White or wholemeal? It depends on your gut bacteria

Is whole wheat sourdough better for you than processed white bread from the supermarket? The perennial question has been studied in detail, and the answer is to listen to your gut.

How we respond to high levels of either bread depends entirely on our individual gut microbiome, according to new research, published this week in Cell Metabolism.

To reach this somewhat inconclusive conclusion, a group of scientists at the Weizmann Institute of Science in Israel gathered 20 healthy participants and subjected them to a sudden increase in bread intake.

For most participants, bread previously made up around 10% of their normal daily calorie intake. During the experiment, this ratio was increased to 25% of their daily intake, for one full week – half the group ate processed white bread, while half ate freshly baked, whole wheat sourdough.

After a full week off bread altogether, the groups then switched their intakes.

Health signals were monitored throughout the experiment, including the participants’ glucose levels upon waking up each morning; levels of calcium, iron and magnesium; levels of fat and cholesterol; markers for inflammation and tissue damage; and, importantly, the state of each participant’s microbiome: the biology of their gut.

Initially, the study-wide data showed no change, surprising researchers, until they realised they were witnessing a balancing-out of data, based on different responses.

“The initial finding, and this was very much contrary to our expectation, was that there were no clinically significant differences between the effects of these two types of bread on any of the parameters that we measured,” says Eran Segal, one of the study’s senior authors.

The results show that around half the participants had a more positive glycemic response to the white bread, while the other cohort responded better to sourdough.

Furthermore, the results suggested responses to each bread type could be predicted by a person’s gut bacteria.

“We present marked personalisation in both bread metabolism and the gut microbiome, suggesting that understanding dietary effects requires integration of person-specific factors,” the researchers write.

Crucially, the results suggest that people experience different responses to different foods, depending on their individual gut microbiome, which has implications for nutrition and diet advice.

“The findings for this study are not only fascinating but potentially very important, because they point toward a new paradigm: different people react differently, even to the same foods,” adds Eran Elinav, another author on the study.

“To date, the nutritional values assigned to food have been based on minimal science, and one-size-fits-all diets have failed miserably.”