The insight could pave the way for fecal transplants of bacteria to help people manage their weight, reduce their risk of certain metabolic diseases or, more simply, encourage them to broaden the bacteria found in their gut -- and therefore their poop -- by eating a healthier, more varied diet.
"We wanted to characterize how the microbiome changes in obese people ... and see which bacteria live in the gut," said Michelle Beaumont,
a research associate in gut microbiome and obesity at Kings College London, and lead author of the study. "This study has shown a clear link between bacterial diversity in feces and markers of obesity and cardiovascular risk."
Beaumont and her team examined stool samples from more than 1,300 sets of twins taking part in the TwinsUK study, the largest twin registry in the UK for the study of aging-related diseases.
DNA was extracted from the twins' samples to identify what fecal microbes were present. The researchers were eager to see the diversity of microbes rather than the volume, and the types of bacteria found were compared between lean and obese twins to spot any differences in gut flora.
When comparing samples, the team used six measures of obesity, including body-mass index, upper to lower body fat ratios and measurements of visceral fat, a form of fat typically found around important organs in the abdomen.
The greatest difference in the types of microbes was seen when comparing visceral fat. People with lower levels of visceral fat, or leaner people, had a more diverse range of bacteria in their feces. Greater fat levels were equally associated with a lower diversity of bacteria, according to the study, published in the journal Genome biology
"Visceral fat is one of the fats that's hardest to remove," Beaumont said, adding that the amount of this fat usually is proportional to weight. "(It) has a much stronger association with the microbiome than BMI."
Influenced by diet?
The team stresses that this is an association and that they cannot state that the differences in bacteria cause people to be more or less obese.
"At the moment, we cannot say anything about causality," Beaumont said. "It could be the diet that's influencing this."
However, she hypothesizes that the microbiome, or gut flora, of obese people may result in them extracting more nutrients from the foods they eat, contributing to more fat being formed.
Stool transplants from a person with a healthy microbiome have been proved to help cure antibiotic-resistant infections
. Beaumont plans to investigate her theory, and other reasons for these differences in gut microbes, by seeing whether fecal transplants from leaner people could change gut flora to help prevent weight gain.
By reducing obesity, these transplants could also prevent metabolic diseases associated with obesity such as cardiovascular disease and diabetes.
Previous studies by the team found that a gut bacterium called Christensenella was more prevalent in leaner people. When it was tested in mice that were overfed, the mice did not gain weight.
Gut microbiomes that contain healthy, inflammation-reducing bacteria have been shown to help reduce the risk of myriad health conditions, such as cancer
, heart disease
Diversifying your flora
"But the microbiome doesn't work in isolation," Beaumont said. "There's more than one bacteria that's working. ... The key thing is diversity."
Other experts agree.
"This is another important study connecting the gut microbiome to obesity," said Dr. Zain Kassam, chief medical officer at OpenBiome
, who was not involved in the study. He is conducting a clinical trial exploring the role of fecal transplants from a lean donor to treat obesity.
"I can imagine a universe in the not-too-distant future where a microbial treatment can complement diet and exercise to deeply impact the obesity epidemic," he said.
Dr. Martin J. Blaser, director of the Human Microbiome Program at New York University, noted that the findings confirm and extend the work of earlier studies.
"To me, the most important finding was that on the large scale of these observations, each of the measures of obesity was associated with reduced diversity of the fecal microbiota," Blaser said. "Analysis of the twins indicates that some of the most heritable taxa have associations with the metabolic abnormalities.
"Thus, inheritance related to obesity is not just human genes but the genes of the microbes we carry."
Still, Beaumont highlighted that the easiest, simplest way to diversify those tiny microbes residing in your gut, and your poop, is diversifying your diet.
She used the example of prehistoric hunter-gatherers who had a much more varied gut flora because their diets were governed by what was available to them -- and therefore were varied through no choice of their own.
"This is really adding to the body of evidence that you should be very, very careful with your diet," she said. "You should be eating a diverse diet."