Overweight? It Could Be Your Intestinal Bacteria

Dec. 20, 2006— -- Overweight individuals may now have a new factor to blame for their excess weight -- the germs living inside their guts.

The trillions of microbes living inside our intestines may play a role in the differences in body weight among people, suggests a new study published in this week's issue of the journal Nature.

"Groundbreaking work like this that opens up a novel hypothesis is important," said Dr. Randy Seeley, associate director of the obesity research center at University of Cincinnati.

Researchers found that the bacteria in the guts of obese mice are better at extracting energy from food than gut bacteria in lean mice. In short, the fat mice take away more calories from their food.

Moreover, when the bacteria from the guts of obese mice were transplanted to slimmer, germ-free mice, these germ-free mice got fatter.

"There is a tremendous battle waged by scientists from many different backgrounds to try to understand this worldwide epidemic of obesity," said Dr. Jeffrey Gordon, director of the genome science center at Washington University in St. Louis and senior author of the study. "We want people to consider the microbial community as a potential factor."

'We Never Dine Alone'

"We had a question about the role our gut microbes play in human health," said Gordon. "Because we never dine alone."

Humans have had a long-standing, evolutionary relationship with bacteria and other microorganisms living in the gut. Many of these bacteria are necessary for well-being because they help us digest food particles that we cannot digest on our own.

"These trillions of microbes provide us with different sets of attributes that are important," said Gordon. "For example, the sugars known as polysaccharides in grains and vegetables have complex structures. Our microbial organisms have the tools to break down these structures and help us extract energy."

The differences in the identity of these gut bacteria may be a clue into why some people are obese and others are lean.

"One interesting idea [from this research] is that our gut microbes may be responding to changes in our body weight," said Seeley.

"This also opens up the possibility that our gut microbes may be paying attention to how much we weigh," Seeley added.

Small Differences Really Add Up

So, how much of a difference do gut bacteria really make in your weight?

Researchers found that while the germ-free mice receiving the gut bacteria from the obese mice gained weight, the weight gain was only modest.

"The ability to change your gut microbes may contribute in small amounts to your weight change," said Seeley.

However, small differences can make a big difference in the long run.

"To gain one pound in a year, a human being only needs to take in an extra 11 calories a day," said Seeley. "[These are] very small differences over long periods of time that can add up to meaningful differences in body weight."

And lifestyle changes can have an effect on what kinds of intestinal residents you have.

"By changing our diet, we can change the kind of bugs found in our colon, from those found in an obese person's colon to that of a lean person's colon," said Dr. Samuel Klein, director of the human nutrition center at Washington University.

Study Opens New Area of Obesity Research

The finding could impact the way researchers look at obesity.

"We are much more obese now than we were 30 years ago, and that is not because the gene pool has changed. It is largely because of the environment," said Seeley.

It is still unclear whether obese individuals start out with a different population of gut bacteria or whether that population of gut bacteria comes from being obese, researchers say. It is also unclear how gut microbes know if they are inside an obese person or a lean person.

"We have to expand our study to other obese populations in this country and to the world to see if this is a consistent theme," said Gordon.

So before you rush out to buy the latest yogurt to change your gut bacteria, you should probably wait for further research.

"Obesity is a chronic disease that causes significant social and medical complications," said Klein. "This is a very exciting observation that opens up a brand new avenue in the science of obesity research."