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The important role of gut microbiota in our health

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The important role of gut microbiota in our health

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In the last few years, nutritional research has moved onto investigating how the gut microbiota, diet, and metabolic health are interlinked. Recent studies are suggesting a complex interplay exists, whereby our gut microbes are associated with our diet and state of health, and those microbes may also change the level of dietary host effects after meals, thus impacting long-term health and weight, as you can see in the simplified diagram below. 

Our gut contains a complex ecosystem of different microorganisms, known as the gut microbiota. These microbes form a very complex ecological entity in our body that plays an important role in many aspects of nutrition and health. 

This includes:

o   Transformation and production of thousands of key metabolites, enzymes, and vitamins (many of which cannot be produced by humans and are not available directly through diet);

o   Extraction of nutrients from our food;

o   Metabolic responses to food.

The structure and community of the gut microbiota have been associated with both health and disease states, including cardiovascular disease, type 2 diabetes, inflammatory bowel disease, cancer, and obesity, according to recent studies (1).

The gut microbiota has been suggested to influence metabolic health through several different host-microbiota interactions mediated by diet both directly (through the availability of diet-dependent metabolites) and indirectly (through modulating the composition of the microbiome). 

The composition of your microbiota varies according to the foods you are eating: changes can take place within days of dietary alteration (2-3), thus by eating the types of foods that increase the presence of “good” microbes and reduce “bad” microbes, we can potentially change the suite of molecules that our gut microbes produce. 

The food we eat influences the our microbiota.

Let’s move on to the most interesting part: what are the foods that can affect our microbiota? We already know that diet is a key modifiable factor in manipulating the microbial community (4).

While nutrients and foods are not consumed in isolation, but rather as part of a broad and varied dietary pattern, there are a few key dietary components that are known to impact our gut microbiota to a greater extent (e.g. dietary fibre, polyphenols, animal products, artificial sweeteners, and dietary fats).

A recent and interesting outcome of research is about the identification of specific foods that correlate with individual microbes (5-6)

DIETARY FIBRE

 

Unsurprisingly, most foods that support the growth of “good” microbes are high in fibre. 

Dietary fibre is used to describe a variety of plant-derived compounds that are not broken down by human enzymes. Without microbes, humans are unable to make full use of both soluble and insoluble dietary fibres. Different microbes are specialised at breaking down different types of fibre, including prebiotic fibres, which are fermented by gut microbiota in the colon. As a result, a wide variety of fibre is important to support a healthy gut microbiota. 

By eating specific high-fibre foods, one can likely increase the population of particular microbes.

As previously mentioned, prebiotic fibres are types of fibre that are fermented by the gut microbiota, supporting the growth of microbes that produce beneficial short-chain fatty acids (SCFAs) including butyrate, acetate, and propionate (7-8). These SCFAs play several important roles in the body, including maintaining the integrity of the large intestine; providing energy to cells in the colon; preventing gut microbiota dysbiosis (9); regulating gluconeogenesis in the liver; hunger and satiety signalling (7); appetite regulation (10); cholesterol metabolism and lipogenesis. 

Low fibre intake negatively impacts beneficial gut microbes, thus reducing the production of SCFAs and leading to the production of harmful metabolites that trigger inflammation (7)

Higher fibre intake, on the other hand, supports the growth of gut microbes that produce SCFAs and correlates with lower diet-induced obesity (11) and insulin resistance (12)

FATS

 

Although non-digestible carbohydrates are gut microbes’ preferred fuel, both the quality and quantity of dietary fats have been shown to impact gut microbiome composition. Research has shown certain fat specific digesting microbes (such as Bilophila) are increased in people on high-fat diets, which are associated with negative long-term health consequences (13). It’s important to note that this association is largely driven by the consumption of ‘bad’ fats (i.e. diets high in saturated and trans fats). A recent systematic review has also highlighted associations between total fat intake (mainly saturated fatty acids), and a reduction in total bacterial number, bacterial richness, and gut microbiome diversity (14).

Bile acids, which are synthesised from cholesterol in the liver, are responsible for facilitating the digestion of dietary fats and oils in the colon. 

MEAT & DAIRY PRODUCTS

 

Evidence suggest that excessive consumption of red meat can also alter gut microbiome composition. In particular, the abundance and activity of Bilophila wadsworthia has been shown to increase in response to a diet high in animal-based products (15).

Interestingly, the production of trimethylamine-N-oxide (TMAO) from dietary phosphatidylcholine and carnitine, which are found in meat and dairy, depends on the gut microbiota. The amount of TMAO found in the bloodstream thus varies widely between people. Trimethylamine is oxidised in the liver to trimethylamine N-oxide, which is positively associated with an increased risk of atherosclerosis, clotting, and major adverse cardiovascular events. This means that some people have more negative consequences of eating meat and dairy than others. 

POLYPHENOLS

 

Polyphenols are a large group of several thousand different chemicals that are found in plant foods. They are generally found in high amounts in brightly coloured fruits and vegetables and those with a slightly bitter or tannin taste (16-17). For example, there are several times more polyphenols in purple carrots or potatoes compared to the common varieties. Polyphenols are also found in nuts, olive oil, and red wine. They provide beneficial gut microbes with a source of energy, enabling them to transform them into bioavailable compounds that are linked to good health. Polyphenols have antioxidant and anti-inflammatory properties, which means that they play a role in preventing oxidative stress and inflammation associated with several common diseases including cardiovascular disease, obesity, and type 2 diabetes (18). Recent studies have also demonstrated that the benefits of plant-rich diets on reducing obesity are due to both the effects of dietary fibre and polyphenol content independently (19).

ARTIFICIAL SWEETENERS

 

Some artificial sweeteners remain undigested as they pass through the digestive system and leave the body unchanged. However, recent preliminary research predominantly in mouse models suggests that they may adversely influence our health by interacting with our gut microbes. Artificial sweeteners have been shown to significantly reduce total gut microbe counts and diversity in mice, and studies of sucralose consumption in humans have shown microbiome changes in those who experienced the largest postprandial insulin responses (20-21-22). More human studies are needed to determine the full extent of their effects on the gut microbiome, but preliminary research is confident in saying that effects are adverse.

 

REFERENCES:

  1.     Singh RK, Chang H-W, Yan D et al. Influence of diet on the gut microbiome and implications for human health. J Transl Med. 2017;15:73. doi:10.1186/s12967-017-1175-y
  2.     Valdes A, Walter J, Segal E et al. Role of the gut microbiota in nutrition and health. BMJ. Jun 2018;361:k2178. doi:10.1136/bmj.k2179
  3.     David LA, Maurice CF, Carmody RN et al. Diet rapidly and reproducibly alters the human gut microbiome. Nature. 2014 Jan 23;505(7484):559-63. doi:10.1038/nature12820
  4.     Tilg H, Moschen AR. Microbiota and diabetes: an evolving relationship. Gut. 2014 Sep;63(9):1513-21. doi:10.1136/gutjnl-2014-306928
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  6.     Spector T, Asnicar F, Berry S et al. Microbiome Signatures of Nutrients, Foods and Dietary Patterns: Potential for Personalized Nutrition from The PREDICT 1 Study. Curr Dev Nutr. 2020 Jun;4(Suppl 2):1587. doi:10.1093/cdn/nzaa062_044
  7.     Wong JM, de Souza R, Kendall CW et al. Colonic health: fermentation and short chain fatty acids. J Clin Gastroenterol. 2006;40(3):235-243. doi:10.1097/00004836-200603000-00015
  8.     Valdes A, Walter J, Segal E et al. Role of the gut microbiota in nutrition and health. BMJ. Jun 2018;361:k2178. doi:10.1136/bmj.k2179
  9.     Byndloss MX, Olsan EE, Rivera-Ch.vez F et al. Microbiota-activated PPAR-γ signalling inhibits dysbiotic Enterobacteriaceae expansion. Science. 2017 Aug;357(6351):570-5. doi:10.1126/science.aam9949
  10. Frost G, Sleeth ML, Sahuri-Arisoylu M, et al. The short-chain fatty acid acetate reduces appetite via a central homeostatic mechanism. Nat Commun. 2014;5:3611. doi:10.1038/ncomms4611
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