Research into the microbiome has increased exponentially over the last 10 years and there are thousands of papers published on the subject every year but what is the relevance for day to day diet and lifestyle choices?
The most effective measure turns out to be cheap and simple: a high intake of a variety of fibres
PROBIOTICS, PREBIOTICS OR BOTH (synbiotics)
The probiotics market was valued at USD 42.66 Billion in 2016, and is projected to reach USD 64.02 Billion by 2022 but the health benefits are still unproven and EFSA has prohibited any health claims associated with the marketing of probiotics. Is it really necessary to eat pro-biotics? small numbers are ever present anyway and just multiply exponentially if they get the right substrates. So far, very few probiotic bacteria or combination of bacteria other than total fecal transplant has been able to modify the microbiome, and why on earth would anyone expect a few bacterial colonies to impact an ecosystem containing thousands of species of micro-organisms and trillions of individual cells. On the other hand, different diets invariably produce profoundly contrasting microbial patterns. We already know how important substrate is for microbial growth, they bloom with appropriate substrate and vanish just as quickly and become dormant without it. A probiotic without an ecological niche and substrate to sustain it is unlikely to survive so synbiotics make a lot more sense. The only things we know for sure are that diet profoundly and rapidly changes the microbiota, that increased species diversity is better than decreased diversity and diverse diets with mixtures of different fibres increase microbiota diversity much more than a single fibre source. So, the most effective measure turns out to be cheap and simple: a high intake of a variety of fibres.
Mechanisms?
But by what means do symbiotic microbiota improve health? First, friendly bacteria are not so friendly if they are not prevented from entering the host by the efficient function and integrity of the gut lining. It’s the byproducts and signaling molecules produced by symbiotic microbes, their metabiotics, that have potential health benefits.
The importance of metabiotics should not be surprising as most plant eating animals rely almost exclusively for their nutrition on byproducts of microbial breakdown of otherwise indigestible plant materials. The most important of these metabiotics that we know of are short chain fatty acids (SCFA) but there are so many more essential building blocks that microbes synthesize as how else do grazing animals build all their body mass eating virtually no protein, carbohydrate or fat. These animals demonstrate a direct relationship between the amount of fibre consumed and microbiota derived nutrients available for growth and energy and this dose response relationship between fibre and metabiotics must hold for humans as well.
Whereas the European Food Safety Authority has so far denied all health claims for “Probiotics and Prebiotics” they have approved several claims for fiber, however the health benefits of fiber go way beyond the simple concept of bowel regularity. Fibre is a powerful antidote to diabetes and its benefits are being linked to many other diseases (Kim, 2014). SCFAs are essential nutrients for intestinal cells particularly butyrate which provides colonic epithelial cells their main energy source as well as cell signaling effects that produce profound changes in the structure and function of the whole hind gut (Riviere). Further beneficial effects of SCFA are mediated through 2 types of free fatty acid receptors present in cells widely distributed in the body such as CNS, immune cells, fat cells, and insulin producing pancreatic islet cells (Priyadarshini, 2016). Colonic entero-endocrine cells increase in number and increase production of incretin hormones GLP1 and PYY on stimulation by SCFA (Konishi, 1984) and these hormones suppress appetite and voluntary food intake (Spreckley, 2015). GLP1 has multiple beneficial actions across the body but particularly regulating glucose metabolism, pancreatic beta cell function and appetite suppression (Zietek, 2016). GLP1 agonist drugs are widely used for the treatment of diabetes and obesity but endogenously produced GLP1 is largely responsible for the remarkable cure rate of type2 diabetes with gastric bypass surgery (Madsbad, 2014). Both GLP and SCFA are involved with preserving intestinal permeability barrier function (Cani, 2009) and butyrate and propionate are powerful anti-inflammatory regulators and may reduce autoimmune diseases, allergies and promote oral tolerance of potential food allergens (Richards, 2016). The so called ‘Leaky Gut Concept’ may be a triggering event in inflammatory conditions and autoimmune diseases such as colitis, arthritis, asthma, celiac disease and type 1 diabetes (Marino, 2017) as well as a factor in age-related diseases that are thought to be increased by systemic inflammation such as heart attacks, strokes, cancer, and dementia (Bourassa, 2016). Recent research links Parkinson’s Disease with “leaky gut” inflammation and recommends fibre pre-biotic nutritional therapies to boost SCFA production (Perez-Pardo, 2017). Systemic inflammation is a prominent factor in type 2 diabetes and the anti-inflammatory effects of butyrate and propionate may be involved in the observed protection from diabetes with high fibre diets (Meijer).
For the paleo afficionados, one thing we know is that pre-historic man relied on much more dietary fibre for sustenance, possibly 100-200 g/day similar to Hadza, East African hunter gatherers recently studied. Anatomically humans are hind gut fermenters and our hominid ancestors relied on hindgut fermentation of biomass for sustenance.
Summary
The gut microbiota produces numerous important nutrients depending on the amount of microbially accessible carbohydrate (MAC or fibre) available to them. Diet is the most important determinant of the microbiome and its beneficial byproducts. Pro-biotics are unlikely to have a lasting effect in the absence of sufficient substrate to sustain them. In any event pro-biotic bacteria are ubiquitous and spontaneously flourish in the presence of appropriate substrate. The prebiotic concept is far too narrow having been driven by manufacturers of proprietary supplements whereas there are established benefits from a wide variety of natural foods containing multiple types of fibres that promote microbial diversity, metabiotic production and a healthy gut environment.