“It’s a landmark study,” Rob Knight, a microbial ecologist at the University of Colorado, Boulder, who was not involved with the work, told ScienceNOW. “It changes our view of how rapidly the microbiome can change.”

In 2009, Turnbaugh’s team had shown a similar effect in mice: a change in diet could affect the murine gut microbiome in a single day, in fact. To see if the results would hold up in people, Turnbaugh, along with Lawrence David from Duke University and their colleagues, fed five volunteers a high-protein diet, with meals of bacon and eggs for breakfast, spareribs and brisket for lunch, and salami and cheese for dinner. This group could also snack on pork rinds and string cheese. The high-fiber group, on the other hand, got fruits, vegetables, grains, and beans. Before, during, and after the diet change, the team collected stool samples from each participant to assess the bacterial composition of their guts, as well as the gene expression activity of those microbes.

Before the week was up, the researchers found noticeable differences in the gut bacteria of the two treatment groups. The meat-eaters harbored more bacteria that are able to tolerate high levels of bile acids, which are secreted by the body to help digest meat. The bacteria isolated from this group also appeared to increase expression of genes involved in breaking down proteins. Plant-eaters, on the other hand, had fewer bile-resisting bacteria and higher expression levels of gene associated with carbohydrate digestion.