Romboutsia ilealis: How is this Bacteria Linked to Healthy Fats?

July 02, 2022 8 min read

Romboutsia ilealis: How is this Bacteria Linked to Healthy Fats? - Layer Origin Nutrition

Romboutsia ilealis: What is it? What Does it Do? How is it Linked to Healthy Fats?

When hearing the words, "the human gut", things such as the colon, intestines, acid, stomach ache, or sickness may spring to mind.

Perhaps words like Bacteroidesor Bifidobacterium may evoke a connection to the stomach.

The latter could well be due to the television advertisements or other forms of media that are becoming more frequent, promoting products that claim to modulate the gut microbiota.

But what about Romboutsia ilealis?  

It wouldn’t be unusual to see many people spending a few moments just trying to pronounce it correctly, but would there be a connection that springs to mind in connection with the human gut?

Many would be forgiven for answering “no”.

Yet, the human gut wouldn’t be the human gut without Romboutsia ilealis and several other spectacularly named gut bacteria that are busy helping to keep you from harm.

This potentially health-promoting bacterium will likely be thrust into the common vernacular soon, as more research is carried out into its beneficial properties.

In this article, we look at one of the good gut microbes identified by the PREDICT 1 study.

This time we say hello to, Romboutsia ilealis and investigate how it works —  particularly its association with polyunsaturated fats.      

What is Romboutsia ilealis?

Romboutsia ilealis is a bacterial species that belongs to the family, Peptostreptococcaceae, a close relative of the Clostridium clusters. It is gram-positive, and rod shaped and resides in the colon with some strains also found in the small intestine.

Research has shown it to have a limited capability to synthesize amino acids but can utilize simple carbohydrates via multiple pathways. It has been shown to adapt to nutrient-rich environments where carbohydrates, amino acids and vitamins are in abundance.

Genome sequencing has shown Romboutsia ilealis to have one of the highest numbers of rRNA gene copies which are believed to give the species the capabilities to adapt to changes in available sources, along with the ability to grow at a faster rate[i].  

What’s So Special About Romboutsia ilealis?

R. ilealishas been identified as one of the 15 good gut microbes in the PREDICT studies

These 15 microbes are associated with better health outcomes such as:
  • lower blood pressure
  • good metabolism
  • lower levels of dietary inflammation
  • better blood sugar control
  • less belly fat
  • lower levels of “bad fats” (triglycerides and cholesterol).

It is thought that R. ilealisis associated with lower levels of inflammation as well as higher levels of polyunsaturated fats. You can be forgiven for thinking that this may be a bad thing — but not all fats are created equal. There are some fats that are beneficial for your health, including polyunsaturated fats.

What are Polyunsaturated Fats?

When it comes to dietary fats, there are two main types: saturated and unsaturated. Whether a fat is classified as saturated or unsaturated depends on its chemical structure. Saturated fats have no double chemical bonds, whereas unsaturated fats have at least one.

Polyunsaturated fats are those that have more than one double chemical bond (if they have one then they are called monounsaturated). Both of these types are considered to be healthy fats because they have numerous benefits. However, they are what’s known as essential because the human body is unable to make them itself, and instead relies on you taking them in via your diet[ii].

There are two major classes of polyunsaturated fats, and you are probably already familiar with them both. They are omega-3 and omega-6 fatty acids.

 Dietary Sources of Polyunsaturated Fats

There are plenty of dietary sources of polyunsaturated fats available. Oily fish is the best source of omega-3 available, but it can also be found in flaxseeds, sunflower seeds, walnuts and pine nuts in lower quantities.

Good fish sources include:

  • salmon
  • herring
  • mackerel
  • trout
  • sardines
  • shrimps

Omega-6 fatty acids, on the other hand, are commonly found in plant-based oils such as:

  • flaxseed oil
  • safflower oil
  • grapeseed oil
  • sunflower oil
  • poppyseed oil

Although plant-based, coconut oil and palm oil have a high percentage of saturated fats in them and so are not good sources of omega-6 fatty acids.

Benefits of Polyunsaturated Fats

Polyunsaturated fats are an essential part of your diet because they are important for blood clotting, brain health, and nerve function.

In a study carried out in 2020, researchers found that increased consumption of both vegetable and polyunsaturated fats was linked to a decreased risk of developing hepatocellular carcinoma (HCC)[iii].

Perhaps polyunsaturated fats are most renowned for their effects on heart health. It was back in the 1960s and 70s that it was first noted that populations who consumed lots of fish had a lower risk of dying from heart disease.

Further studies, later on, linked an increased fish consumption with high omega-3 blood levels and a low risk of heart disease and heart-related deaths. In fact, a 1998 study found that men who ate fish at least once per week were at a reduced risk of sudden cardiac death[iv].

Another study involving Japanese men and women also found that consuming fish regularly may have a protective effect on the heart and help to prevent cardiovascular disease[v]. However, there are mixed results when it comes to using fish oil or omega-3 supplements. For example, some studies have found that supplements are not associated with significant reductions in coronary heart disease or heart attacks and strokes in people who were already at risk of the disease[vi].

However, this study was repeated in October 2019 with an increased sample size of almost 65%. The re-analysis of the data found that supplementation with omega-3 fish oils does lower the risk of heart attack and coronary heart disease, but the study concluded that this is dose-dependent[vii]. So, the higher the dose, the lower the risk is.  

That’s not all, there’s also research available linking omega-3 and omega-6 fatty acids to better management of attention deficit hyperactivity disorder symptoms (ADHD)[viii].

Polyunsaturated Fats and the Gut Microbiome

Diet is a major determinant of the composition and function of your gut microbiome. For example, indigestible dietary fibre is the favored energy source of your gut microbes and there’s lots of research available regarding the importance of non-digestible carbohydrates.

However, scientists are also interested in the effects of other dietary nutrients on this unique ecosystem, including fats.

It's maybe no surprise that a diet high in saturated fats can have a negative effect on the diversity of the microbiome as well as its composition[ix]. Observing the gut microbiomes of individuals who follow a Western diet and comparing their gut microbiota compositions with people who eat a predominantly Mediterranean diet, shows scientists there is a significant difference between the two[x].

A Mediterranean diet, for example, has an increased diversity whereas a typical western diet that’s rich in energy-dense and processed foods is associated with poor health outcomes[xi]. A major component of the Mediterranean diet is fish, particularly the types that are rich in omega-3s.

Research shows that omega-3 fatty acids can affect the gut microbiome in three ways, by:

  • modulating its abundance and composition
  • inhibiting the production of proinflammatory mediators or promoting anti-inflammatory types
  • regulating the production of short chain fatty acids[xii].

Some studies have shown that increased consumption of omega-3s leads to a reduction in Faecalibacterium but an increase in butyrate-producers[xiii]. However, when omega-6 fatty acids are added to the mix, a careful balance must be struck because scientists have theorised that a diet high in omega-6 but low in omega-3 may increase inflammation[xiv].

The western diet can be high in omega-6 compared to omega-3 fatty acids which may contribute to some of the negative health outcomes that it is associated with. However, more research needs to be carried out to investigate the health effects of an excessive omega-6 intake.

You can help to keep things balanced and support your gut by:

  • eating fish once or twice per week to increase your omega-3 intake
  • limit your use of vegetable oils that are high in omega-6
  • eat a healthy, balanced diet that includes a variety of naturally coloured foods.


There is currently little research available into the beneficial effects of Romboutsia ilealis,but it has been circled as one of the 15 good gut microbes by the PREDICT studies. The researchers state that people who had R. ilealispresent in their gut had lower levels of inflammation and increased polyunsaturated fat levels.

Polyunsaturated fats are classified as “healthy” fats that your body needs to carry out a number of functions and to support various aspects of your health.

The two main types are omega-3 and omega-6 fatty acids. As well as supporting nerve function and heart health, omega-3 also has a positive effect on your overall gut health.  

That’s why it is important to ensure that you incorporate at least 2 portions of oily fish into your diet each week (or use supplements if you are unable to eat fish).

Remember, diet is an important regulator of gut health, but you can also support it with prebiotic supplements, including HMOs.


Written By: 

Leanne Edermaniger

Leanne is a science writer who specializes in human health and enjoys writing about all things related to the gut microbiome.

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[i] Gerritsen J, Hornung B, Renckens B, van Hijum SAFT, Martins Dos Santos VAP, Rijkers GT, Schaap PJ, de Vos WM, Smidt H. Genomic and functional analysis of Romboutsia ilealis CRIBT reveals adaptation to the small intestine. PeerJ. 2017 Sep 11;5:e3698. doi: 10.7717/peerj.3698. PMID: 28924494; PMCID: PMC5598433.

[ii] Di Pasquale MG. The essentials of essential fatty acids. J Diet Suppl. 2009;6(2):143-61. doi: 10.1080/19390210902861841. PMID: 22435414.

[iii] Yang W et al. High dietary intake of vegetable or polyunsaturated fats is associated with reduced risk of hepatocellular carcinoma. Clinical Gastroenterology and Hepatology. November 2020;(18(12): 2775-2783.

[iv] Albert CM, Hennekens CH, O'Donnell CJ, Ajani UA, Carey VJ, Willett WC, Ruskin JN, Manson JE. Fish consumption and risk of sudden cardiac death. JAMA. 1998 Jan 7;279(1):23-8. doi: 10.1001/jama.279.1.23. PMID: 9424039.

[v] Yamagishi K, Iso H, Date C, Fukui M, Wakai K, Kikuchi S, Inaba Y, Tanabe N, Tamakoshi A; Japan Collaborative Cohort Study for Evaluation of Cancer Risk Study Group. Fish, omega-3 polyunsaturated fatty acids, and mortality from cardiovascular diseases in a nationwide community-based cohort of Japanese men and women the JACC (Japan Collaborative Cohort Study for Evaluation of Cancer Risk) Study. J Am Coll Cardiol. 2008 Sep 16;52(12):988-96. doi: 10.1016/j.jacc.2008.06.018. PMID: 18786479.

[vi] Aung T, Halsey J, Kromhout D, Gerstein HC, Marchioli R, Tavazzi L, Geleijnse JM, Rauch B, Ness A, Galan P, Chew EY, Bosch J, Collins R, Lewington S, Armitage J, Clarke R; Omega-3 Treatment Trialists’ Collaboration. Associations of Omega-3 Fatty Acid Supplement Use with Cardiovascular Disease Risks: Meta-analysis of 10 Trials Involving 77 917 Individuals. JAMA Cardiol. 2018 Mar 1;3(3):225-234. doi: 10.1001/jamacardio.2017.5205. PMID: 29387889; PMCID: PMC5885893.

[vii] Hu Y, Hu, FB, Manson JE. Marine omega-3 supplementation and cardiovascular disease: An updated meta-analysis of 13 randomized controlled trials involving 127 477 participants. September 2019; 8(19).

[viii] Königs A, Kiliaan AJ. Critical appraisal of omega-3 fatty acids in attention-deficit/hyperactivity disorder treatment. Neuropsychiatr Dis Treat. 2016 Jul 26;12:1869-82. doi: 10.2147/NDT.S68652. PMID: 27555775; PMCID: PMC4968854.

[ix] Zhang M, Yang XJ. Effects of a high fat diet on intestinal microbiota and gastrointestinal diseases. World J Gastroenterol. 2016 Oct 28;22(40):8905-8909. doi: 10.3748/wjg.v22.i40.8905. PMID: 27833381; PMCID: PMC5083795.

[x] Merra G, Noce A, Marrone G, Cintoni M, Tarsitano MG, Capacci A, De Lorenzo A. Influence of Mediterranean Diet on Human Gut Microbiota. Nutrients. 2020 Dec 22;13(1):7. doi: 10.3390/nu13010007. PMID: 33375042; PMCID: PMC7822000.

[xi] Shi Z. Gut Microbiota: An Important Link between Western Diet and Chronic Diseases. Nutrients. 2019 Sep 24;11(10):2287. doi: 10.3390/nu11102287. PMID: 31554269; PMCID: PMC6835660.

[xii] Fu Y, Wang Y, Gao H, Li D, Jiang R, Ge L, Tong C, Xu K. Associations among Dietary Omega-3 Polyunsaturated Fatty Acids, the Gut Microbiota, and Intestinal Immunity. Mediators Inflamm. 2021 Jan 2;2021:8879227. doi: 10.1155/2021/8879227. PMID: 33488295; PMCID: PMC7801035.

[xiii] Costantini L, Molinari R, Farinon B, Merendino N. Impact of Omega-3 Fatty Acids on the Gut Microbiota. Int J Mol Sci. 2017 Dec 7;18(12):2645. doi: 10.3390/ijms18122645. PMID: 29215589; PMCID: PMC5751248.

[xiv] Russo GL. Dietary n-6 and n-3 polyunsaturated fatty acids: from biochemistry to clinical implications in cardiovascular prevention. Biochem Pharmacol. 2009 Mar 15;77(6):937-46. doi: 10.1016/j.bcp.2008.10.020. Epub 2008 Oct 28. PMID: 19022225.

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