Wednesday, Mar. 17, 2010

The Bugs in Your Gut

Our intestines are teeming with trillions of bacteria that help us to digest food and keep us regular. And a growing body of research suggests that having just the right population of these gut bugs may even help keep the pounds off.

Scientists are discovering that the gut flora of obese and normal-weight individuals are different, and that their particular makeup is associated with whether calories taken in are turned into fat or fuel. It's not yet clear, however, whether a person's gut-bug profile causes weight gain or is a result of it. In obese patients undergoing gastric bypass surgery, for example, gut bug populations change to become more like those of normal-weight individuals after the operation. In studies of genetically identical twins, bacterial populations have been found to differ, depending on whether the twin is lean or obese.

But the evidence seems to be pointing to a strong causal role for the microbes in obesity. In the latest research, in mice, scientists led by Andrew Gewirtz at Emory University showed that normal-weight mice who were transplanted with the gut microbes from obese mice ended up getting heavier; the once lean mice gained weight, even without eating any extra food. And the mice with the obese gut profile also developed signs of metabolic syndrome, the constellation of symptoms including high cholesterol, hypertension and diabetes that are associated with excessive weight gain.

Previous mouse studies by Jeffrey Gordon at Washington University in St. Louis, Mo., suggest that changing the diet can change the makeup of bugs. When one group of mice was fed a typical Western diet, high in fat and sugars, they tended to gain weight and grow more of a type of gut bacteria called Firmicutes and fewer of a type called Bacteroidetes. In mice given a low-fat plant-based chow, the distribution of the two groups of bugs flipped and the animals remained lean. The shifts were dramatic and rapid. Switching a mouse from low-fat plant chow to a high-fat Western diet resulted in an explosion of Firmicutes in less than a day.

Taken together the findings, while still preliminary, suggest that a "gut profile" could potentially serve as a diagnostic tool for identifying who might have a propensity for obesity. If, for instance, your gut environment contains a preponderance of Firmicutes, then your body may be predisposed to digest calories in a way that leads to greater fat storage. (Gordon's earlier work with identical twins of different weights found that the obese twin tended to have more Firmicutes colonies than the leaner one.)

Genetic researchers have also recently sequenced the genomes of microbial gut bacteria. That work, combined with data on gut-bug populations in obese versus lean mice and people, could lead to a new way of regulating weight by changing the gut microbe populations of obese individuals to resemble those of leaner people.