We’ve all heard of it, but few of us know exactly what it is and what it does. We’re talking about fibre. Fibre is an important component of a healthy and balanced diet that plays an important role in our bodies. It is a large, non-degradable molecule that cannot be absorbed from our intestines into our bloodstream and thus passes through the intestine and is excreted without being taken into our body. The nature of fibre made it so that we used to believe it to be a useless component of food. If your body can’t absorb it, what’s the point? Today we know that dietary fibre actually has many beneficial effects for our bodies and our health; some such effects include the regulation of digestion, suppression of cardiovascular diseases and diabetes, stimulating the immune system, and (according to some sources) prevention of cancer.
Despite these important needs, the U.S. Department of Health and Human Services reports that fibre is one of four nutrients (besides calcium, potassium, and vitamin D) that are chronically deficient in the average Western diet.
What is fibre and how does it help digestion?
As we said, fibre is a large molecule in food that is not broken down by the digestive enzymes in our bodies and thus does not enter our bloodstreams. Most often they are made up of chain-linked sugars, thus placing them into the group of substances known as polysaccharides. That being said, there is one type of polysaccharide that the human body CAN digest: starch. Other than starch though, all other polysaccharides are counted as fibre. Though starch and cellulose are very similar in chemical structure (both being composed of a glucose chain), the different types of interatomic bonds make fibre unable to be taken apart in our gut while starches can be torn apart by our digestive enzymes. In addition to indigestible polysaccharides, there are several other major molecules that are also considered to be fibre, such as suberin from certain plant tissues (such as potato peels) and lignin from woody tissues (broccoli and asparagus stalks, for example), but they are less prominent on our plates.
So, digestible polysaccharides (starches) are broken down in the small intestine into individual sugar units and are absorbed into our bloodstream. Simple enough. Things get tricky though when we look at the fibres, however. These units pass through our body without being degraded, but their effects depend on a range of variables such as their solubility, viscosity, and whether or not our beneficial gut bacteria can use them for food.
Fibres that stimulate the development of our intestinal flora (just a fancy way of saying the good bacteria that we all have living inside us. Worry not, no one is starting a botanical garden in your belly!) are called prebiotics (not to be confused with probiotics, which refers to beneficial living bacteria in foods). When bacteria in the colon digest (ferment) the fibres, short fatty acids such as acetic, propionic, and butyric acid are produced; these also have significant health effects.
Why consume fibre?
Dietary fibre has many beneficial effects on the human body:
- It regulates digestion and prevents constipation as well as diarrhoea and leakage (personally, we are pretty glad for this one).
- It inhibits cardiovascular disease, mainly by reducing blood cholesterol levels.
- It inhibits diabetes, mainly by slowing glucose uptake and thus reducing the need for insulin.
- It stimulates the immune system directly by binding fibres to the immune cells in the gut (such as Peyer’s patches) or indirectly by stimulating the gut flora, which in turn stimulates the immune system.
- According to some sources, fibre is even involved in cancer prevention, but research has not yet provided a concrete effect.
The fibres with the best laxative effect are those that swell in water and are known as soluble fibre. Flaxseed and psyllium have both long been known to be useful for this.
One major epidemiological study (nearly 100 000 people!) on the impact of dietary fibre on cardiovascular risk found that the risk of such diseases was 50% lower in people consuming higher amounts of fibre (22.9 g/day as compared to 11.5 g/day). Not only did fibre have this overall effect, but those who consumed more fibre also tended to have healthier lifestyles. Smokers in the high fibre group accounted for 14% of the people and tended to consume 4.4 g of alcohol per day. In contrast, the low fibre group was 38.6% smokers and they tended to consume 11.8 g of alcohol per day. This study also suggested that this protective effect was shown mainly from fibres from cereals but not those from fruits and vegetables. Increasing cereal fibre intake by 5 g per day showed a reduction in the risk of cardiovascular diseases by as much as 37%. Though dropping levels of cholesterol may be part of this, researchers believe that this effect is too great to simply be caused by that. Prolonged macronutrient digestion time (and thus increased insulin sensitivity and decreased triglyceride levels) has also been suggested.
The direct effect of dietary fibre on cholesterol levels has also been investigated in several controlled clinical studies. Doses of 5-40 g/day resulted in a 5-17% decrease in cholesterol levels. In one double blind, randomised trail, 51 volunteers with slightly elevated cholesterol (200-260 mg/dl) were given 15 g of a mixture of various soluble fibres such as psyllium, pectin, guar (Cyamopsis spp. seed gum), and locust (Ceratonia seed gum). After two months on this diet, the average cholesterol level of the test group decreased by 7% and the LDL cholesterol level dropped by 10%. These levels remained low for the duration of the study (an additional 4 months). HDL cholesterol levels, the “cholesterol pathway, triglycerides, and weight did not change significantly. The control group that obtained no fibre supplement also saw no significant changes.
The impact of fibre on the development of cancer, unfortunately, is not as well understood. Some studies have shown good inhibition of cancer and others have shown zero effect, depending on the study design and the type of fibre studied. This shows the importance of a diet of varied fibre; not only do we need to eat enough fibre, but it also has to be diverse (are you surprised though? Diversity is the name of the game when it comes to nutrition). Two mechanisms have been proposed for a potential protective effect from fibre in regards to cancer: one is that fibre absorbs carcinogenic substances in food and thus prevents them being absorbed into our bodies while the other suggests and anti-tumour effect due to butyric acid produced by fermentation of fibres in the large intestine.
Daily dose of fibre
The recommended daily amount of fibre is at least 14 g of fibre per 1000 kcal, which for most people is at least 20-35 g of fibre per day. European legislation permits foodstuffs to be claimed as a source of fibre when the product contains at least 3 g of dietary fibre per 100 g of food or at least 1.5 g of fibre per 100 kcal. The claim that a food has a high fibre content is permissible when the product contains 2 times more dietary fibre than is required for the dietary fibre source claim.
Is too much fibre harmful?
At present, no international professional organisation has set an upper limit on the amount of fibre we should consume. Problems can, however, occur when people who eat too little fibre rapidly increase their intake. This can cause bloating, gas, and more frequent and softer stools (but not diarrhoea). To avoid this, it is best to increase fibre intake gradually over a few weeks. Large amounts of fibre can also decrease the efficiency absorption for certain minerals from food. As long as we are eating enough of these minerals though, this will not be a problem.
Digestible and indigestible starch?
As we previouly mentioned, starch is a digestible form of polysaccharide, but this does not mean the entire starch is digestible! There are multiple types: starches that digest rapidly (degrade in 20 minutes), starches that digest slowly (degrade in 100 minutes), and non-digestible starches (do not degrade in 100 minutes). For our health, it is best to have a slow digesting starch since this puts less strain on our insulin system as well as providing energy to the body for a longer period of time; this keeps us feeling fed longer and helps keep us from overeating. Indigestible starch, in contrast, enters the large intestine and acts as fibre. A starch may be indigestible due to inaccessibility to digestive enzymes or due to having a crystalline structure that prevents the enzymes from reaching it. Cooking and chewing influence these factors.
Inulin is a polysaccharide composed of 10-50 fructose sugars. Some plants store inulin in the roots as a backup food, much like many plants store starch. Inulin cannot be broken down by the human digestive enzymes and thus cannot be absorbed into our bloodstream. When the undigested inulin reaches the colon our little bacteria friends that live there consume it. These bacteria then secrete fatty acids and other products, which CAN be absorbed into our bloodstream (thanks, bacteria!).
Even though it cannot be digested in our small intestine, inulin is not completely devoid of caloric content. That being said, the calorie content is thought to be about 4 times lower than in starch.
In plants, inulin is normally found in underground parts. Plants such as dandelion (Taraxacum officinale), Jerusalem artichoke (Helianthus tuberosus), chicory (Cichorium intybus), artichoke (Cynara scolmus), onion (Allium cepa), garlic (Allium sativum), leek (Allium porum), and asparagus fern (Asparagus densiflorus) are all places you can find inulin.
Inulin has a sweet taste and increases the viscosity of food; as a result, it is often added to yoghurts and dairy products. That being said, the ,ain dietary source of inulin in Western diets is from cereals. These cereals, however, only cotnain about 1% inulin, but we eat so much of them that it makes it the main source. Onions have about 6%, but unless you’re trying to get some smelly breath you probably don’t eat onions nearly as much as cereals. On average, people probably consume about 2-4 g of inulin each day.
Research has proved that inulin has a beneficial effect on reducing serum lipids, but its effect on digestion remains a mystery. As we mentioned, large amounts of fibre intake can slightly reduce mineral uptake, but inulin actually has the opposite effect and boosts calcium absorption.
In a double-blind randomised study on 12 men with elevated blood cholesterol levels (more than 200 mg/dl), interesting results were found on the effects of inulin on lipid and glucose levels in blood. The participants received either a regular diet for 3 weeks or a diet containing 20 g of chicory inulin for 3 weeks. The diets were composed in such a way that fats were less than 30% of all energy consumed, of which saturated fats represented less than 10% of all energy consumed. The total intake of cholesterol was also less than 300 mg each day. 6 days before the end of the 3 week test period, the participants ingested 20 plasticised metal beads in order to determine the passage of intestinal contents through the body. All faeces was then collected and x-rayed to determine the pace at which the beads were excreted. The results of concurrent blood tests, however, showed that inulin had a significant effect only on the triglyceride content of the participants (283 mg/dl for the control diet and 243 mg/dl for the inulin diet). Cholesterol levels were lower on average for those with the inulin diet (228 mg/dl for the non-inulin diet and 220 mg/dl for the inulin diet), but the difference was not statistically significant. Likewise, dietary inulin did not affect the amount or composition of the faeces or its passage through the gut.
The effect on calcium absorption has also been investigated, in this case in a randomised study where 50 children received 8 g of inulin daily while the control group was given an equal amount of maltodextrin. Using stable 46Ca isotopes, the proportion of calcium absorbed from food into the blood was measured. Initially both groups displayed absorption of about 30% of the calcium they consumed, but after 8 weeks this increased to 38% in the inulin group and remained unchanged for those in the control group. In influence in the Fok1 subset of the FF genotype was even more pronounced. In these, absorption increased to 45% (versus 33% in the control group). The situation remained similar after one year. Mineral bone density was also significantly increased in the inulin group.
Pectin is a constituent of the cell wall in all plant tissues and therefore is present in all foods of plant origin. It is especially abundant in unripe apples and oranges. Within the food industry it is actually used for gel formation. In fruit juice though it is undesirable as it precipitates and makes the juice cloudy (although it can be removed with the addition of pectinases).
Pectin is a branched polysaccharide composed of 100-2000 sugar units (yikes!) such as D-galacturonic acid, L-rhamnose, D-galactose, L-arabinose, D-xylose, and L-fucose. Of all the effects of pectin, its antilipemic effects (lowering of lipids in the blood) is the most studied. A mixture of guar, apple pectin, and apple pulp in a study was shown to decrease serum cholesterol by 12% while the triglyceride content dropped by 17%. HDL cholesterol, however, remained unchanged.
1. Haralampu S.G.: Resistant starch – a review of the physiological properties and biological impact of RS3. Carbohydrate Polymers 41 (2000) 285-292.
2. Den Hond E, Geypens B, Ghoos Y: Effect of high performance chichory inulin on constipation. Nutrition research 2000, Vol. 20, pp 731-736.
3. Causey JL, Feirtag JM, Gallaher DD et al: Effects of dietary inulin on serum lipids, blood glucose and the gastrointestinal environment in hyperholesterolemic men. Nutrition Research 2000, Vol. 20, pp 191-201.
4. Williams CM: Effects of inulin on lipid parameters in humans. J Nutr 1999 Jul;129(7 Suppl):1471S-3S.
5. Taper HS, Roberfroid M: Influence of inulin and oligofructose on breast cancer and tumor growth. J Nutr 1999 Jul;129(7 Suppl):1488S-91S.
Pokorn D: Balastne snovi pri Slovencih, JAMA [Elektronski vir]. – ISSN 1408-8754; 7, 5 (1999), 5-6.
6. Fuchs C.S., Giovannucci E.L., Colditz G.A., Hunter D.J., Stampfer M.J., Rosner B., Speizer F.E., Willett W.C.: Dietary fiber and the risk of colorectal cancer and adenoma in women, N. Engl. J. Med. 340 (1999) 169-176.
7. Wolk A., Manson J.E., Stampfer M.J., Coldiz G.A., Hu F.B., Speier F.E., Hennekens C.H., Willet W.C.: Dolgoročno uživanje balastnih snovi in zmanjšano tveganje za koronarno bolezen pri ženskah, JAMA 281 (1999) 1998-2004. (http://www.mf.uni-lj.si/jama/jama99-5/html/dolgorocno.html)
8. Jansen C.D., Haskell W., Whittam J.H.: Long-term effects of water-solble dietary fiber in the management of hypercholesterolemia in healthy men and women, Am J Cardioll 79 (1997) 34-37.
9. EU 2006a: UREDBA (ES) št. 1924/2006 EVROPSKEGA PARLEMENTA IN SVETA z dne 20. decembra 2006 o prehranskih in zdravstvenih trditvah na živilih; SL Uradni list Evropske unije L 404/9, 30.12.2006
10. EU 2006: PREHRANSKE TRDITVE IN POGOJI, KI VELJAJO ZANJE, Uradni list Evropske unije C 297 E/95, 7.12.2006.
11. FAO 2010: Codex alimentarius commission. ftp://ftp.fao.org/codex/ccmas31/ma31_05e.pdf
12. Garcimartin M; Vidal-Valverde C; Martinez-Castro I; et al. Evaluation of dietary fiber in potato crisps: influence of the analytical method used. Journal of Food Quality 18(1) 33-43.
13. Abrams S, Griffin I, Hawthorne K, Liang L, Gunn S, Darlington G, Ellis K (2005). “A combination of prebiotic short- and long-chain inulin-type fructans enhances calcium absorption and bone mineralization in young adolescents”. Am J Clin Nutr 82 (2): 471–6.
14. Coudray C, Demigné C, Rayssiguier Y (2003). “Effects of dietary fibers on magnesium absorption in animals and humans”. J Nutr 133 (1): 1–4.