Research into the gut microbiota has continued to accelerate in the last decade or so; and we now know for sure that the effects of a healthy / unhealthy gut microbiota are not limited to the gut. What happens in the gut certainly doesn’t stay in the gut and it is true to say that every other bodily system may be impacted in some way by alterations (good or bad) in this internal ecosystem. The old adage ‘you are what you eat’ has been expanded in the past to ‘you are what you eat, and can digest and absorb’. And perhaps now needs to read, ‘you are what you eat, and can digest and absorb, and this is dependent on a healthy gut microbiota.’

More recently, there has been heightened interest in the relationship between the gut microbiota and a healthy weight. In part, because obesity has been linked to an increased risk of disease severity in COVID-19,1 and increasingly, evidence has shown an interaction between COVID-19 and homeostasis of the gut microbiota.2 Join the dots and the relationship between obesity and the gut microbiota becomes incredibly important to explore. As you will see however, it’s not straightforward. Yet there are some simple takeaways - let’s take a closer look.

Features of a healthy gut microbiota
Diversity is a word that comes up time and again in relation to good health. It is now generally agreed that including in the diet, a diverse variety of plant foods is an important foundation of health. And one major proposed benefit of consuming plant food variety is to feed a diverse gut microbiota; one where many different species of beneficial gut microbes co-exist synergistically to form an intestinal ecosystem that can positively support health.

We know that there are many different species of beneficial microbes within the gut microbiota, each with their own unique health-supportive roles and likely many more that are yet to be identified. What is also known is that other aspects such as ‘diversity’ and ‘richness’ of species, and ‘microbiome stability’ are important too.3 A disrupted gut microbiota, or ‘gut dysbiosis’ may be due to loss of beneficial microbes, proliferation of potentially harmful microbes, and reduction of microbial diversity or richness. Numerous studies have found that reduced microbiota diversity may be associated with health problems, for example in conditions such as Crohn’s Disease and Ulcerative Colitis.4-6 The gut microbiota also appears to contribute to the course and severity of COVID-19.2 This March 2021 review on the subject published in Frontiers in Medicine is a great read if you’re interested in a deeper dive on this. 

Gut microbiota and obesity
The relationship between the gut microbiota and obesity is complex, continually evolving and far from being fully understood. Many studies to date however have reported differences in microbiota composition between obese and lean humans.7-9

The gut microbiota is often likened to an endocrine organ, involved in the maintenance of host immunity and energy homeostasis. Any alterations in the composition of the gut microbiota may alter the relationship between microbes and host and may result in low grade chronic inflammatory processes such as is typically seen in metabolic disease and obesity. Indeed, alterations in the gut microbiota has been identified as an important factor in the development of metabolic diseases.10 Since we are facing a global health crisis of metabolic diseases and obesity, the involvement of the gut microbiota is a crucial area to consider.

Reduced microbiome ‘richness’
One noticeable alteration that has been observed in some studies of obesity is a reduced ‘richness’ of microbes or their genes in some obese individuals. In one 2013 study published in Nature, researchers evaluated the gut microbial composition in 123 non-obese and 169 obese Danish individuals. The researchers found that “individuals with a low bacterial richness (23% of the population) are characterized by more marked overall adiposity, insulin resistance and dyslipidaemia and a more pronounced inflammatory phenotype when compared with high bacterial richness individuals.” It is important to note however that low richness was not observed in the majority of obese individuals, indicating that low richness may only have the potential to be important in a subset of obese individuals.11

Population variation in gut microbiota-obesity relationship
And indeed researchers have started to explore gut microbiota phenotypes of obesity across different populations. In a 2019 study on Gut microbiota phenotypes of obesity, and published in Nature npg Biofilms and Microbiomes, the researchers explored the hypothesis that population heterogeneity, particularly race/ethnicity, may impact the gut microbiota-obesity relationship. They found that gut microbiota phenotypes of obesity may differ with race/ethnicity or its correlates, such as dietary components or socioeconomic status. The researchers also commented that, “non-white individuals are greatly underrepresented (in microbiome cohorts), creating substantial challenges to understanding population-level patterns in the microbiome-obesity relationship. Further study of how population heterogeneity influences the relationship between the gut microbiota and obesity is warranted”.12

Complex picture, simple takeaway
As you can see, the picture is not straightforward; and the more you delve into this subject, it may seem like more questions are thrown up than answers. Rather than drifting into overwhelm however, the key here is to acknowledge the incredible potential for the gut microbiota to influence every aspect of health, from achieving and maintaining a healthy weight to optimal metabolic health, immune regulation and so much more. Nurturing and supporting a healthy internal ecosystem will always be a good idea and yet it is crucial to acknowledge, account for and embrace variation for every individual. Just as with any aspect of health, there is no ‘one size fits all’ approach. It’s an exciting time of research for the gut microbiota; and we are likely only seeing the tip of the iceberg when it comes to understanding it more fully. The clinical takeaway however is clear; stay open and curious, yet confident in the fact that looking after the gut microbiota is relevant, and necessary for every individual at every age.

Useful resources:

Click here for inspiration on how to include more plant diversity into the diet.

Click here for how to nurture healthy gut bacteria for good. 

Click here for a fully referenced Research Summary on Probiotics.

References
1. Ho JSY, Fernando DI, et al. Obesity in Covid-19: A systematic review and meta-analysis. Ann Acad Med Singap. 2020 Dec; 49 (12): 996-1008.
2. Burchill E, Lymberopoulos E, et al. The unique impact of Covid-19 on human gut microbiome research. Frontiers in Medicine, 16 March 2021. https://doi.org/10.3389/fmed.2021.652464
3. Johnson KVA, Burnet PWJ, et al. Microbiome: Should we diversify from diversity? Gut Microbes. 2016; 7(6): 455-458
4. Manichanh C, Rigottier-Gois L, Bonnaud E, Gloux K, Pelletier E, Frangeul L, Nalin R, Jarrin C, Chardon P, Marteau P, et al.. Reduced diversity of faecal microbiota in Crohn's disease revealed by a metagenomic approach. Gut 2006; 55:205-11;
5. Noor SO, Ridgway K, Scovell L, et al. Ulcerative colitis and irritable bowel patients exhibit distinct abnormalities of the gut microbiota. BMC Gastroenterol 2010; 10:134.
6. Falony G, Joossens M, Vieira-Silva S, et al. Population-level analysis of gut microbiome variation. Science 2016; 352:560-4.
7. Zupancic, M. L. et al. Analysis of the gut microbiota in the old order amish and its relation to the metabolic syndrome. PLoS ONE 7, e43052 (2012).
8. Turnbaugh, P. J. et al. A core gut microbiome in obese and lean twins. Nature 457, 480–484 (2008).
9. Hermes, G. D. A., Zoetendal, E. G. & Smidt, H. Molecular ecological tools to decipher the role of our microbial mass in obesity. Benef. Microbes 6, 61–81 (2015).
10. Marchesi JR, Adams DH, Fava F, et al. The gut microbiota and host health: a new clinical frontier. Gut. 2016; 65:330–9
11. Le Chatelier, E. et al. Richness of human gut microbiome correlates with metabolic markers. Nature 500, 541–546 (2013).
12. Stanislawski, M.A., Dabelea, D., Lange, L.A. et al. Gut microbiota phenotypes of obesity. npj Biofilms Microbiomes 5, 18 (2019).

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