A team from the Weizmann Institute of Science in Israel says generalisations about which type of bread is better for our guts don't actually apply to individuals - instead, what's healthy for us to eat is a much more personal question related to our own unique biology.
"We were sure that the sourdough bread would come out a healthier choice, but much to our surprise, we found no difference between the health effects of the two types of bread," says one of the researchers, Eran Segal.
Sourdough is typically made using microbes dating back to ancient times - it's what provides that distinctive tang - whereas white loaves are baked with strains of yeast that date back just 150 years.
That kind of heritage and a lack of artificial processing have lead many to believe that sourdoughs are healthier and better for our bodies. But maybe not, this study suggests.
Segal and his team recruited 20 participants, and observed them over the course of two weeks - 10 spent a week eating sourdough bread and then a week eating white bread, while the other 10 did the reverse.
The experiment found that levels of blood sugar, minerals, and liver enzymes were all affected by eating bread, but, oddly enough, the type of bread made no real difference.
Take the level of blood sugar, which whole grain bread is often said to help moderate - about half the volunteers ended up with higher blood sugar levels after eating white bread as expected, but the other half had higher blood sugar levels after eating the sourdough.
"That's probably because the body's response to bread is a highly personal matter, so the differences between people in the study averaged themselves out," says one of the team, Eran Elinav.
The researchers aren't saying whole grain bread is suddenly not a healthy choice - but they are suggesting that each person's body responds to bread types differently.
The study also looked at the microbiome of the participants, and found that the particular make-up of each person's gut bacteria could predict some of their reactions to the two types of bread.
It's worth pointing out that the study relied on a very small sample size, and only ran for a short period of time, so it's too early to apply these findings universally. But the results challenge some major assumptions about a fundamental part of many people's diets, and is worth following up on.
Another limitation of the study is that what constitutes a typical bread recipe can also vary a great deal across different parts of the world, which could influence the specific nutritional values from those featured in this experiment.
The researchers suggest that further studies look at using microbes in the body as a way of personalising healthy eating plans.
In the meantime, don't be so sure that buying a whole grain sourdough loaf is going to be any better for your overall health than sticking to a cheap white one - pay attention to how your body responds to a varied diet, and try to base your eating habits on whatever gives you the best result.
As chef Anthony Warner, who wasn't involved in the research, writes for New Scientist: "Healthfulness is defined by chemical composition, not by the price tag or the kitchen or factory in which it was made."
The study has been published in Cell Metabolism.
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