Effects of Dietary Fibre on Metabolic Health

Dietary fibre and its types

Dietary fibre has been very well studied over the past many years for its health rewards.1 Dietary fibre, a plant derived substance resistant to enzymatic digestion includes cellulose, non-cellulosic polysaccharides (hemicellulose, pectins, gums, mucilages) and lignin.2 The Academy of Nutrition and Dietetics (2015) has put forth the importance of taking dietary fibre in adequate amount from various plant sources.3 The Institute of Medicine (IOM), National Academy of Sciences defines dietary fibre as non-digestible carbohydrate and lignin that is intrinsic and intact in plants, whereas functional fibre is isolated non-digestible carbohydrates that have beneficial physiological effects in humans. Total fiber is the sum of dietary fiber and functional fiber.1

Dietary Guidelines Advisory Committee (DGAC), 2015 characterized dietary fibre as one of the shortfall nutrients. According to this report, the under-consumption of dietary fibre in the general public should be considered as a ‘public health concern’ because of the adverse health outcomes.4 In most of the nations across the globe, the present dietary fibre consumption levels are far below the recommended levels.1 The IOM has not set any upper limit for fibre intake. In recent years, there is a trend to find new sources of dietary fibre that can be used in the food industry and supplementation has been used to enhance fibre content of foods.2

Dietary fibres are basically of two types2:

Metabolism of dietary fibre

Cereals, vegetables, nuts and fruits are the natural sources of dietary fibre.2 Dietary fibre sources, in addition to fibre, provide additional micronutrients and phytochemicals, thereby further improving the health. Also, the dietary fibre-rich foods have low saturated and trans-fatty acids, sodium and sugar contents. Therefore, consumption of adequate dietary fibre is associated with improved health outcomes and reduces the risk of various chronic diseases, such as cardiovascular diseases, type II diabetes mellitus, and some types of cancers. It also helps in keeping digestion process healthy and reduces the risk of weight gain.3

Even though fibre does not add much to the energy, intake of fibre plays various significant roles. The bacteria residing in the colon are responsible mainly for the metabolism of fibre. The susceptibility to the fermentation depends on the structure of the fibre.5

Acetate, butyrate and propionate are the common SCFAs produced by fermentation of fibres. The colonic bacterial milieu and the diet display a dynamic interplay. These fatty acids play important role in colonic physiology by stimulating colonic blood flow and promoting fluid and electrolyte uptake. Butyrate is fuel for the colonic cells, and perhaps it helps to maintain the normal phenotype of these cells.5

Benefits of dietary fibre

The important roles of dietary fibre are as following2,5:

  • Dietary fibre aids in laxation as it increases the volume of faecal bulk and helps in expulsion of toxins through the faecal stream. It maintains and forms stool bulk because of its resistant to colonic degradation. It also regulates intraluminal pressure and colonic wall resistance. In other words, intake of adequate dietary fibre prevents constipation and diverticulosis. These effects are due to the water-binding ability of the dietary fibre and fermentation resulting in changes in osmotic balance, thus increasing faecal biomass.3
  • Dietary fibre dilutes toxins, carcinogens and tumour promoters by decreasing intestinal transit time, and thus the colonic mucosal exposure. The inverse relationship of dietary fibre intake and total death rates, specifically cardiovascular, infectious, and respiratory deaths in both men and women, and cancer deaths in men has been well shown by the data from the National Institutes of Health- AARP Diet and Health Study.3
  • Dietary fibre balances intestinal pH and stimulates production of short-chain fatty acids and thereby reduce the risk of colorectal cancers. A study by Dahm et al. showed a strong inverse relationship between intake of dietary fibre and colorectal cancer. The risk for developing the cancer in individuals consuming the highest quintile of fibre (24g) was 30% less than those consuming the lowest quintile of fibre intake (10g). This study further supports the dietary fibre intake recommendations to decrease the incidence of colorectal cancers.6 This association may be attributed to mechanisms such as reduction in colonic transit time, changes in bile acid metabolism, dilution of gut contents, and short-chain fatty acid production.6
  • Dietary fibre also helps in regulating blood sugar levels by decreasing the rate of release and absorption of simple sugars. This results in lower postprandial blood sugars levels. It also has a positive impact on insulin sensitivity, attributing to its low glycaemic index. A meta-analysis of observational studies by Yao et al. concluded that the risk of type 2 diabetes decreased by 6% with more dietary fibre intake (2g/ day increment cereal fiber intake).7
  • Dietary fibre intake is associated with lower blood pressures, particularly systolic blood pressure. Soluble fibre types such as pectin lowers the serum cholesterol, while the insoluble fibres reduce serum triglycerides. Dietary fibre also controls appetite by slowing down gastric emptying and promoting satiety, and thus, has been linked to lower body weights. Thus dietary fibre clearly has a role in prevention of hypercholesterolemia and obesity and thus reduces the chances of stroke. Findings from a meta-analysis by Threapleton et al. showed that increased dietary fibre intake is significantly associated with decrease risk of primary stroke. This study also supports the current recommendations for increased dietary fibre intake.8

Recommendations

According to the 2010 Dietary Guidelines for Americans (DGA), the recommended AI levels for total fibre intake by age and gender are1:

The American Academy of Paediatrics suggests two different guidelines for fibre intake in children:

  1. Age of the child in years + 5 g
  2. 5 g fibre/(kg body weight × d) up to 35 g/d.

The DRI for dietary fibre as recommended by IOM is 14 g fibre/1000 kcal consumed or between 19 and 38 g/d in children, depending on age.

According to NHANES 2013-2014 data, the mean dietary fibre intake of most children and adults is not enough. It is 16 g per day by children aged 2 and older, 18 g per day by men and 15 g per day by women.10 This means the intake levels are one-half to one-third of the recommended levels.11

The medical practitioners should advise patients to incorporate fibre-rich foods in their routine diet in moderate amounts throughout the day.3 Consumption of dietary fibre can fill the gap between the recommended intake levels and the prevalent intake levels.1

New researches around role in metabolic health

A recent study by Mackie et al. has revealed the benefits of dietary fibre in the upper GI tract. Due to the viscosity of fibres, gastric emptying is altered and therefore, satiety and fullness as well are affected subsequent to nutrient release and nutrient sensing.12

As dietary fibre is a key substrate for microbiota of gut, regulation of dietary fibre intake has an immediate and direct impact on the microbial population of the gut.3 Dietary fibres are able to modulate the gut microbiota and thus play a critical role in the composition and metabolism of the microbiomes, which in turn affects intestinal health.3 Dietary fibre intake is also known to have effects on immune function via the gut microbiota.3

Conclusion

The recommendation of the dietary fibre intake is underscored across the globe because of the poor fibre intake. According to NHANES 2013-2014, there is a large gap between the actual intake and the recommended levels. To address this large gap and in order to meet the recommended intake dietary fibre levels, it is critical to increase the dietary fibre consumption. The health benefits of dietary fibres are very well researched and well documented over the past decades. Higher dietary fibre intake reduces the risk of developing various chronic diseases such as cardiovascular disease, type II diabetes mellitus, and some cancers. Such populations are also associated with lower body weights. Apart from these, it is strongly suggested that the occurrence of conditions such as constipation, diverticular disease, and gallstones could be reduced with the adequate dietary fibre intake.

References

  1. Li YO, Komarek AR. Dietary fibre basics: Health, nutrition, analysis, and applications. Food Qual Safety. 2017;1:47-59.
  2. Dhingra D, Michael M, Rajput H, Patil RT. Dietary fibre in foods: a review. J Food Sci Technol. 2012;49(3):255-66.
  3. Wendy J. Dahl PhD, RD and Maria L. Stewart PhD. Position of the Academy of Nutrition and Dietetics: health implications of dietary fiber. J Acad Nutr Diet. 2015;115:1861-70.
  4. ODPHP. Scientific report of the 2015 Dietary Guidelines Advisory Committee. Part A: executive summary. 2015. Available from https://health.gov/dietaryguidelines/2015-scientific-report/02-executive-summary.asp.
  5. Parks EP, Maqbool A, Shaikhkhalil A, Groleau V, Dougherty KA, Stallings VA. Nutritional requirements. In: Kliegman RM, Stanton BF, St Geme JW, Schor NF, editors. Nelson textbook of paediatrics. 20th ed., Chapter 44. Philadelphia, PA: Elsevier, Inc.; 2016. p. 268-86.e1.
  6. Dahm CC, Keogh RH, Spencer EA, et al. Dietary fiber and colorectal cancer risk: a nested case–control study using food diaries. J Natl Cancer Inst 2010;102:614-26.
  7. Yao B, Fang H, Xu W, et al. Dietary fiber intake and risk of type 2 diabetes: a dose–response analysis of prospective studies. Eur J Epidemiol. 2014;29(2):79-88.
  8. Threapleton DE, Greenwood DC, Evans CEL, et al. Dietary Fiber intake and risk of first stroke: a systematic review and meta-analysis. Stroke. 2013;44:1360-68.
  9. Kranz S, Brauchla M, Slavin JL, Miller KB. What do we know about dietary fiber intake in children and health? The effects of fiber intake on constipation, obesity, and diabetes in children. Adv Nutr. 2012;3:47–53.
  10. Centers for Disease Control and Prevention. NCHS fact sheet. March 2017. Available from https://www.cdc.gov/nchs.
  11. Jones JM. Dietary fiber future directions: integrating new definitions and findings to inform nutrition research and communication. Adv Nutr. 2013;4:8-15.
  12. Mackie A, Bajka B, Rigby N. Roles for dietary fibre in the upper GI tract: the importance of viscosity. Food Res Int. 8(Part B);234-8.
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