New research provides stronger evidence that the composition of gut 
bacteria influences obesity.

Three new studies involving mice and humans provide new and stronger 
evidence that the composition of gut bacteria influences obesity.

In a U.S. study, fecal material from four human twin pairs (each set 
included one obese and one lean twin) was transplanted into 
genetically identical mice that were raised in a germ-free 
environment. The mice that received gut microbiota from obese humans 
became obese and developed obesity-related metabolism traits: 
dyslipidemia, insulin resistance, and inflammatory markers. In 
contrast, mice that received microbiota from lean humans stayed lean. 
Because mice routinely eat each other’s feces, researchers tried 
housing obese mice with lean mice — the obese mice gradually became 
lean and lost the obesity-related metabolic changes, and their gut 
flora changed to resemble that of the lean mice.

In a study from France, researchers compared genes from the gut 
microbiomes of 123 nonobese and 169 obese humans. The nonobese humans 
typically exhibited a different composition of gut bacteria, although 
the differences involved just a few species. Certain bacterial species 
were linked to insulin resistance, fatty liver, and inflammation. In 
another French study, investigators found the same “obesity-
related” bacteria to be present in their 49 obese participants, 
particularly those with obesity-related metabolism traits. When 
participants were placed on energy-restricted diets for 6 weeks, their 
microbiome changed toward that seen in lean individuals. Returning to 
their typical diets for 6 weeks moved the participants’ microbiomes 
back toward the initial composition.

COMMENT

Collectively, these three studies provide strong evidence that the 
composition of bacteria in our gut affects whether we are obese — and 
whether we develop obesity-related metabolism traits. They also 
indicate that what we eat can affect our microbiome. The studies do 
not prove that the gut microbiome will predict which people will 
become obese or develop obesity-related metabolism. And they surely 
don’t prove that directly altering the gut microbiome of obese people 
will improve their health. However, many investigators already are 
interested in pursuing those possibilities.

Ridaura VK et al. Science 2013 Sep 6. Le Chatelier E et al. Nature 2013 Aug 29. Cotillard A et al. Nature 2013 Aug 29.