In a scientific surprise that may change many people's view of carbohydrates, a Japanese study revealed that eating bread and rice may lead to weight gain, even without increasing the total calories consumed.
Researchers at Osaka Metropolitan University, led by Professor Shigenobu Matsumura, decided to go beyond traditional theories that focus on fat as the main driver of obesity, and highlight carbohydrates that top the daily tables around the world, from bread to rice to pasta.
Scientists have long used high-fat diets in animal studies because fat was considered the primary cause of weight gain. However, the Japanese team identified a significant research gap: carbohydrates are consumed daily in large quantities, yet their role in obesity and metabolism has not been studied with the same depth. Everyone believes that "bread causes obesity" or that "carbohydrates should be limited," but the most important question remains unanswered: Is the problem with carbohydrates themselves, or with how people choose and consume them?
To answer this question, the researchers designed a clever experiment on mice. They divided them into different groups: one group ate standard food only, another ate standard food with added bread, a third with wheat flour, a fourth with rice flour, in addition to two additional groups, one of which ate high-fat food alone, and the other with added wheat flour.
The team then carefully monitored changes in body weight, energy consumption, blood metabolites, and even gene expression in the liver.
The mice showed such a strong preference for carbohydrate-rich foods that they completely stopped eating their regular, standard diet. Even more surprisingly, although their total calorie intake did not increase significantly, their body mass and fat percentage did.
Even more strangely, the mice that ate rice flour gained weight in the same way as the mice that ate wheat flour, meaning that the problem is not exclusive to wheat.
It turned out that mice that ate a high-fat diet with added wheat flour gained less weight than those that ate only the high-fat diet.
Researchers used a sophisticated technique called indirect calorimetry combined with breath gas analysis to accurately measure energy expenditure. The biggest surprise was that the weight gain wasn't due to "overeating," as commonly believed, but rather a decrease in the amount of energy the body burns (energy expenditure). In other words, the body was burning fewer calories than before. This was accompanied by metabolic changes including elevated blood fatty acid levels and fat accumulation in the liver. Furthermore, the activity of genes responsible for fatty acid production and transport increased.
All these signs together paint a clear picture: a diet rich in carbohydrates, even without an increase in calories, reprograms metabolism towards fat storage and reduced energy burning.
The good news is that these changes aren't permanent. When researchers removed wheat flour from the diet, both body weight and metabolic disorders improved significantly. This suggests that moving away from a carbohydrate-heavy diet and toward a more balanced one could be an effective key to weight management.
Now, the Japanese team plans to move their research from the laboratory to real life, through human studies to verify the applicability of these findings to actual dietary habits.
These important findings were published in the journal Molecular Nutrition & Food Research.
