Carb-heavy foods like bread and rice may trigger weight gain in unexpected ways.

Scientists discover why bread can cause weight gain without extra calories

Osaka Metropolitan University

Bread has long been a dietary cornerstone, sustaining societies for generations. It is deeply woven into everyday life. But with obesity rates continuing to climb, researchers are beginning to question whether this reliance on staple carbohydrates still makes sense in modern diets.

Obesity increases the risk of many lifestyle-related diseases, making prevention a major public health priority. Traditionally, research has focused on high fat consumption as the main driver of weight gain. This is why many animal studies rely on high fat diets.

However, carbohydrates such as bread, rice, and noodles are consumed daily around the world, yet their role in obesity and metabolism has not been explored as thoroughly. While many people believe that "bread makes you gain weight" or that "carbohydrates should be limited," it has been unclear whether the issue lies in the foods themselves or in how people choose and consume them.

Study Explores Carb Preference and Metabolic Effects

To better understand these questions, a research team led by Professor Shigenobu Matsumura at Osaka Metropolitan University's Graduate School of Human Life and Ecology studied how carbohydrates influence eating behavior and metabolism in mice.

The researchers examined whether mice preferred foods like wheat, bread, and rice over standard chow, and how these choices affected body weight and energy use. The animals were divided into several feeding groups, including Chow, Chow + Bread, Chow + Wheat flour, Chow + Rice flour, High-fat diet (HFD) + Chow, and HFD + Wheat flour. The team tracked changes in body weight, energy expenditure, blood metabolites, and liver gene expression.

Carb Preference Linked to Weight Gain Without More Calories

The findings showed that mice strongly favored carbohydrate-rich foods and stopped eating their standard chow altogether. Even though their total calorie intake did not increase significantly, both body weight and fat mass went up.

Mice that consumed rice flour gained weight in a similar way to those that ate wheat flour. In contrast, mice given a High-fat diet (HFD) + Wheat flour gained less weight than those on a High-fat diet (HFD) + Chow.

"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," said Professor Matsumura.

Slower Energy Use May Drive Weight Gain

The team also used indirect calorimetry with respiratory gas analysis to better understand energy use. The results showed that weight gain was not caused by "overeating," but by a reduction in energy expenditure.

Further analysis revealed higher levels of fatty acids in the blood and lower levels of essential amino acids. In the liver, fat accumulation increased, along with the activity of genes linked to fatty acid production and lipid transport.

When wheat flour was removed from the diet, both body weight and metabolic abnormalities improved quickly. This suggests that moving away from a wheat-heavy diet and toward a more balanced one may help regulate body weight more effectively.

Next Steps: Applying Findings to Human Diets

"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," stated Professor Matsumura. "We also intend to investigate how factors such as whole grains, unrefined grains, and foods rich in dietary fiber, as well as their combinations with proteins and fats, food processing methods, and timing of consumption, affect metabolic responses to carbohydrate intake. In the future, we hope this will serve as a scientific foundation for achieving a balance between "taste" and "health" in the fields of nutritional guidance, food education, and food development."

The findings were published in Molecular Nutrition & Food Research.

Journal Reference:

1. Shigenobu Matsumura, Miona Marutani, Eri Nousou, Nagisa Murakami, Saki Mizobata, Miyu Fujisawa, Mizuki Fujiwara, Nanase Iki, Soyoka Horie, Yuka Yamato, Azumi Yamamoto, Mina Fujitani, Teppei Fujikawa, Chinami Ishibashi, Shigeo Takenaka. Wheat Flour Intake Promotes Weight Gain and Metabolic Changes in Mice. Molecular Nutrition, 2026; 70 (2) DOI: 10.1002/mnfr.70394