The flourishing market for healthy bread and boutique bakeries may be in for a shock, as new research from scientists at the Weizmann Institute of Science shows that plain white bread may not deserve its unhealthy reputation after all — at least not for about half the population.
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The scientists have found that the human digestive system does not necessarily prefer the so-called healthy breads, such as those made with whole wheat or exotic flours.
The research, published on Tuesday in the scientific journal Cell Metabolism, examined how industrially manufactured white bread affects the human body, as compared to artisanal sourdough whole-grain bread, which has been considered to be much healthier. Surprisingly, even to them, the scientists found the type of bread consumed did not affect the health of the participants.
In fact, what they discovered was that on average people responded about the same to the “healthy” bread and the processed bread. But a deeper analysis found this conclusion was misleading: Half the subjects responded better to the whole wheat sourdough – and the other half reacted better to the processed white flour.
The researchers used the regular subsidized white bread sold in Israel, known as “achid” for the study. The sourdough-leavened bread was made in an artisanal bakery from freshly stone-milled whole grain wheat flour, baked in a stone hearth oven and delivered fresh to the subjects: It was prepared especially for the study and was assumed to possess superior properties.
Sourdough put to the test
The study included 20 healthy men and women, who normally consume about 10 percent of their regular calories from bread. At the beginning of the random trial, the subjects underwent a full mapping of their gut microbiomes – all the microorganisms, especially bacteria, found in their digestive system. In addition, the participants underwent full blood tests before, during and after the study, including levels of blood sugar; essential minerals, such as calcium, iron and magnesium; cholesterol and fats; various kidney and liver enzymes and markers for inflammation and tissue damage. The microbiome too was sampled again during and after the study.
The subjects were randomly divided into two groups. After a three-day period to set a baseline for the subjects, the first group was asked to eat processed white bread for an entire week and to consume 25 percent of their total calories from this bread. The second group did the same – only with the whole-wheat sourdough-leavened bread. Their food consumption was carefully supervised and all participants were forbidden to eat any other type of wheat products.
At the end of the first week of the study they had a two-week break from all bread and then spent another week eating the other type of bread used in the study.
The findings were quite varied, individual and unexpected. For example, the blood sugar levels of one participant, who ate the sourdough bread, rose significantly, but returned to normal after a few hours. But ingesting white bread caused his blood sugar to rise only moderately and return to normal levels much more quickly. In other participants the effect was just the opposite.
High blood sugar and sourdough
“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 Prof. Eran Segal of Weizmann’s Computer Science and Applied Mathematics Department.
“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 Dr. Eran Elinav of the Immunology Department, who headed the study with Segal and Prof. Avraham Levy of the Plant and Environmental Sciences Department. “Different people react differently, even to the same foods,” added Elinav.
“We planned the experiment so that everyone would consume the same amount of available carbohydrates from both bread types. Because whole wheat bread contains relatively fewer carbohydrates, this meant that people ate more of it compared to the white bread. This difference in carbohydrate levels should also be taken into consideration when planning a diet,” said Levy.
Part of the difference in responses is due to the differences in bacteria in the gut, and those with similar microbiomes responded similarly, they said.
The scientists also found major differences in the subjects depending on when they were eating bread and when they were abstaining from it, regardless of the type of bread.
The scientists developed an algorithm linking the microbiome composition with the person’s response to the type of bread. “Using this algorithm, we managed to predict who will have high blood sugar after eating white bread, and who will have high blood sugar after eating the sourdough,” says researcher Tal Korem.
The sourdough bread had more bacteria and fungi because that is how it is leavened, as opposed to the baker’s yeast in the white bread, and has more nutrients because of the whole wheat flour; but that does not make it better for everyone, and some people may even be better off receiving their nutrients from other sources, said Segal.
At the same time, the scientists did not find any long-term or cumulative health effects from the breads, despite having measured a large number of medical parameters. They did not find any impact on feelings of satiation or heaviness, or a relationship between the medical parameters and the digestive processes, after eating either of the breads.
Other scientists cautioned about the study’s results, wondering whether a week was enough time to accurately judge the results, or whether 20 people was a large enough sample. In any case, people should not give up on eating healthy grains just because of the study, and for half the people the healthy grains seems to be better, and could have other effects in the long term.
Previous research conducted by Elinav and Segal has shown that microbiomes have a major influence on the development of various diseases. The bacterial profile in the body, and particularly in the digestive system, has a critical influence on how the body responds to various foods, their research has shown.
“To date, the nutritional values assigned to food have been based on minimal science, and one-size-fits-all diets have failed miserably These findings could lead to a more rational approach for telling people which foods are a better fit for them, based on their microbiomes,” says Elinav.
The article is entitled: “Bread Affects Clinical Parameters and Induces Gut Microbiome-Associated Personal Glycemic Responses.”