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expert reaction to study investigating the effects of consuming white processed bread vs sourdough bread

Publishing in Cell Metabolism researchers report that there are no significant differences between two 1-week-long dietary interventions, one including consumption of white bread and the other wholewheat sourdough bread. However, upon further investigation, the results showed people responded to the bread differently.


Prof. Susan Jebb, Professor of Diet and Population Health, University of Oxford, said:

“This paper describes a randomised crossover trial with 20 people who were asked to eat either white bread, or wholewheat sourdough bread in random order with a two week gap between the two periods.  The findings are clear – there was no effect of the type of bread on over 20 clinical measurements. It may be that this is too small a group to detect potentially small differences but the fact remains that there was no measureable effect on health.  The investigators then proceeded to conduct a completely different type of analysis. They looked at individuals before and after consuming each type of bread. They acknowledge in the paper, but not the press release, that since there is no control group, any changes observed may be due to other factors unrelated to the bread. People participating in trials commonly change their behavior in a host of ways and if we ask people to change their diet by eating a certain amount of bread they may well make other changes in their diet too. It is just as likely that these changes, which have not been measured, may explain any changes observed. As such, it is very difficult to draw any robust conclusions specifically about bread from this analysis. The more general observation that the responses to food vary by individual is hardly surprising or new.”


Dr Elizabeth Lund, Independent Consultant in Nutrition and Gastrointestinal Health, said:

“Although the results of this study are interesting, it should be noted that they are not conclusive and the study itself is small (only 20 participants). The research set out to look at many different biomarkers but their main focus was on markers of risk related to type 2 diabetes. For example, they investigated the glycaemic response to consuming bread by measuring the levels of glucose in the blood after participants consumed either mass-produced white bread or wholegrain sourdough bread.

“The intervention was very short at only one week for each type of bread, especially compared to previous studies that have showed a benefit of whole grains in the control of blood glucose. It is therefore not surprising that no effect in eating the different types of bread was observed. The more interesting result was that the levels of glucose in the blood after eating the sourdough whole wheat bread was in some cases higher than when the same person ate the more mass produced style of bread, and in many cases the two were very similar. This may suggest that there was no overall difference in the glycaemic index (which indicates a food’s effect on a person’s blood glucose) between the two types of bread but that there is a lot of variation between people in how they respond to the bread. It is well know that people’s responses to food is very variable, hence nutritional guidelines are simply guidelines based on averages and people will not always respond to the advice in the same way.

“It is very interesting that the bacteria in the gut seemed to predict how individuals may respond to bread consumption, but this part of the study needs to be checked in larger studies that are explicitly designed to investigate this aspect as it was not the primary aim here. It should not be forgotten that the health benefits of whole grains may be much longer-term than a one week study can show, especially in relation to gut health and prevention of conditions like bowel cancer. Therefore this study does not imply that people should give up eating whole grain foods based on these results.”


* ‘Bread Affects Clinical Parameters and Induces Gut Microbiome-Associated Personal Glycemic Responses’ by Korem et al. published in Cell Metabolism on Tuesday 6th June.


Declared interests

Prof. Susan Jebb: Prof Susan Jebb is employed by the University of Oxford and receives no personal funding from the food industry.  Susan Jebb is conducting research into the treatment of obesity, some of which include support from WeightWatchers, Slimming World and the Cambridge Weight Plan.  Susan was the independent Chair of the Public Health Responsibility Deal Food Network and was a science advisor to the Foresight obesity report.  From 2007-10 she was the principal investigator for a research study funded by the food industry to investigate the potential for a functional beverage to help weight loss. The results of this work have been published:

Dr Elizabeth Lund: “I am an independent consultant, previously a research leader at The Institute of Food Research (now Quadram Institute Bioscience), Norwich, UK.  I have previously undertaken research on oats and rye bread as well as the microbiome but have no direct conflicts of interest in relation to this study.”

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