol or modulation of blood glucose (Source: Food Standards Australia and New Zealand or FSANZ). In basic terms, dietary fibre is the total roughage or bulk that is found in plant-based foods. Occasionally fibre may be divided into two sub-categories, namely 'soluble' and 'insoluble' fibre.

FSANZ requires manufacturers to declare the 'dietary fibre' content of a food on the nutrition information panel only if a nutrition claim is made about the carbohydrate, sugar or fibre content of the product.  

Role in Disease Prevention

Fibre's Laxative Effect and Bowel Disorders

Dietary fibre has positive effects on both faecal bulk and consistency - producing softer and bulkier stools that pass more quickly through the colon. This means dietary fibre performs an effective role in both preventing and treating constipation. The laxative effect of dietary fibre also helps to explain its important role in treating bowel disorders such as hiatus hernia, diverticular disease and haemorrhoids. It should be noted that sufficient fluid intake is also required with dietary fibre to maximise these effects.

An increased intake of fibre is associated with a decreased risk of diverticulosis. Furthermore, individuals diagnosed with diverticulosis who increase intake of both soluble and insoluble fibre are at lower risk of developing diverticulitis (the inflamed state of this health condition). If diverticulitis is present it is recommended that patients avoid insoluble fibre and instead focus on foods rich in soluble fibre. In either case, it is recommended that individuals with an underlying bowel condition seek advice from a medical practitioner before altering their fibre intake.

Lowering Cholesterol Levels and Cardiovascular Disease

A number of controlled clinical trials have shown that an increased intake of soluble fibre is associated with decreased LDL and total cholesterol levels (see below for soluble fibre sources). The evidence for this action on cholesterol levels is so reliable that FSANZ actually defines some forms of dietary fibre by its ability to lower cholesterol levels. Given that raised cholesterol levels increase the risk of coronary heart disease, it is not surprising that an increased intake of soluble fibre is also associated with a reduced risk of coronary heart disease. With a 1% reduction in cholesterol levels equating to an approximately 2% reduction in CHD. 

Lowering Blood Sugar Levels and Diabetes Risk/Management

The inclusion of dietary fibre in a meal has been shown to significantly improve blood glucose and insulin levels. High fibre foods typically slow absorption of food from a carbohydrate-containing meal and it is likely that this effect is primarily responsible for the improvements in carbohydrate metabolism. Whether increasing fibre intake can result in a reduced risk of Type 2 diabetes is debatable. Ecological studies have suggested this may be the case, but more research is required. On the other hand, much recent research has focused on the ability of dietary fibre to positively assist in the management of Type 1 and Type 2 diabetes. 

Can Fibre Fight Cancer?

The 2007 Second Expert Report Food, Nutrition, Physical Activity, and the Prevention of Cancer: A Global Perspective, produced by the World Cancer Research Fund and American Institute of Cancer Research, states that foods containing dietary fibre probably protect against colorectal cancer; and there is limited evidence to suggest that foods containing dietary fibre may also protect against oesophageal cancer.

Food Sources

Natural sources of fibre include wholegrain and wholemeal breads and cereals; fruits and vegetables; nuts, legumes and seeds.

Soluble fibre is particularly found in oats, barley, dried peas, beans, lentils, vegetables (broccoli, brussel sprouts, carrots, potato, kumara) and fruit (apples, pears, citrus, stone and berry fruit). Whereas, insoluble fibre is found in wholegrain, mixed grain, wholemeal and corn based breads, cereals, pasta and other products, as well as in varying degrees in the above foods. 

It should be noted that fibre-rich meals interact with the absorption of a number of essential minerals, reducing the body's ability to absorb them (e.g. iron, calcium, zinc and magnesium). This is due to the high phytate content in fibre-rich foods rather than the fibre itself.

Processed food and Dietary Fibre

Generally speaking the less processed a food is the greater the likely dietary fibre content.

Typical Contributions from Food

Food	Total Dietary Fibre (g)
Bread, multigrain (1 slice)	1.2
Bread, white (1 slice)	0.7
Muffin, bran (medium size)	6.3
Pasta, fresh (1 cup)	4.8
Rice, white (1/2 cup)	0.45
Rice, brown (1/2 cup)	1.85
Oats, rolled, raw (1/2 cup)	4.15
Porridge, prepared with milk (1/2 cup)	1.05
Broccoli, boiled (1/2 cup)	2.6
Brussel sprouts, boiled (1/2 cup)	1.65
Carrot, boiled (one)	1.6
Potato, baked (one medium - 90g)	1.8
Apple (one)	1.9
Pear (one)	3.1
Orange (one)	2.2
Peach (one)	2.3

Source: The Concise New Zealand Food Composition Tables 5th Edition.

Adequate Intake (AI) Levels

There is currently insufficient data available to set a 'recommended dietary intake' level for dietary fibre. Hence, only an indication of what would be considered an 'adequate' intake level is provided. Following are the adequate intake (AI) levels for New Zealand and Australia. Note: There are no separate recommendations for soluble and insoluble fibre intake. Rather, a total ‘dietary fibre’ target is provided.

Dietary Fibre AIs – New Zealand and Australia

Life Stage	Age	Males (g/day)	Females (g/day)
Children	1-3 yr	14	14
	4-8 yr	18	18
	9-13 yr	24	20
	14-18 yr	28	22
Adults	19+ yr	30	25
Pregnancy	14-18 yr	-	25
	19+ yr	-	28
Breastfeeding	14-18 yr	-	27
	19+ yr	-	30

Suggested Dietary Target for Reducing Chronic Disease Risk

The New Zealand and Australian governments have set a suggested dietary target (SDT) for dietary fibre intake of 28g/day for women and 38g/day for men. There is some evidence to suggest that this higher intake level of dietary fibre may reduce the risk of various chronic diseases, in the opinion of the NZ and Australian health authorities. Hence, this higher SDT target is provided for the information purposes of individuals and is based on current scientific evidence relating diet to chronic disease risk.

Deficiency

No deficiency state, as such, exists for low intakes of dietary fibre. However, individuals with a low dietary fibre intake are likely to have smaller faecal bulk and a longer transit time for food through the colon, potentially resulting in constipation.

Toxicity

A high intake of fibre will not cause any specific effects, thus an upper limit of intake has not been set. Nonetheless, when considering increases in fibre intake it is recommended to make changes gradually as some people may experience bloating, gas and abdominal cramping if they increase their intake too quickly. 

References

Ministry of Health. (2006). Nutrient reference values for Australia and New Zealand. Canberra: Commonwealth of Australia.

Last Updated: 16 October 2008

The material provided by Thinking Nutrition Ltd on this website is for information purposes only. It is not a substitute for appropriate health advice from a qualified medical practitioner.