Choosing white or whole-grain bread may depend on what lives in your gut

Whether standard white bread or an artisanal sourdough loaf is “healthier” depends on the microbes living in a person’s intestines, a new study suggests.

Averaging results from 20 people who ate white and whole wheat sourdough bread for one week each, researchers found no difference in people’s response to the breads, which includes changes in blood sugar levels. But when researchers examined each person individually, a different pattern emerged. Some people’s blood sugar levels climbed more after eating white bread compared with sourdough bread. For others, the opposite was true, the team reports June 6 in Cell Metabolism.

The results are part of a growing body of evidence that nutrition advice should be personalized. Previous work by the same group at the Weizmann Institute of Science in Rehovot, Israel, showed that different people’s responses to eating a variety of foods vary considerably (SN: 1/9/16, p. 8). Research with mice also suggested that genetic differences may cause one strain of mouse to gain weight on a diet that helps another strain slim down (SN: 8/20/16, p. 13).

In the new study, researchers Eran Elinav, Eran Segal and colleagues analyzed the study participants’ genetic makeup along with the mix of microbes in stool samples from each person. The team could predict a participant’s response to the two types of bread based only on which microbes were present in the stool — particularly the amounts of two types of bacteria called Coprobacter fastidiosus and Lachnospiraceae bacterium 3_1_46FAA. The researchers don’t yet know how the bacteria change blood sugar levels.