Researchers found that mice given gut (microbes) from obese humans became fatter than those that got microbes carried by slim folks. When the husky and lean mice shared microbes with each other, the bigger ones picked up some of the beneficial gut flora—and had (improved) metabolisms.

But this shift only occurred if the mice were on a high-fiber, low-saturated fat diet. If they were on a junk food diet, no improvement. The findings are in the journal Science. [Vanessa K. Ridaura et al, Gut Microbiota from Twins Discordant for Obesity Modulate Metabolism in Mice]

It’s not clear how humans might remodel our microbial communities to change health or weight class. The mice in the study were raised in germ-free (environments) and had no native microbiomes of their own.

In people, so-called fecal transplants have been reserved for more severe conditions than a bulging belly. And (probiotic) products, such as yogurt, are minimally effective. But flat or fat, what your belly looks like on the outside might have a lot to do with what's on the inside.