I mentioned in a previous article that most of us are not aware of how all the little constituents that make up our food work to keep us alive and thriving, let alone how complicated our physiology is. Some of these constituents get publicity because they have some connection to a phenomenon that affects many of us; vitamin D is associated with the sunlight so when vitamin d deficiency is widely noted, we are told to get more sun, or cold and flu season is among us so you better stock up on vitamin C. However, there is one peculiar component of our food that is known for its connection to a topic that many of us find to be taboo: bowel movements, and what do most of us associate bowel movements with?… fibre… but besides helping us in the bathroom, fibre has numerous benefits that many of us are not even aware of so let’s uncover the unknown about fibre.

In simple terms, fibre represents the carbohydrate part of food that cannot be digested through the upper gastrointestinal tract, such as by stomach acid or pancreatic enzymes, but can be partly metabolized by flora in the microbiome, whilst the rest is excreted in stool. Classifying a type of dietary fibre depends on the structure of the carbohydrate that makes up that fibre;

Less than 10 monomeric (simple molecule) units	More than 10 monomeric units
Non-starch polysaccharides Cellulose/hemicellulose Gums/pectin  Mucilage  Inulin Psyllium   -glucan	Resistant oligosaccharides Galacto-oligosaccharide (GOS) Fructo-oligosaccharide (FOS)

Fibre is either soluble or insoluble, referring to the capacity to dissolve in water, and these characteristics partly dictate how it exerts its benefits. Other important characteristics include the viscosity and fermentability of the fibre, or the ability to form a gel-like consistency in water and the degree to which host microbes can metabolize it (Cronin et al, 2021). Fruits and vegetables (including nuts and seeds), or plant sources of food, contain almost all the dietary fibre that humans consume except for breast milk, which contains some oligosaccharides. Due to the absorption qualities of fibre and its ability to modulate the gut microbiome, dietary fibre has numerous benefits related to preventing and treating obesity, cancer, diabetes, and heart disease (Maurya et al, 2015). 

Prevention of cardiovascular disease

• As soluble fibre passes through the stomach, it forms a gel-like consistency, which slows gastric emptying, stimulates peristalsis, and controls nutrient absorption, and these effects can reduce postprandial blood glucose and lipid increases   
  • Reduces total and low-density lipoprotein (LDL) cholesterol by stimulating bile flow absorbing cholesterol in the small intestine 
    • A meta-analysis of 23 trials assessing the effect of dietary fibre intake on cardiovascular risk parameters found a significant beneficial effect of increased fibre on total cholesterol levels 
  • Short-chain fatty acids, by-products of fibre fermentation in the colon, can inhibit cholesterol synthesis (Hartley et al, 2016)

Prevention and treatment of obesity and diabetes  

• As previously mentioned, fibre intake has been shown to improve glucose and insulin metabolism in type 2 diabetes patients as fibre can reduce the rate of glucose absorption and breakdown
  • Management of postprandial spikes in blood glucose are an important factor in the management of T2 diabetes as adequate insulin production may be compromised (Brennan, 2005)
• Given the effects of dietary fibre on reducing nutrient absorption in the GI tract, it is evident that fibre has the positive effect of increasing satiety after a meal, which is important in the treatment of obesity as it prevents the likelihood of overeating
• Dietary polyphenols, present in sources of fibre, also exert benefits in managing obesity via reducing carbohydrate/fat absorption, fat oxidation, chronic inflammation and activating thermogenesis (I will dedicate a separate article to polyphenols)
  • Additionally, polyphenols can reduce circulating levels of ghrelin, a hunger-stimulating hormone that is inversely related to body mass, adding to fibre’s ability to improve satiety (Moyano et al, 2016)

Prevention of colon cancer 

• The effects of dietary fibre that contribute to the prevention of colon cancer are mostly attributed to fibre’s ability to modulate the microbiome (as the microbiome is such a dense topic, I will dedicate an article to elaborate on how important it is)
  • In particular, both inulin-type fructans (chain of fructose molecules), and oligosaccharides have been shown to increase Bifido and Lactobacillus species and short-chain fatty acid (SCFA) production
    • SCFAs and protective species of flora can modulate the immune system to prevent inflammation and auto-immune-related conditions such as asthma and dermatitis by enhancing T regulatory cells and consequently decreasing inflammatory cytokines (IL-1, IL-6, and TNF-) 
    • Enhancing natural killer cell and its tumoricidal (tumour cell destruction) activity whilst inhibiting infiltration of inflammatory cells in the colon (Cai et al, 2019)
    • SCFAs exert anti-carcinogenic effects by reducing colonic pH, which prevents the conversion of bile acid metabolites into more toxic forms and butyrate, in particular, reduces cell proliferation and induces apoptosis
    • Additionally, fibre also binds to bile acids and dilutes faecal carcinogenic substances (lipopolysaccharides) whilst enhancing their excretion by reducing bowel transit time (Gianfredi et al, 2018)  

Sources of fibre and their fibre content  

Source	Total (g/100 grams)	Insoluble	Soluble
Grains
Barley	17.3	–	–
Corn	13.4	–	–
Oats	10.3	6.5	3.8
Rice (dry)	1.3	1.0	0.3
Rice (cooked	0.7	0.7	–
Wheat (whole grain)	12.6	10.2	2.3
Wheat germ	14.0	12.9	1.1
Legumes & pulses
Green beans	1.90	1.40	0.50
Soy	15.0	–	–
Peas, green frozen	3.5	3.2	0.3
Kidney beans, canned	6.3	4.7	1.6
Lentils, raw	11.4	10.3	1.1
Lima beans, canned	4.2	3.8	0.4
White beans, raw	17.7	13.4	4.3
Vegetables
Potato, no skin	1.30	1.0	0.30
Bitter gourd	16.6	13.5	3.1
Beetroot	7.8	5.4	2.4
Fenugreek leaves	4.9	4.2	0.7
Ladyfinger	4.3	3.0	1.3
Spinach, raw	2.6	2.1	0.5
Turnips	2.0	1.5	0.5
Tomato, raw	1.2	0.8	0.4
Green onions, raw	2.2	2.2	0.0
Eggplant	6.6	5.3	1.3
Cucumbers, peeled	0.6	0.5	0.1
Cauliflower, raw	1.8	1.1	0.7
Celery, raw	1.5	1.0	0.5
Carrot, raw	2.5	2.30	0.20
Broccoli, raw	3.29	3.00	0.29
Fruits
Apple, unpeeled	2.0	1.8	0.2
Kiwi	3.39	2.61	0.80
Mango	1.80	1.06	0.74
Pineapple	1.20	1.10	0.10
Pomegranate	0.60	0.49	0.11
Watermelon	0.50	0.30	0.20
Grapes	1.2	0.7	0.5
Oranges	1.8	0.7	1.1
Plums	1.6	0.7	0.9
Strawberry	2.2	1.3	0.9
Bananas	1.7	1.2	0.5
Peach	1.9	1.0	0.9
Pear	3.0	2.0	1.0
Nuts and seeds
Almonds	11.20	10.10	1.10
Coconut, raw	9.0	8.5	0.5
Peanut, dry roasted	8.0	7.5	0.5
Cashew, oil roasted	6.0	–	–
Sesame seed	7.79	5.89	1.90
Flaxseed	22.33	10.15	12.18 (Dhingra et al, 2012)

The above graph highlights the amount of fibre found in most of the fibre sources we consume and hopefully it makes your shopping a little easier depending on what type of fibre you feel you need to prioritize. Fibre is a frequently overlooked and neglected component of our diet and I hope that I’ve highlighted why it’s invaluable in preventing chronic illness and overall health and we don’t need to approach our relationship with food the same way we do with things like supplements, thinking about ingredients and components because whole food chemistry is complex enough but with a little added awareness, a simple decision like adding new sources of fibre in your diet will make improvements in your health that you will be grateful for in the future. 

References

Brennan, C.S. (2005). Dietary fibre, glycaemic response, and diabetes. Molecular nutrition and food research, 49(6), 560-570. https://doi.org/10.1002/mnfr.200500025

Cai, Y., Folkerts, J., Folkerts, G., Maurer, M., et al. (2019). Microbiota-dependent and -independent effects of dietary fibre on human health. British journal of pharmacology, 177(6), 1363-1381. https://doi.org/10.1111/bph.14871

Cronin, P., Joyce, S.A., O’Toole, P.W. & O’Connor, E.M. (2021). Dietary fibre modulates the gut microbiota. Nutrients, 13(5), 1655. https://doi.org/10.3390/nu13051655

Dhingra, D., Michael, M., Rajput, H. & Patil, R.T. (2012). Dietary fibre in foods: a review. Journal of food science and technology, 49(3), 255-266. https://doi.org/10.1007%2Fs13197-011-0365-5

Gianfredi, V., Salvatori, T., Villarini, M. & Moretti, M., et al. (2018). Is dietary fibre truly protective against colon cancer? A systematic review and meta-analysis. International journal of food sciences and nutrition, 69(8), 904-915. https://doi.org/10.1080/09637486.2018.1446917

Hartley, L., May, M.D., Loveman, E., Colquitt, J.L., et al. (2016). Dietary fibre for the primary prevention of cardiovascular disease. Cochrane database of systematic reviews, 2016(1). https://doi.org/10.1002%2F14651858.CD011472.pub2

Maurya, A.K., Pandey, R.K., Rai, D., Porwal, P., et al. (2015). Waste products of fruits and vegetables processing as a source of dietary fibre: a review. Trends in biosciences, 8(19), 5129-5140.https://www.researchgate.net/profile/Paras-Porwal/publication/319350706_Waste_Product_of_Fruits_and_Vegetables_Processing_as_A_Source_of_Dietary_Fibre_A_Review/links/59a655ab4585156873cecb69/Waste-Product-of-Fruits-and-Vegetables-Processing-as-A-Source-of-Dietary-Fibre-A-Review.pdf

Moyano, G., Sayago-Ayerdi, S.G., Largo, C., Caz, V., et al. (2016). Potential use of dietary fibre from Hibiscus sabdariffa and Agave tequilana in obesity management. Journal of functional foods, 21, 1-9. https://doi.org/10.1016/j.jff.2015.11.011