Health guides recommend whole flour bread over white, based on the fiber and nutrients removed in the processing. Some research has backed this up. However, a new study found no overall benefit at the community level of one type of bread over another. Instead, some individuals did better on whole wheat, while others benefited more from white bread. The findings reinforce the fact that nutrition is exceptionally complex, and anyone claiming a “wonder-diet” that will benefit everyone is probably talking nonsense.

Some dietary effects take years to show up, but a glycemic response is much quicker, allowing scientists from the Weizmann Institute of Science, Israel, to conduct a controlled study. They selected 20 healthy participants who were regular bread eaters – getting about 10 percent of their calories from the bread, a globally typical figure.

Half were put on a diet heavy in supermarket-bought white bread, getting a quarter of their calories this way for two weeks. The other half were given whole wheat sourdough so artisanal it was made especially for the study. Sourdough is thought to have nutritional benefits independent of the choice of flour. At the end of the trial period, the two groups switched.

Glucose fat and cholesterol levels were monitored throughout the study, and for a period beforehand for comparison. Essential minerals, kidney and liver enzymes were also tracked. "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," said Professor Eran Segal, one of the senior authors of the study published in Cell Metabolism, in a statement. 

However, when the researchers looked deeper they found this was because some of the participants had a better glycemic response, reflecting blood sugar levels after eating showed, to white bread. For others, it was the reverse. The sample as a whole had a non-insignificant average difference.

The researchers analyzed bacteria found in participants' guts and found they could predict which bread would suit an individual better based on their intestinal flora alone.

The authors acknowledge the diets used for the study were unrealistic. People who eat whole wheat generally eat less of it, because the high fiber content fills them up. Tthe study actually gave people more sourdough, however, since it was designed to have equal amounts of carbohydrates in each two week period, and whole wheat has lower concentrations of available carbohydrates for a given weight.

"To date, the nutritional values assigned to food have been based on minimal science, and one-size-fits-all diets have failed miserably," said co-author Dr Eran Elinav. This might be about to change.