What’s the background?

Bread makes up about 10% of the daily calories consumed by adults. Now a study by researchers at the Weizmann Institute of Science in Israel has delved into whether fresh wholegrain sourdough is better for you than industrially produced white bread containing refined wheat flour as well as preservatives and emulsifiers.

The team were looking at the impact of eating the different types of bread on bacteria and other gut flora (collectively known as the “microbiome”) and various clinical measures, such as levels of cholesterol, blood sugar and minerals.

What did the study involve?

Twenty participants had their gut microbiomes analysed and blood tests taken before they were randomly assigned to one of two regimes. One group of 10 were given specially prepared wholegrain sourdough for a week, while the other group were asked to eat industrially produced white bread. After a two-week break, the groups switched their bread type. During the study, the participants increased their consumption of bread, so that on average it accounted for 22% of their daily calorie intake. Participants did not eat wheat from any other source.

I love sliced white bread. It’s the best thing since … er, sliced white bread Read more

Let me guess, sourdough was better for you?

In a word, no. “The really shocking result was that on everything that we looked at, we didn’t find any difference between the effects of the two breads,” said Eran Segal, a co-author of the research. In addition, the participants’ gut microbiomes appeared unchanged by munching on different types of bread.

So it doesn’t matter which bread I eat?

Not quite. When the team looked closer at the results, they found that there were considerable differences between participants’ responses to the different breads – meaning that when the results were pooled together, on average no differences were seen. “We found that half of the people had a higher blood glucose response to white bread, the other half had a higher blood glucose response to the sourdough bread,” said Segal. The team were also able to predict, based on the makeup of the participants’ microbiomes, to which bread they would have the stronger response. “White bread might not be bad for you. In fact, for some people it might be better than sourdough bread. In other words it is really personal,” said Segal.

Are there any drawbacks to the study?

While all 20 participants ate both types of bread, it is still a small number of individuals and each type of bread was only eaten for a week – it might be that longer is needed to see differences. “Also [the study] doesn’t clearly describe the rest of the diet ... what other vegetables and things people might be eating, which might have an effect,” said Duane Mellor, a dietitian and spokesperson for the British Dietetic Association, who added that different types of sourdough, and ways of making the bread, could lead to different results.

Is your gut making you sick? Read more

That said, when the researchers looked at the impact of the increased bread consumption, regardless of type, they saw noticeable changes over a host of measures, from the levels of “bad” cholesterol to levels of various enzymes. “Adding a significant amount of bread to your diet basically even for one week is already going to impact your blood parameters,” said Segal. However, it might have been that participants changed other areas of their diet while taking part in the study, and there was no control group of individuals who did not change their bread consumption.

What’s the upshot of this?

Primarily, the study highlights that individuals can differ considerably in their response to the same food, in this case bread. “The reason is due to the individual differences in our gut microbes and this is important,” said Tim Spector, professor of genetic epidemiology at King’s College London and author of The Diet Myth. “While glucose spikes are controversial as to whether they matter in most people, [the study] shows us that our healthy metabolism is best controlled by looking inside our guts and not at the food label.”

The findings tie in to other work from the same team at the Weizmann Institute of Science that has suggested that personalised diets, tailored to individuals’ gut microbes could offer health benefits.

“If you are of the type that responds in terms of [blood glucose levels] much more strongly to the sourdough bread then maybe you might want to consider getting these other fibres and mineral and nutrients not from sourdough bread but by other means,” said Segal.