New study shows how different people react to various breads.

In a new study, researchers discovered that different people’s bodies react differently to the same foods, which could be a breakthrough in understanding why dieting, for millions, hasn’t worked.

Researchers at the Weizmann Institute of Science in Israel, based their study on the nutritional and glycemic effects of eating two different types of bread. Their findings were published on June 6 in the journal Cell Metabolism.

After decades of studies on which breads are healthiest, it remained unclear what effect bread and different bread types have on different systems in the body, especially the microbiome, which encompasses the millions of microorganisms that naturally live on and in the human body.

One of the researchers’ new findings is that there is no clinical difference in the effects of ingesting white or wheat bread.

The researchers came to this conclusion after performing a crossover study of 20 adults. Processed white bread was introduced into the diets of half of the subjects, while the other half ate handmade, whole-wheat sourdough bread.

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In addition, researchers found that the composition of the subjects’ microbiomes was generally resilient to the dietary intervention of bread, and that the glycemic response (the effect on glucose, or blood sugar, levels) to the two bread types varies greatly among the population.

Dr. Eran Elinav, a researcher in the Department of Immunology at the Weizmann Institute, and one of the study’s senior authors, said these findings were “fascinating” and “potentially very important.”

“To date, the nutritional values assigned to food have been based on minimal science, and one-size-fits-all diets have failed miserably,” he said.

Eran Segal, PhD, a computational biologist at Weizmann, and another senior author, told Healthline they also performed a crossover clinical trial where subjects were compared with themselves. The results were “very powerful” as it compared short-term effects of interventions.

“Subjects were compared to themselves,” he explained. “We compared increased short-term (one week) consumption of industrial white bread vs. matched consumption of artisanal sourdough-leavened whole-wheat bread, which we originally viewed as radical opposites in terms of their health benefits.”

The researchers also measured various clinical end points, including weight, blood pressure, various blood tests, and the gut microbiome.

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To their surprise, Segal said they found no difference between the effects those two breads had on the various end points they measured. They combined and analyzed data on the two bread types, testing whether bread of any type had an effect.

The scientists found that just one week of bread consumption after eating no bread resulted in statistically significant changes to multiple clinical parameters, he said.

“We saw a reduction in essential minerals in the blood (calcium, magnesium, iron) and an increase in LDH (lactate dehydrogenase, a marker of tissue damage),” Segal said. “But we also saw an improvement in markers of liver and kidney function, inflammation markers, and cholesterol levels.”

In the microbiome, he said they found only a minimal difference between the effects of the different breads — two microbial taxa (groups of organisms), that were increased with white bread. But, generally they saw that the microbiome was very resilient to this intervention.

“This is surprising, as the current paradigm in the field is that a change in nutrition rapidly changes the makeup of the microbiome,” Segal said. “This is probably dependent on the kind of change. We had a nutritional change that was significant enough to change clinical parameters, which we tend to think of as very stable. And yet it had a minimal effect on the microbiome.”

The researchers were also co-authors of a paper published in 2015 in the journal Cell. In that study they observed the nutritional habits of 900 people. The researchers found that bread was the single most consumed food item in their diets, making up roughly 10 percent of their caloric intake.

In their latest study, participants also normally received about 10 percent of their calories from bread, Segal said. Half were assigned to consume an increased amount of processed, packaged white bread for a week (about 25 percent of their calories), and half were assigned to eat an increased amount of whole-wheat sourdough. The fresh wheat bread was baked specifically for the participants and delivered to them. Then, after two weeks without bread, the diets for each group were reversed.

Segal said they monitored numerous health effects before and during the study. These included the subjects’ glucose levels upon waking up; their levels of the essential minerals calcium, iron, and magnesium; fat and cholesterol levels; kidney and liver enzymes; and markers for inflammation and tissue damage.

The team also measured the composition of the subjects’ microbiomes before, during, and after the study.

“In fact, half the people had higher glycemic responses to white bread, and the other half had higher responses to sourdough bread,” Segal said. “We also proved rigorously that this was statistically significant and not a result of random fluctuations.”

“So, having very personal, often opposite responses, to the same kind of bread poses a problem. How would we know, in advance, which type of food is better for each person?”

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The scientists created a prediction algorithm: “We showed that we could have predicted, with fairly good accuracy, which bread induces lower glycemic responses for each subject personally, and did that based on their initial microbiome configurations,” Segal said.

“This is one very important way in which the food we eat affects our metabolism,” he said. “High glucose responses are a risk factor for the development of type 2 diabetes, cardiovascular disease, and liver cirrhosis. It is also associated with obesity, and enhanced all-cause mortality in both type 2 diabetes and cancer.”

Using personalized medicine has become increasingly popular in medicine, but using this technique for diets could potentially mark a shift in how nutritionists work with patients.

Kristin Kirkpatrick, MS, RD, LD, told Healthline that rather than giving universal dietary recommendations, nutrition advice is most effective when tailored specifically to the person, “considering metabolic characteristics, microbiota, food allergies or sensitivities, insulin and glucose sensitivities, and genes, if applicable.”

Kirkpatrick, also the manager of Wellness Nutrition Services at the Cleveland Clinic Wellness Institute in Ohio, has co-authored “Skinny Liver: A Proven Program to Prevent and Reverse the New Silent Epidemic – Fatty Liver Disease.” She said despite the findings in this small study, a longer-term study is needed.

“The findings in this study are based on two 1-week-long interventions. A small snapshot in time,” she said. “It may not be indicative of the potential nutrition effects that can take weeks, months, or even years to be seen and quantified.”

The study also brings up a question. Which is better bread: processed white or fresh, whole-wheat sourdough?

There are certain facts about whole-grain bread vs. white bread that support healthier overall nutrition, regardless of glycemic response, Kirkpatrick said.

“We know that the processing [milling] of intact grains to white flour removes layers of essential nutrition: B vitamins, minerals, proteins, healthy fats, and fiber in the bran and germ layers removed,” she said. “This leaves the white flour with only the endosperm, containing all the starch without a lot of nutrient density.”

So, even if glycemic responses after ingestion were the same, she added, study participants most likely would still miss out on those these vital nutrients if they chose white bread over whole wheat.

How did the Weizmann team measure the makeup of microbiomes? Some trips to the bathroom and a little help from their smartphones.

Stool samples were collected from participants at several points during the study. Segal said they extracted DNA from the samples, and analyzed the DNA sequence of the microbes in the stool.

“To identify the source of each of these DNA sequences, we matched it with databases of known DNA sequences of different bacteria known to reside in the gut,” he said.

Participants also used a smartphone app, developed by the scientists, to log their bread intake in real-time.

Called the Personalized Nutrition Project, the app analyzes the microbiome to predict sugar responses to thousands of different foods. Originally developed for the team’s previous 2012 study, the app was licensed and is now marketed by DayTwo.

The study raised questions that Segal, Elinav, and their colleagues are exploring now. Which genetic mechanisms drive differences between people? What biological mechanisms in the microbiome drive differences between people?

“If ‘one-size-fits-all’ diets do not work,” Segal said, “how can we better personalize diets? We are currently conducting research to answer some of these questions.”

“We need more research to establish precisely how the microbiome affects how people respond to food. But, we envision a future where each of us would have their microbiome profiled, and then receive personal nutrition advice based on it.”