Can you really 'catch' obesity?

"Obesity could be contagious like superbug C diff, suggest scientists," The Daily Telegraph reports. This rather alarming headline follows a study that explored characteristics of bacteria living in the human gut.

The study did not, however, look at any link to obesity. There's no reason to think that you can "catch" obesity from spending time with people who are overweight.

The colony of bacteria in the human gut (known as the microbiome) affects how we digest food, our immune system, how our body temperature remains stable, and other bodily functions. Little is known about the hundreds of species of bacteria living in our guts, because they were thought to be difficult to culture in the laboratory.

In this study researchers showed that about 40% of the gut bacteria known to scientists could be cultured. Further investigation found some can live and be transferred outside the body by producing spores, which are germinated by gut acids when they reach a new host – in this case another human. The superbug Clostridium difficile (C diff), which causes diarrhoea, is known to spread from person to person in this way.

Researchers did not find (or look for) any bacteria that might be linked with obesity. But in their press release, they speculated that bowel conditions such as inflammatory bowel disease, or obesity, could be caused by an imbalance of gut bacteria.

Where did the story come from?

The study was carried out by researchers from the Wellcome Trust Sanger Institute in the UK, Hudson Institute of Medical Research and Monash University in Australia. It was funded by the Wellcome Trust, the UK Medical Research Council, the Australian National Health and Medical Research Council, and the Victorian Government.

The study was published in the peer-reviewed journal Nature, on an open-access basis, so it is free to read online.

The Telegraph and Daily Mail both jump on the suggestion that obesity could be caused by gut bacteria and could be spread like an infection from person to person, even though the study does not look at obesity. We don't know the effects of the bacteria identified and cultured in the study.

It would be sad if this study led to obese people being labelled as "contagious", as the headlines might suggest.

The Wellcome Trust Sanger Institute's press release says: "Imbalances in our gut microbiome can contribute to complex conditions and diseases such as obesity, Inflammatory Bowel Disease, Irritable Bowel Syndrome and allergies." However, it does not suggest these imbalances are contagious.

What kind of research was this?

This was a laboratory-based study using samples of faeces from six healthy people. Researchers used genetic profiling techniques and worked with cultures on agar plates to investigate the types of bacteria found in the samples.

What did the research involve?

Researchers took stool samples from six healthy people and used gene sequencing combined with bacterial culturing to grow cultures of bacteria and identify the species found. They treated samples with ethanol, to separate out those bacteria (like C diff) which were resistant to ethanol because they formed spores.

They then looked to see how long the bacteria lived outside the human body. Most gut bacteria live in conditions with no oxygen, so they don't live long when exposed to oxygen. The researchers exposed the bacteria to acids produced in the body's bile duct, to see whether this induced the spores to "germinate", in the way that temperature and moisture induces germination in the seeds of plants.

Finally, the researchers used metagenomic sequencing (the study of genetic material) to work out what proportion of bacterial colonies in the gut are likely to be spore-forming.

What were the basic results?

The researchers said they had been able to culture 39% of bacteria identified in a database of known gut bacteria, and 73.5% of the bacteria identified in the samples in this study. They also identified new species.

They found about one third of the bacteria from their samples formed spores, and that these spores could last at least 21 days (the length of the study) of exposure to oxygen, while most non-spore forming bacteria lived for only two to six days.

When researchers exposed the bacteria to bile acids (which form part of our digestive system), the spore-forming bacteria germinated, enabling the bacteria to be cultured, while the non-spore forming bacteria were not affected.

How did the researchers interpret the results?

The researchers said they had shown that spore-formation among gut bacteria was "widespread" and that these bacteria shared characteristics with C diff, which could make them "highly transmissible for long periods" outside the body, and "have the potential to spread rapidly over long distances".

They say their research "unlocks the human intestinal microbiota" for further investigation. In their press release, they suggest they could develop treatments for conditions such as C diff infection, by creating pills with mixtures of desirable gut bacteria to compete with the bacteria causing problems.

Conclusion

The human microbiome is a fascinating field of research, and we are just beginning to learn how this colony of bacteria in our guts affects our health. This research widens our knowledge of these bacteria, and suggests ways they may survive and spread from person to person.

It also shows that many bacterial spores are resistant to ethanol, the main ingredient of hygienic hand gels. This reinforces the importance of using soap to wash your hands and not to rely on hand gels, especially in hospitals.

Because of the headlines in some newspapers, it's important to be clear about what the research hasn't found. It hasn't found bacteria in the gut that are responsible for causing obesity, or a link between obesity and C diff. It also hasn't found evidence that obesity spreads from person to person by bacterial transfer.

The study has simply found that about 30% of bacteria in our guts are likely to be capable of spreading from person to person. We don't know what effect that has, because we don't yet understand what role these bacteria play in the gut.

If you are worried about your weight, take a look at our weight loss guide; you can find out what weight is healthy for your height, and get advice on how to lose weight sensibly if you need to.