The study was carried out by researchers from the Department of Twin Research and Genetic Epidemiology at King’s College London in the UK, the Department of Microbiology and the Department of Molecular Biology and Genetics at Cornell University, and the University of Colorado in the US, and the Max Planck Institute for Developmental Biology in Germany.

It was funded by the US National Institutes for Health (NIH), the Cornell Center for Comparative Population Genomics, the Wellcome Trust, the European Community’s Seventh Framework Programme, the European Research Council, and the National Institute for Health Research (NIHR).

The authors declared no conflicts of interest.

The study was published in the peer-reviewed journal, Genome Biology. It is open access, so it’s freely available to read online.

The media reported the story accurately, with the BBC acknowledging that, although there is a link between bacteria in human faeces and levels of obesity, there is no known explanation yet.

What kind of research was this?

This was a cross-sectional observational study carried out on healthy sets of twins.

It aimed to explore the association between the bacteria present in human faeces and obesity.

This type of study can’t prove cause and effect, but is useful for looking at associations between risk factors and outcomes.

So while this study doesn’t prove that bacteria found in human faeces cause visceral fat, it does show there is a relationship between the two.

What did the research involve?

Researchers studied health volunteers involved in the TwinsUK Adult Twin Registry. Data on body fat was collected from a sample of 3,666 twins.

They looked at the links between bacteria found in faeces and six different measures of body fat.

The sample was mostly of European descent, and the average age was 63.

Faeces samples were collected from 1,313 individuals, and the bacteria in these investigated. Almost all those sampled were female.

The information from study participants on the bacteria present in their faeces was compared with body fat levels.

The six body fat measures included three of visceral fat, two of body fat distribution, and one of body mass index (BMI).

Excess visceral fat in particular is a risk factor for cardiovascular disease and metabolic disease, such as type 2 diabetes.

What were the basic results?

All but one measure of obesity were significantly associated with a lack of diversity of bacteria in the faeces.

However, the association was strongest for visceral fat, which is found around the internal organs and is a bigger risk factor for cardiovascular and metabolic diseases.

Researchers found the higher the diversity of bacteria in faeces, the lower the level of visceral fat.

The reverse was also shown: the less diverse the bacteria, the more likely participants were to have more visceral fat.

Visceral fat was found to be highly heritable (relative risk [RR] = 0.70, 95% confidence interval [CI] = 0.58 to 0.74). This was true even when adjustments for body mass index (BMI) were made.

How did the researchers interpret the results?

Dr Michelle Beaumont, lead author of the study from the Department of Twin Research and Genetic Epidemiology at King’s, said: “This study has shown a clear link between bacterial diversity in faeces and markers of obesity and cardiovascular risk, particularly for visceral fat.

“However, as this was an observational study we cannot say precisely how communities of bacteria in the gut might influence the storage of fat in the body, or whether a different mechanism is involved in weight gain.”

Senior author, Dr Jordana Bell, also from the Department of Twin Research and Genetic Epidemiology, said: “There is a growing body of evidence to suggest that gut bacteria may play a role in obesity, and a number of studies are now exploring this in more detail.

“Further scientific investigation is needed to understand how precisely our gut microbes can influence human health, and if interventions such as faecal transplants can have safe, beneficial, and effective impacts on this process.”

Conclusion

This cross-sectional study found a strong association between visceral fat and bacteria diversity in faeces.

The use of measures other than BMI was one of this study’s strengths, as BMI doesn’t reveal whether weight is from fat tissue or muscle.

The findings suggest that body fat levels may be passed down through families.

However, this is early research and there are a number of things to consider:

• We don’t know how the bacteria in our gut and faeces influence levels of fat in the body.
• The study doesn’t prove that having less diverse bacteria in our faeces causes visceral fat around the organs.
• Participants’ diets weren’t taken into consideration.
• Participants were mainly female and from the UK, so the findings can’t be applied across genders or globally.

It’s important to eat a balanced diet and have an active lifestyle to maintain a healthy weight.