ADA 2010: Gut bacteria can affect obesity

Obesity and leanness are more than calories in and energy out. The amount of adipose tissue an individual carries is also a function of his or her intestinal microbiota. Research in mice and humans suggests that individuals with more Firmicutes and fewer Bacteroidetes tend to be more obese.

"We should take into account microbes in the gut, diet, and other factors in considering energy balance and obesity, " said Peter Turnbaugh, PhD, Harvard FAS Center for Systems Biology, Harvard University, Cambridge, Massachusetts. "There is an interaction between host and microbes."

Turnbaugh explored the latest data on relationships between intestinal microbial communities and obesity during Nutrient-Gut-Brain Modulators, a Monday morning symposium at the 70th Scientific Sessions of the American Diabetes Association meeting in Orlando, Fla. There are approximately 10 times more microbial cells than human cells in the typical individual, he noted, amounting to anywhere between 10 and 100 trillion cells. Most reside in the intestinal tract.

The gut microflora are composed of about 10 major lineages, with Bacteroidetes, Firmicutes, and Actinobacteria accounting for more than 90% of the total population. Every individual hosts more than 1,000 distinct species of bacteria, Turnbaugh added, and the bacterial population living in each individual is unique.

Mice raised in a sterile, germ-free environment eat more but weigh less than mice raised in a normal bacteria-filled environment, but their weight quickly changes when bacterial communities are introduced. Genetically obese mice colonized with bacteria from obese humans become obese, whereas genetically identical mice colonized by bacteria from lean humans become lean.

"There was no difference in the amount of chow these mice consumed before and after colonization," Turnbaugh said. "The only difference was the relative population of Bacteroidetes and Firmicutes."

Using a broad-spectrum antibiotic to kill gut microbes reversed the weight gained by the obese mice that had been colonized with bacteria from obese humans, he continued. Changing the microbial balance can also attenuate the effects of type 1 diabetes and reverse metabolic syndrome in mice.

Studies of obese and lean human twin pairs found 383 microbial genes associated with obesity, Turnbaugh said. Many were associated with carbohydrate metabolism. That matches the findings that obese mice extract more calories from their diets than lean mice. Analysis of mice stools also found evidence of increased microbial fermentation in obese mice, a process that would provide additional calories.