CARBOHYDRATES THAT ESCAPE DIGESTION
Humans can only digest a few different types of carbohydrates via enzyme action. For example, polysaccharides, found in foods like potato and rice, are digested via the action of pancreatic and salivary amylase. And disaccharides, such as sucrose (e.g. table sugar) and lactose (e.g. cows milk) are digested by the brush border enzymes sucrase and lactase, respectively.
In regard to lactose, the ability to digest it varies amongst different people around the world, particularly different cultural groups. In Australia, up to 5% of Caucasians, and 75% of non-Caucasians cannot digest it. This is referred to as lactose intolerance. Furthermore, as people get older their ability to digest lactose often decreases.
Carbohydrates that escape digestion via enzymes, or cannot be digested by enzymes, end up becoming substrates for bacterial fermentation in the lower part of the intestinal tract (aka colon). These types of carbohydrates are referred to as prebiotics and include inulin, fructo-oligosaccharides (fructans, FOS) and galacto-oligosaccharides (GOS).
When an individuals diet is rich in fruits, vegetables, whole grains, nuts and legumes, an abundant source of plant polysaccharides (carbohydrates) that contain different types of prebiotics (fermentable fibres) will be provided.
Prebiotics are defined as “selectively fermented ingredients that result in specific changes in the composition and/or activity of the gastrointestinal microbiota, thus conferring benefit(s) upon host health.”
That is, when you consume prebiotics, your gut microbiota will be altered, and you will reap health benefits. For example, when certain prebiotics are consumed they can increase butyrate-producing bacteria, such as Faecalibacterium prausnitzii. Butyrate-producing bacteria are associated with reducing gut inflammation. Prebiotics can also increase the bacteria Akkermansia muciniphila, a mucin degrading bacterium that improves gut barrier function.
In summary, prebiotics improve gut health via increasing both the diversity and bacterial numbers of the gut microbiota. By consuming foods rich in both dietary fibre and prebiotics, your gut microbes will grow and metabolise. And just like you, they need to be fed daily.
It is important to note that not all fibres are classified as prebiotics, but majority of prebiotics are classified as fibres.
Bacterial fermentation of undigested carbohydrates (prebiotics) results in the production of various end products, particularly the Short Chain Fatty Acids (SCFAs): Acetate, Propionate and Butyrate; and gases, Hydrogen and Carbon dioxide.
Among the SCFAs produced, Butyrate is likely the most important, as this SCFA is the key energy source for both your colonocytes (colon cells) and enterocytes (small intestine cells).
Nearly all of the SCFAs produced (up to 99%) by your microbiota are absorbed in the gut or actually used by your gut microbiota (either the ones producing them or their neighbours). The residual SCFAs, mostly propionate and acetate, will go on to circulate around the body, with some (Acetate) even crossing the blood-brain barrier.
SCFAs deliver a number of benefits, including:
Strengthen the integrity of the gut lining (epithelial cell integrity)
Influence lipid metabolism
Influence glucose homeostasis
Strengthen immune function
Help to regulate appetite
Many many more benefits still being discovered...
Consuming a variety of prebiotic fibres (e.g. inulin, fructans, FOS and GOS), as well as resistant starches, is the ideal way to support a diverse gut microbial community.
This is done by eating lots of different plant foods.
If a typical Western diet is consumed, the high intake of animal products and processed carbohydrates will often push out the high fibre wholefoods, resulting in a significantly reduced substrate load for the gut microbiota. As a consequence, a reduction in gut bacteria numbers and diversity will occur. This is a major cause of dysbiosis, which is defined as a dis-balance of good and bad bacteria in the gut. Dysbiosis can cause an array of digestive issues, and increase the risk for a number of chronic health conditions, including weight gain and colon cancer.
EAT FOR GUT HEALTH
To improve your gut microbial health, aim to eat a variety of plant-based wholefoods rich in fibre, including prebiotics, on a daily basis.
Prebiotic rich foods include legumes, certain vegetables, fruits, grains and nuts.
Garlic, onion, leek, shallots & spring onion
Asparagus, beetroot & fennel bulb
Green peas, snow peas & sweetcorn
Nectarines, white peaches
Dried fruit (eg. figs, dates)
Rye bread, Rye crackers
Wheat bran, wheat bread