Photo: Mikal Schlosser

Q&A - Do gut bacteria cause obesity?

Monday 16 Jul 18


Tine Rask Licht
Professor, Head of Research Group
National Food Institute
+45 35 88 71 86

Research into gut bacteria has exploded over the past ten years. Scientists are investigating the role these bacteria play for our health, including how they affect our weight.

Professor Tine Rask Licht from DTU has been involved in the research from the beginning.

Q: Can bacteria influence our weight?


A: Yes, the latest research suggests they can. We know there are variations in how well bacteria extract nutrition from a person’s diet. We have shown this through studies at DTU Food, and other studies have shown the same.

Our studies were able to confirm that gut bacterial communities from people with obesity were better at metabolizing dietary fibre than bacterial communities from people with normal weight. This means that the bacteria can have a direct influence on our ability to extract energy from food. However, it is important to understand that the bacteria contribute only a small proportion (perhaps 5-10 per cent) of our energy metabolism. They therefore cannot be the only cause of obesity.

Q: Do people with obesity have special gut bacteria?

A: No, not necessarily. But the bacterial communities in their guts—also known as the gut microbiota—may have a different composition of bacterial species, with some species being more dominant.

It is important to emphasise that this does not mean the bacteria cause their obesity. The gut microbiota are the result of our diet. Perhaps the special composition of gut microbiota in people with obesity is primarily due to them having other dietary habits—habits that have led to obesity and also contributed to the growth of certain bacterial species.

Q: Can people with obesity lose weight by modifying their microbiota?

A: I am not aware of any evidence-based examples that show that gut bacteria can be changed in people with obesity so that they lose weight. Unless they change their diet—but it’s nothing new that dietary changes can lead to weight loss.

We have found that gut microbiota are incredibly stable throughout our lives. They are fully formed at the age of four. Each person has around 160 bacterial species in their gut, out of 500-600 species known to occur in human faeces.

If you make drastic changes to your diet, you can slightly change the microbiota in your gut. But your samples from before and after the dietary changes will still be more similar than if you compare your sample with another person’s. The gut microbiota are roughly as individual as a fingerprint.

Q: How can bacteria regulate appetite?

A: The gut is closely linked to the immune system, the endocrine system, and the nervous system. The bacteria can communicate with these systems through substances they secrete, which interact with receptors in the gut wall. These can trigger certain reactions in the immune and endocrine systems. The bacteria’s products—metabolites—can also be absorbed through the gut wall and then into the bloodstream.

These metabolites can act as signalling substances that affect receptors in other organs, such as the brain or liver. The bacteria’s surface structures can also be recognized by receptors in the gut lining, triggering a response in the body.

Q: What are the biggest challenges in your research?

A: It is a very new research field that has taken off as a result of entirely new methods in the field of gene sequencing technology. We have only taken the first very tentative steps to identify the role of gut microbiota in our health.

The gut bacteria’s interaction with the body’s systems is incredibly complex. The bacteria are involved in a lot of mechanisms that we have not yet begun to understand. There is a lot of focus on gut bacteria, and it can be difficult to communicate to the public that while gut bacteria can sometimes be part of the problem, they are rarely the whole explanation.

I have seen a food blogger write that we could just eat cream cakes because obesity is due to gut bacteria. But if you want to lose weight, it’s probably a bad idea to eat cream cakes, no matter what the composition of your gut bacteria.

Q: Where will we be in ten years time?

A: I believe we may be able to use measurements of gut microbiota as a diagnostic tool for the earlier detection of diseases.

We might also be able to give more personalized advice on diet and medication, optimized in relation to the individual’s microbiota. For example, we might be able to say which people can benefit from which probiotics (bacteria in pill form), and to develop better strategies for prevention and treatment that are based on changes in the gut bacterial community. I doubt that we will be able to cure depression or obesity by modifying the gut microbiota.

Research at DTU

• Professor Tine Rask Licht is head of the research group for Microbes, Health, and Allergy at DTU Food.

• The group’s research into gut microbiota includes studies of the effects of diet and dietary components on gut bacteria populations, and derived effects on the immune system and metabolism.

• The group is also involved in a number of large microbiome research collaboration projects. The group contributes sequencing-based analyses of gut bacterial communities, cultivation-dependent trials, experiments with the design of gut microbiota in original germ-free animals, and experiments aimed at understanding the physiological state of bacteria and the significance of this in the complex bacterial communities.

• Tine Rask Licht is also involved a new research project together with Anja Boisen, a professor at DTU Nanotech and head of the IDUN research centre, which aims to examine the possibility of improving health using small capsules containing bacteria for the gut system. The project is being financed by the Novo Nordisk Foundation.

Professor Tine Rask Licht from DTU Food expects that we will one day be able to use measurements of the gut bacterial composition in diagnosing disease.