What is dietary fibre?
Dietary fibre is used to describe the majority of the carbohydrates that enter the colon, such as non-starch polysaccharides, resistant starch and indigested oligosaccharides. This fraction of carbohydrates cannot be digested in the small intestine and is fermented by bacteria of the colon, producing short-chain fatty acids (SCFA, acetate, propionate and butyrate) and gases (carbon dioxide, hydrogen and methane).
Tips to increase fibre intake ?
- Increase your bean intake. All beans are good sources of fibre, whether baked beans, beans like kidney beans in chilli or beans in salads. Half a tin of black beans (200g –> 120g drained weight) is about 8 g of fibre. As well as beans, chickpeas and lentils are rich in fibre, high sources of protein and low fat.
- Wholegrain and wholemeal. Skip white bread and pasta, look out for wholegrain and wholemeal on the labels. Brown or wholegrain rice? Choose wholegrain rice over white rice, white rice doesn’t offer as much fibre.
- Nuts: Choose almonds, pecans, and walnuts as they have more fibre than other nuts.
- Fruit and vegetables:Consume at least your 5-a-day portions and the crunchier, the better. A medium-sized apple alone is 2g fibre. If fresh fruit isn’t available, dried fruit can offer a good alternative to boost your fibre intake but beware of the sugar content. A 50g portion of dried figs is 4g fibre.
5.Choose bran based cereals. As a rule of thumb, a high-fibre food will contain more than 6g of fibre per 100g. A 30g bowl of Bran Flakes delivers 4g of fibre. As an alternative try porridge, which is made from oats and is a great source of fibre.
How much fibre should we consume?
The Recommended Daily Intake for fibre in the United Kingdom until recently was 18 g/day and in Australia it is even higher, recommending daily consumption of 25-30 g. Following Australia this recommendation was raised in July 2015 to 30g/day (SACN report on Carbohydrates and Health). In the UK most people are not consuming enough, with the average intakes being 12.8 g/day for women (19-64 years) and 14.7 g/day for men (19-64 years) (6). A number of studies have shown that an increase in the intake of dietary fibre has been associated with the treatment and prevention of a number of multifactorial diseases, such as colon cancer, obesity and diabetes and coronary heart diseases.
The role in promoting health:
In order to fully understand the role of fibre in the promotion of health it is useful to outline the effects SCFA and gases have in the body.
- Colonic Health
As fibre enters the colon water flows in by osmosis, which increases faecal bulk and makes it softer. This results to the laxative effect, which is associated with a decrease in the transit time, increase in faecal bulk and an increase in the frequency of defecation (1) (See Graphs 1,2).
The decrease in transit time reduces the time toxins are in contact with colonocytes, hence reducing carcinogenesis in the colon, an effect originally introduced by Burkitt (1969) (8). A high fibre diet also contributes to the decrease of diseases related to constipation and abdominal pressure, such as diverticulitis (1). A number of prebiotic health effects have been associated with the balance of gut microflora and in particular with that of bifidobacteria (9) (See Figure 1).
As mentioned above, butyrate is the preferred substrate for colonocytes and is used in microbial growth. The SCFA produced by the microbial fermentation reduce the pH of the colon, which prevents the growth of pathogenic organisms in the colon (10). The reduced pH of the colon increases the bioavailability of calcium and that leads to greater mineral density (9).
- Obesity and Type 2 Diabetes
The health benefits of unavailable carbohydrates with respect to obesity and diabetes are associated with the effect they have on the gastrointestinal tract, the digestion of all carbohydrates and their absorption. High-fibre diets usually require longer chewing time compared to low-fibre diets (1). This promotes the secretion of more saliva and gastric juices, decreases gastric emptying and increases gastric distension promoting satiety (4). With the decrease of gastric emptying, chyme enters the ileum at a slower rate decreasing the rate of nutrient absorption. This is useful regarding the management of type 2 diabetes. A high fibre diet slows the absorption of glucose and consequently reduces the release of insulin, lowering the glycaemic index of foods. Carbohydrate-rich diets with low-glycaemic index have been associated with lower risk of diabetes (12). High peaks of insulin levels have been linked to the aetiology of both diabetes and coronary heart diseases (4). Unavailable carbohydrates have a lower energy density, varying from 1.4 – 2.4 kcal/g, are less palatable and hence are associated with reduced food intake and satiety (6). This yield depends upon the degree of fermentation and the proportion of the short-chain fatty acids produced (3). A high-fibre diet would prove beneficial in the management of obesity as it is closely linked to both Type 2 diabetes and coronary heart diseases.
- Cardiovascular diseases (CVD)
High intakes of fibre have been associated with a reduced risk of CVD (1). A study on Japanese Men and Women with no previous predisposition of cancer, stroke or coronary heart disease, showed that dietary intakes of fibre, in particular fruits and cereal fibres, could have a reduction in the mortality of coronary heart diseases (13). High levels of blood cholesterol are closely linked to CVD (14). As cholesterol is a precursor of bile, an increase in bile synthesis would reduce the cholesterol level in the blood. Unavailable carbohydrates bind to bile acids, increasing their excretion in faeces while increasing the bile synthesis in the gallbladder (5).
As seen in Table 1, the SCFA produced from the fermentation of the unavailable carbohydrates have antagonistic effects in the hepatic synthesis of cholesterol. This will depend on the rate of production of acetate with respect to propionate, which will either stimulate or inhibit cholesterol synthesis respectively. A meta-analysis looking at four types of primary sources of fibre, oat products, psyllium, pectin and guar gum showed they had a similar effect on lowering cholesterol levels (14).
The health benefits associated with the increase intake of dietary fibre are an active area of research. The concern still remains why people consume less fibre than is currently recommended and what can be done to change this?
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- Scientific Advisory Committee on Nutrition, Report on Nutrition recommendations on carbohydrates, including sugars and fibre. London: TSO;2015.