Study: Gut Bacteria Determine Fat or Thin

June 19, 2006 — A microbe that converts waste into methane for daily flatulence could make the difference between fat people and thin people, according to a new study that looks into the menagerie of microorganisms living in our colon.

Called Methanobrevibacter smithii (M. smithii), the bacterium is key to how well we process calories, reports Jeffrey Gordon, director of the Center for Genome Sciences at Washington University School of Medicine in St. Louis, Missouri, and and his graduate student Buck Samuel.

The bacteria are essentially the body’s waste-removal workhorses. They mop up hydrogen and byproducts released by the gut bacteria, helping them digest complex sugars.

In this way, the stored calories are liberated and absorbed.

"We are superorganisms containing a mixture of not just human cells but also bacterial cells and cells of another microscopic domain of life known as archaea," Gordon said in a statement.

"The genes present in this community of 10-100 trillion bugs vastly outnumber our own genes and are a key part of our genetic landscape, providing us with attributes we have not had to evolve on our own."

One such attribute is the ability to break down non-digestible complex carbohydrates known as polysaccharides.

Gut bacteria such as Bacteroides thetaiotaomicronchop these complex sugars into short-chain fatty acids that account for up to 10 percent of a person's daily calorie intake.

Gordon and Samuel tested whether M. smithii, which was recently recognized as the most common archaeon in human intestines, could speed up digestion within a group of germ-free mice.

The researchers colonized one group of these mice with the polysaccharide-digesting bacterium B. thetaiotaomicron. Another group was colonized with M. smithii, while a third group received both B. thetaiotaomicron and M. smithii.

Mice from a control group were colonized with the sulfate-reducing bacterium Desulfovibrio piger, which is the most abundant species in healthy adults.

All the mice then received the same amount of food – a standard rodent chow rich in plant polysaccharides. Mice colonized with both B. thetaiotaomicron and M. smithii had significantly more fat than animals colonized with either microbe alone or with the other bacteria.

The researchers concluded that M. smithii collaborate with B. thetaiotaomicron to increase calorie intake from food.

"M. smithii acts as a ‘power broker’ in the distal gut community," they wrote in the Proceedings of the National Academy of Sciences, where the study was published..

Indeed, without M. smithii’s activity and synergy with B. thetaiotaomicron, accumulation of byproducts would slow sugar digestion, basically inhibiting the absorption of calories.

The finding would explain why different people gain different amounts of calories from identical foods.

"Further studies are needed to understand how to manipulate the representation of M. smithii and or other archaeons in our gut microbiota: the results could lead to a novel means for preventing obesity in the overfed or increasing caloric harvest in the underfed," wrote the researchers.

According to microbiologist Martin Blaser of New York University, the study is very important.

"It begins to describe the complex interactions between the microbes adapted to the intestinal tract and the metabolism of the host animal," Blaser told Discovery News. "Although the study was done in mice, it is a model system for human biology."