Once upon a time, the roughage in food, now called dietary fibre, was regarded as a nuisance, something that might slow the growth of children, just like it did in animals. But in the 1970s two British nutrition scientists, Dennis Burkett and Hugh Trowell, proposed something radical – that dietary fibre might actually be helpful – preventing the development of diseases like type 2 diabetes and colon cancer. In fact, they suggested that nearly every “Western” disease might respond favourably to eating more roughage.
I can still remember raising my eyebrow while listening to Dennis Burkett’s presentation to Sydney scientists in 1980. He showed us a plate-size ‘cow pat’ next to a match box. In the next slide, he showed a typical human stool of a westerner, a few hard pellets, again next to a match box (If you’d like to see more description of human stools, you’ll love the Bristol stool chart (see below)). Today, 50 years later, we know that dietary fibre increases the water content, volume and weight of faeces, and along with that the ability to alter human metabolism in ways we never thought possible.
Last year, I was lucky enough to join a group of 40 eminent scientists calling themselves the International Carbohydrate Quality Consortium, who met in Palinuro, Italy, to draw up a consensus statement on what we know and don’t know about dietary fibre. The ensuing paper was published in the journal Nutrients recently (see below).
There is still some confusion and disagreement about the definition of dietary fibre and how it should be measured. Many of us still use the original definition: carbohydrates (and relatively small amounts of other substances) derived from plants that are not digested by our digestive enzymes or absorbed in our small intestine. Hence, they reach the large intestine where a substantial amount is digested by the microbes in our microbiome. Helpful bacteria produce small organic acids that are absorbed into the body, used for our metabolism, and promote insulin sensitivity.
However, there is much that is not known about dietary fibre, in part because the structure of the plant cell wall which makes up the majority of our dietary fibre has not been fully explored. Added to this, what occurs during chewing and movement through the digestive tract is not clear. The nature and actions of the microbiome are just beginning to be investigated.
What do we know? Dietary fibre can alter gut function starting in the mouth and ending where the intestinal tract exits the body (the anus). The actual physiological effects depend critically on the properties of individual plant polysaccharides and oligosaccharides (i.e., types of fibre) and also the structural integrity of cell walls, an important part of the architecture of the plant tissue. These effects may include increasing or decreasing salivation, viscosity of the food as it travels down the gut, stomach emptying rate, extent of nutrient digestion and absorption, transit time, faecal bulking, laxation, fermentation, colonic acidity, microbiota amount and composition, binding of mucus, enzymes, hormones, bile acids and other metabolites, which may also be bioactive. Whew!
Beyond the gut, the established metabolic effects of specific types of fibre include lowering of blood cholesterol and postprandial (after meal) blood glucose, and fasting blood glucose in people with diabetes. In particular, these effects have been observed with isolated viscous fibres such as psyllium, β-glucans, guar gum, glucomannan and pectic polysaccharides. Another plant isolate, inulin, though not viscous, can lower fasting glucose and insulin and fasting LDL-cholesterol while increasing good cholesterol (HDL) in people with diabetes and to a lesser extent in overweight and obese persons. Some manufactures add inulin to yogurt and other products.
These observations show that fibre is capable of modifying our metabolism. Moreover, fibre-rich sources of edible plants such as pulses, nuts, barley, oats, some vegetables and fruits have been shown to improve long-term control of established chronic-disease risk factors, like blood fats (i.e., cholesterol and triglycerides), glucose, blood pressure and body weight. Many of these beneficial health effects have been attributed to the presence of fibre in these foods. But a distinction needs to be made between natural sources of dietary fibre and purified or modified forms of fibre. The purified forms may not have the same properties that they did as part of the original food.
Recommendations from the ICQC:
- Dietary fibre is an important part of a sustainable, balanced healthy diet. Consumption is below recommended intake levels for optimal health in many parts of the world and may be decreasing.
- We should eat a total of 14 g /1000 Calories (3.3 g / 1000 kilojoules) of dietary fibre a day, ideally from naturally-occurring fibre.
- Dietary fibre should be a compulsory part of a food label’s Nutrition facts / information panel.
- Labelling should distinguish between fibre that is intrinsic to foods and that added as a functional supplement.
- Burkitt DP and Trowell HC. Refined carbohydrate food and disease. Academic Press, London, 1975.
- The Bristol stool chart
- Dietary Fibre Consensus from the International Carbohydrate Quality Consortium
- Capuano E. The behavior of dietary fiber in the gastrointestinal tract determines its physiological effect. Crit Rev Food Sci Nutr. 2017
Professor Jennie Brand-Miller holds a Personal Chair in Human Nutrition in the Charles Perkins Centre and the School of Life and Environmental Sciences, at the University of Sydney. She is recognised around the world for her work on carbohydrates and the glycemic index (or GI) of foods, with over 300 scientific publications. Her books about the glycemic index have been bestsellers and made the GI a household word.