A new animal study is raising questions about how refined carbohydrates may affect metabolism, but it does not support the sweeping claim that bread itself causes weight gain.
In research published in Molecular Nutrition & Food Research, scientists at Osaka Metropolitan University found that mice given access to foods like bread, wheat flour and rice flour gained weight and body fat even though their total calorie intake did not increase significantly. The researchers say the results point to changes in energy expenditure rather than simple overeating.
Because the study was conducted in mice under tightly controlled feeding conditions, it cannot show that the same thing happens in humans.
The study focused on a common nutrition debate. High-fat diets are often used in obesity research, but carbohydrates such as bread, rice and noodles are everyday staples in many diets. To test how carbohydrate-rich foods affected metabolism, the researchers divided mice into several feeding groups, including standard chow alone, chow plus bread, chow plus wheat flour, chow plus rice flour, a high-fat diet plus chow and a high-fat diet plus wheat flour.
The mice consistently preferred the carbohydrate-rich foods over standard chow and largely stopped eating the chow when both were available. Mice in the bread, wheat flour and rice flour groups gained weight and body fat despite no major rise in overall calorie intake. By contrast, mice in the high-fat diet plus wheat flour group gained less weight than those in the high-fat diet plus chow group, which suggests the findings were not simply about wheat being uniquely harmful.
“These findings suggest that weight gain may not be due to wheat-specific effects, but rather to a strong preference for carbohydrates and the associated metabolic changes,” study author Shigenobu Matsumura said.
The researchers also used indirect calorimetry, a method that estimates how much energy the body is using by measuring respiratory gases. That testing suggested the mice were burning less energy, which may help explain why they gained weight even without a large increase in calorie intake. Blood testing showed higher fatty acid levels and lower levels of essential amino acids. The researchers also found fat buildup in the liver and increased activity in genes linked to fat synthesis and lipid transport.
Those changes improved after the mice stopped eating wheat flour, with body weight and metabolic markers moving back toward baseline. That suggests the effect was tied to the feeding pattern used in the experiment, not necessarily to permanent metabolic damage.
Still, the mice were not eating meals that resemble a normal human diet. They were given refined carbohydrate sources alongside standard chow, and they strongly favored the more palatable option. The study also did not test whole grains, mixed meals or fiber-rich carbohydrate foods eaten with protein and fat.
In other words, it does not show that bread, rice or carbohydrates in general cause weight gain in people.
Instead, the study points to a narrower and more useful takeaway: highly refined, appealing carbohydrate foods may affect metabolism differently depending on what else is in the diet, how the food is processed and how strongly it drives food preference.
“Going forward, we plan to shift our research focus to humans to verify the extent to which the metabolic changes identified in this study apply to actual dietary habits,” Matsumura said.
This study was supported by the Public Foundation of Elizabeth Arnold-Fuji, Sugiyama Sangyo-Kagaku General Incorporated Foundation, Tojuro Iijima Foundation for Food Science and Technology and the Japan Society for the Promotion of Science (JSPS KAKENHI).