Fibre and health
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Fibre and health

The relationship between fibre and health has been appreciated for centuries, specifically as it relates to digestive health. More recently, scientists have set to investigate the role of fibre in other aspects of health.

What is fibre

There are multiple definitions of fibre and it’s difficult to write a concise one. Most fibre is carbohydrate, with the exception of lignin (1).

Fibre includes non-starch polysaccharides (cellulose, hemicelluloses, pectins, hydrocolloids), resistant oligosaccharides, resistant starch and lignin (2).

By other definitions, fibre is a polysaccharide with ten or more monomeric units which is not hydrolysed by endogenous hormones in the small intestine (3). In other words, multiple sugars held together that cannot be broken down (i.e. digested) in the small intestine.

Fibre and prebiotics are often used interchangeably, however it’s important to note that all prebiotics are fibre but not all fibre is prebiotic. This is because not all fibre passes undigested to the lower intestinal tract to be fermented by gut bacteria providing health benefits to the host (4).

Types of fibre

There are a few ways to categorise fibre.

Polysaccharides

  • Non-starch polysaccharides
  • Resistant starch (RS)
  • Resistant oligosaccharides (6)

Soluble vs insoluble

  • Soluble fibres: dissolve in water, form gels and are fermentable, e.g. pectins, gums, inulin-type fructans and some hemicelluloses, β-glucan, wheat dextrin, psyllium
  • Insoluble fibres: do not dissolve in water, do not form gels and are not fully fermentable, e.g. lignin, cellulose, some pectins and some hemicelluloses (3, 4, 5)

Both are present in most fibre-containing foods at a ratio of 1:2 soluble to insoluble (3).

Natural vs artificial

  • Natural fibre, e.g. inulin, psyllium, β-glucan from oats
  • Artificial fibre, e.g. polydextrose, wheat dextrin, methylcellulose (7)

Dietary vs functional

  • Dietary fibre: non-digestible carbohydrates and lignin from plants occurring intrinsically in plants, e.g. lignin, cellulose, β-glucans, hemicelluloses, pectins, gums, RS
  • Functional fibre: isolated non-digestible carbohydrates, e.g. resistant dextrins, psyllium, fructo-oligosaccharides (FOS), polydextrose, isolated gums, isolated RS (4, 7)

Fermentable vs non-fermentable

  • Fermentable, e.g. wheat dextrin, pectins, β-glucans, guar gum, inulin
  • Non-fermentable, e.g. cellulose, lignin (4)

Fibre can be further classified into highly, intermediate, minimally or non-fermentable fibre (5).

Viscous vs non-viscous

  • Viscous, e.g. pectins, β-glucans, some gums (e.g. guar gum), psyllium
  • Non-viscous, e.g. polydextrose, inulin (4)

Food sources of fibre

Soluble and insoluble fibres are found in different food sources such as legumes, vegetables, nuts, seeds, fruits, and cereals in different proportions (6). For example, fruits and vegetables have between 0.6 to 5.1 grams of fibre per serve (10).

FOS and galacto-oligosaccharides (GOS) come from cereal grains, onions, leeks, garlic, asparagus, bananas, raisins, chicory sprouts, soybeans, acacia gums, psyllium and other plants (2).

Inulin is present in onions, garlic, wheat, artichokes and bananas (3).

β-glucan is found in the endosperm of cereal grains, primarily barley and oats (3).

Pectin is present in fruit, for example in the peel of citrus fruit (3).

RS is found in starchy foods such as cereals, legumes, tubers, and non-mature fruits like green bananas (6).

Arabinoxylans are present in cereals (6).

How much do we need?

The daily adequate intake (AI) in Australia depends on gender, age and life stage as per the tables below (1).

Age (years) AI (males) AI (females)
1-3 14 g/d 14 g/d
4-3 18 g/d 18 g/d
9-13 24 g/d 20 g/d
14-18 28 g/d 22 g/d
14-18 (pregnant) 25 g/d
14-18 (lactating) 27 g/d
19+ 30 g/d 25 g/d
19-50 (pregnant) 28 g/d
19-50 (lactating) 30 g/d

Is there an upper limit?

The Australian Nutrient Reference Values do not specify an upper limit for dietary fibre (1). However, some fibres such as polydextrose and RS are better tolerated than FOS. For example, a relatively low dose of FOS can cause digestive discomfort in some individuals, while 50g of RS may be well tolerated (4).

How much do we eat?

As seen in my previous post about NRVs and actual intake, we don’t eat enough fibre.

Fibre and health

It is difficult to determine whether beneficial health effects are due to the fibre or other nutritional characteristics of food, such as vitamins, minerals, phytochemicals, and total energy (7).

Gut health

The fibre we eat has a direct effect on the composition and numbers of our gut microbiome (2, 6).

Some fibres such as psyllium and wheat bran increase water content in faeces. This, together with the weight of the fibre itself and the bacteria it feeds, contribute to regular stools (4, 7). On the other hand, other fibres such as wheat dextrin and finely ground wheat bran decrease water content of stools, leading to constipation (7).

Likewise, soluble fibre, such as psyllium can help mitigate symptoms of irritable bowel syndrome (IBS), while insoluble fibre can actually make them worse. In particular, patients with chronic constipation and IBS with constipation may benefit from natural fibre or fibre supplement intake (5). Moreover, fibre intake may prevent irritable bowel disease and Crohn’s disease (6).

Bacteria are also metabolically active. The most important metabolites produced by bacterial fermentation of fibre are short-chain fatty acids (SCFAs), which include acetate, propionate and butyrate. Butyrate is especially important for gut health, as it is metabolised in the gut lining. The metabolic products of prebiotic fermentation also lower the pH (5, 6), protects the mucus layer of the gut (6) and may impact gut permeability and inflammation (5).

Oligosaccharides and GOS promote the growth of Bifidobacteria (2, 4, 6) and Lactobacilli (2, 4). RS stimulates the growth of certain bacteria, including Bifidobacterium adolescentis (6). Inulin stimulates the growth of Bifidobacteria (beneficial bacteria) and limits the growth of pathogenic bacteria (3).

Immune health

SCFAs produced by fermentation of prebiotics also regulate both the innate and adaptive immune system. For example, they promote the generation of T cells in the colon (6).

Metabolic health

Some types of fibre, such as RS may have beneficial effects on individuals with metabolic syndrome (2).

Blood sugar control

According to the literature, fibre seems to be effective in preventing the development of type 2 diabetes (3).

Certain fibres, such as psyllium, β-glucan and non-hydrolysed guar gum, form gels when mixed with water. When taken before a meal, they increase the viscosity of the food bolus, slowing down the absorption of sugar in the digestive tract. This can help lower post-prandial sugar and insulin spikes (4, 6, 7).

RS has also been shown to reduce post-prandial blood sugar and insulin levels (3).

Rye bran may be helpful in lowering post-prandial blood sugar levels, although at a large dose (31g) (3).

Heart health

Increasing fibre intake may modestly lower the risk of coronary heart disease and cardiovascular disease (4, 8). This has been observed for total fibre, insoluble, cereal, fruit and vegetable fibre (8).

Gel-forming fibres can lower total and LDL-cholesterol levels. In particular, psyllium has been shown to be effective itself or in combination with other therapies (4, 7). β-glucan and guar gum have also been shown to significantly decrease both total and LDL-cholesterol (3, 4, 6).

Fermentation of inulin produces propionate, shifting the acetate to propionate ratio and reducing both total and LDL-cholesterol (3).

Pectin can change the quality of fibrin, a protein involved in blood clotting, potentially reducing the risk for atherosclerosis, stroke and coronary heart disease (3, 4, 6).

Rice bran may also decrease total and LDL-cholesterol, and triglycerides. Modified cellulose may decrease total and LDL-cholesterol. RS may decrease total cholesterol and triglycerides (3).

RS and inulin, both soluble fibres, do not appear to lower blood cholesterol, and the effect of insoluble fibre on stool weight is highly variable (4).

Weight loss

There seems to be a relationship between dietary fibre intake and weight loss (3, 4, 6). This could be explained through increased satiety and/or a decrease in energy intake (3).

There are several ways in which fibre may increase satiety:

  • the fermentation of fibre leads to the production of gut hormones that trigger satiety (3)
  • chewing fibre-rich foods takes longer than chewing fibre-free foods, leading to increased saliva and stomach acid production, leading to stomach distention, which signals fullness (4)
  • soluble and viscous fibres bind water, which also cause stomach distention, which signals fullness (4)

A high fibre intake is usually associated with a low energy intake due to the fact that people who eat more fibre tend to have healthier diets, which are usually lower in fat and energy (4).

There is no consensus regarding whether soluble or insoluble fibres are more effective in reducing body weight. It seems that insoluble fibre seems to be more effective for individuals consuming a high fat diet (3). Among soluble fibre, psyllium may have a larger effect than guar gum because it’s not fermentable and, therefore, doesn’t contribute to total energy intake (7). Similarly, inulin may be effective in preventing and treating obesity (3).

Cancer

There is mixed evidence that high fibre intake protects against colorectal cancer (2).

Likewise, there is evidence from animal studies that pectin could play a role in cancer prevention (3).

Diverticular disease

High intakes of dietary fibre may reduce the risk of diverticular disease, with fibre from fruit being more effective than total fibre and cereal fibre (9).

Respiratory health

Fibre intake is inversely associated with asthma and chronic obstructive pulmonary disease (6).

Summary and recommendations

There are multiple types of fibre which have different effects on health. Because we eat foods and not nutrients, it is hard to determine whether the health benefits are due to the is the fibre or the other nutrients in fibre-containing foods, or an overall healthier lifestyle.

Regardless, the intake of fibre-rich whole unprocessed such as whole grains, vegetables, fruit, legumes and nuts is likely to lead to better health outcomes for most people.

If you have a digestive condition, it is a good idea to talk to a dietitian as some types of fibre could be detrimental or you may need to find your tolerance threshold.

References

  1. National Health and Medical Research Council. Nutrient Reference Values for Australia and New Zealand [Internet]. Canberra: National Health and Medical Research Council; 2006. Available from: http://www.nhmrc.gov.au/_files_nhmrc/publications/attachments/n35.pdf
  2. Hijová E, Bertková I, Štofilová J. Dietary fibre as prebiotics in nutrition. Cent Eur J Public Health. 2019 Sep;27(3):251–5.
  3. Lattimer JM, Haub MD. Effects of dietary fiber and its components on metabolic health. Nutrients. 2010 Dec;2(12):1266–89.
  4. Slavin J. Fiber and prebiotics: mechanisms and health benefits. Nutrients. 2013 Apr;5(4):1417–35.
  5. Rao SSC, Yu S, Fedewa A. Systematic review: dietary fibre and FODMAP-restricted diet in the management of constipation and irritable bowel syndrome. Aliment Pharmacol Ther. 2015 Jun;41(12):1256–70.
  6. Makki K, Deehan EC, Walter J, Bäckhed F. The Impact of Dietary Fiber on Gut Microbiota in Host Health and Disease. Cell Host Microbe. 2018 Jun;23(6):705–15.
  7. Lambeau K V, McRorie JWJ. Fiber supplements and clinically proven health benefits: How to recognize and recommend an effective fiber therapy. J Am Assoc Nurse Pract. 2017 Apr;29(4):216–23.
  8. Evans CEL. Dietary fibre and cardiovascular health: a review of current evidence and policy. Proc Nutr Soc. 2020 Feb;79(1):61–7.
  9. Aune D, Sen A, Norat T, Riboli E. Dietary fibre intake and the risk of diverticular disease: a systematic review and meta-analysis of prospective studies. Eur J Nutr. 2020 Mar;59(2):421–32.
  10. Slavin JL, Lloyd B. Health benefits of fruits and vegetables. Adv Nutr. 2012 Jul;3(4):506–16.

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