Whole wheat bread is not necessarily a healthier option for everyone, suggests a new study that looked at how different people’s bodies react to increased consumption of both white and whole wheat bread.

The study, published Tuesday in the journal Cell Metabolism, compared how processed white bread and artisanal whole wheat sourdough bread affected the bodies of 20 healthy subjects.

Half of the study participants were assigned to consume an increased amount of processed, packaged white bread for a week, representing about 25 per cent of their calorie intake.

The other half was told to consume an increased amount of whole wheat sourdough, which was baked especially for the study and delivered fresh to the participants. After a two-week period without bread, the diets for the two groups were reversed.

Before and during the study, researchers monitored the participants’ glucose, fat and cholesterol levels. They also tested the subjects’ minerals, calcium, iron and magnesium levels, checked their kidney and liver enzymes, as well as several markers for inflammation and tissue damage.

The researchers also measured the makeup of the participants' microbiomes before, during, and after the study.

"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," Eran Segal, one of the study’s senior authors and a biologist at the Weizmann Institute of Science in Israel, said in a news release.

"We looked at a number of markers, and there was no measurable difference in the effect that this type of dietary intervention had."

But when the researchers looked closely at the study participants’ glycemic responses, they realized that about half the people had a better response to the white bread, and the other half had a better response to the whole wheat sourdough.

The glycemic response generally refers to the changes in blood glucose after consuming foods that contain carbohydrates.

“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,” said Eran Elinav, another senior study author and a researcher in the department of immunology at the Weizmann Institute.

Researchers say the findings could help tell people “which foods are a better fit for them, based on their microbiomes." But more research is needed to understand exactly how different types of bread – and food in general – affect different people.

Dietician Rosie Schwartz says that “not all whole grains are created equal,” and she wishes that the researchers broadened the scope of the study to consider other grains.

“It might have gotten different results if we were looking at rye, barley, oats for example, all of which contain soluble fibre and could change what happens with the blood sugar,” she said.

Eating whole grains can provide some health benefits, Schwartz said, and a person’s glycemic response to bread is only one piece of the puzzle.

“There’s a decreased risk, for example, for colon cancer … so we need to look at the whole picture rather than just blood sugar,” she said.

Everyone has a different health profile that may be influenced by several factors, such as genetics and family history. Schwartz underscored the importance of maintaining beneficial gut bacteria to a person’s overall health.

“The one thing I think is clear is that eating more plant foods, eating more whole grains, eating more fibre will promote a healthy microbiota – and that may be the key to how we react to foods,” she said.