The gut microbiota and its key role in nutritional health

Studies that have characterized the gut microbiota have also explored its primary functions. One such function is the digestion of dietary fibre through bacterial fermentation (6, 19). This results in the synthesis of short-chain fatty acids (SCFAs), which support overall health in a number of ways (6, 19). SCFAs such as acetate, propionate and butyrate serve as sources of energy for intestinal epithelial cells (6, 19). They also help maintain the integrity of the intestinal barrier, which is essential for reducing inflammatory responses (6, 14). SCFAs also contribute to satiety by stimulating the release of incretin hormones (2, 5). For example, butyrate and propionate increase the production and secretion of GLP-1 (2, 5). More broadly, the second brain also supports the immune system (12). The “good” bacteria of the gut microbiota play a protective role by occupying intestinal surfaces, preventing pathogenic micro-organisms from entering the body (6). The microbiota is also involved in mental and neurological functions, communicating with the brain and aiding in the synthesis of neurotransmitters (6, 7, 12, 14). It also facilitates the absorption of minerals that promote bone health, such as magnesium, iron, calcium and phosphorus (6, 12), and synthesizes various B-complex, K-complex and other vitamins (6, 11, 12).

In examining these many functions of the gut microbiota, scientists have found that certain bacterial profiles do more than others to promote overall health. Although there is no official consensus, a “healthy” microbiota seems to be characterized by both qualitative and quantitative bacterial diversity. Conversely, an imbalance in the gut microbiota can lead to what is known as intestinal dysbiosis (19). This has been observed in a number of different pathologies, including depression, obesity, type 2 diabetes, irritable bowel syndrome, and inflammatory bowel diseases such as Crohn’s disease and ulcerative colitis (6, 11, 12, 14, 17, 19). Science has not yet determined whether dysbiosis is a cause or a consequence of these pathologies. However, having an altered microbiota undeniably plays a role, particularly as it increases the presence of proinflammatory bacteria and causes changes in the intestinal barrier (17, 19).

As mentioned above, one of the keys to a healthy gut microbiota is diet. Studies have shown that dietary changes have a fast-acting impact on the composition of the gut microbiota (18). For example, switching from a low-fibre, meat-based diet to one high in fibre (30 g per day) caused a change in the composition and function of the microbiota after only one to two days (18). Of course, the time it takes for bacteria to react to dietary changes varies between individuals depending on their microbiota (18).

There are certain food groups and components known to benefit intestinal bacteria. Unsurprisingly, fibre is an essential nutrient for a balanced microbiota (6, 10, 14), as it provides the fuel for intestinal bacteria to produce SCFAs by fermentation (6). Studies show that a diet rich in fibre and vegetables promotes a rich and diverse bacterial ecosystem and reduces the presence of opportunistic bacteria and inflammatory markers (10, 16).

Sources of fibre: vegetables, fruits, nuts, seeds, legumes, soybeans, whole grains (oats, quinoa, brown rice, wild rice, buckwheat, millet)

Another way to benefit intestinal bacteria is by eating foods rich in prebiotics, which can also be high in fibre. Prebiotics are essentially food for bacteria and include compounds such as inulin, resistant starch, and polyphenols (3, 6, 20). Consuming prebiotic-rich foods is associated with a healthy microbiota, well-maintained integrity of the intestinal barrier, and increased production of SCFAs (3, 6). The importance of prebiotics is illustrated by the fact that low-FODMAP diets, which are particularly low in prebiotic foods, have been shown to have detrimental effects on the microbiota (1, 4, 15). Prolonged use of these diets is associated with reduced diversity of the microbiota and of certain beneficial bacteria, including Bifidobacteria (1, 4, 15). Despite the clinical benefits of low-FODMAP diets in managing gastrointestinal symptoms, their effect on microbial health demonstrates that they should not be applied on a long-term basis.

Sources of prebiotics: beets, asparagus, garlic, onions, legumes, green bananas, whole grains, oats, potatoes, nuts, seeds, berries, coffee, tea

Another dietary component that benefits a healthy microbiota is probiotics. These are defined as “live micro-organisms that, when administered in adequate amounts, confer a health benefit on the host” (9). Probiotics are currently being studied in a variety of contexts and populations. Currently, the main scientific challenge is the high level of heterogeneity in the data, with a great deal of variation in bacterial strains and quantities, duration of use, and target populations. With so many factors, it is difficult to draw conclusions from studies—not to mention the fact that each person’s microbiota is unique. The management of gastrointestinal symptoms for specific pathologies is somewhat better documented. Fortunately, there is a clinical guide available to help healthcare providers choose the right probiotic products for their patients’ symptoms (8).

Certain foods can be recommended in a clinical setting to promote microbial health for a number of clienteles. As mentioned above, diet is an important factor in balancing the microbiota, with many links to overall health. There is currently no scientific consensus on what kinds of tests can best assess the health of the microbiota, and several companies are trying to carve out a niche to help professionals provide more personalized care. Among them, the ones using shotgun metagenomics are recognized as having more scientific value, rather than merely aiming to sell their own supplements to the clients of these analyses. At present, the main sources of information used to formulate specific nutritional recommendations are pathologies, nutritional assessments, medical data, and symptoms.

In terms of eating regimens, the Mediterranean diet is associated with good microbial health. It has been linked to an increase in bacterial and SCFA diversity (3, 14, 16). This is thought to be because the Mediterranean diet is rich in fibre, plants, and omega-3 fatty acids, and low in animal proteins and processed foods (3, 14, 16).

Limiting the consumption of animal protein specifically has been found to be important for gut health (3, 14, 16). Diets high in animal protein have been associated with an increase in harmful bacteria and TMAO, an inflammatory marker that plays a role in cardiovascular disease (3, 10, 16). Conversely, consuming plant proteins is associated with an increase in good bacteria (3, 16).

Food variety is also important for microbial health. One study of over 10,000 subjects found a healthier bacterial profile in those consuming more than 30 different types of plants per week, compared to those consuming fewer than 10 (13). These included vegetables, fruits, seeds, nuts, whole grains, and legumes (13). Eating a wide variety of foods cannot be recommended enough.

Eating mostly unprocessed and minimally processed foods is also important for healthy gut bacteria (14, 16, 20). Consuming food additives such as sweeteners (sucralose, aspartame, saccharin) and emulsifiers (carboxymethylcellulose, polysorbate 80) has been associated with alterations in the gut microbiota (14, 16, 20).

In sum, the nutritional recommendations for a healthy microbiota are (16):

• Eat mostly plants: vegetables, fruits, plant proteins, whole grains
• Eat more minimally processed foods and cook more at home
• Eat less red meat and processed foods

It is clear that the gut microbiota plays an important role in overall health. It performs many essential functions throughout the body. Intestinal bacteria in particular also play a key role in a number of health conditions. It will be interesting to see how future studies help us better understand the concept of the “microbial signature,” which seems to be emerging as a predisposing factor for allergies, obesity, certain mental health disorders, and more. Since the topic has become so popular, we are also learning about many associations between gut bacteria and hormones in the context of fertility and perimenopause. It will be interesting to see what role the microbiota plays in managing these critical aspects of human health.

Clearly, there’s still much to learn from the world of micro-organisms. Future studies assessing the impact of diet on the microbiota could lead to even more personalized recommendations based on individual bacterial signatures.