Bread, rice and noodles are eaten every day around the world. Yet when scientists study obesity, they usually look elsewhere. Obesity raises the risk of many lifestyle-related diseases, and research has traditionally treated high fat intake as the main driver of weight gain. That is why many animal studies rely on high-fat diets.
Carbohydrates have received far less attention. Many people believe that "bread makes you gain weight" or that "carbohydrates should be limited," but it has been unclear whether the problem lies in the foods themselves or in how people choose and eat them.
To look into this, a research team led by Professor Shigenobu Matsumura at Osaka Metropolitan University studied how carbohydrates affect eating behavior and metabolism in mice. Some mice received bread, wheat flour or rice flour alongside their standard feed, known as chow. Others were given a high-fat diet, with either chow or wheat flour on the side. The team then tracked the animals' body weight, how much energy they used, substances in their blood and gene activity in their livers.
The mice showed a strong preference. Given the choice, they ate the carbohydrate-rich foods and stopped eating their chow altogether. Their total calorie intake did not rise significantly, but their body weight and fat mass did.
The type of grain did not seem to matter much. Mice that ate rice flour gained weight in much the same way as those that ate wheat flour. And among mice on a high-fat diet, those given wheat flour gained less weight than those given 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.
If the mice were not eating more, why did they gain weight? To find out, the team measured the gases the animals breathed out, a method that shows how much energy the body is burning. The mice were not overeating. They were using less energy.
Other tests pointed in the same direction. The mice had more fatty acids in their blood and fewer essential amino acids, the building blocks of protein that the body cannot make on its own. Fat built up in their livers, and genes involved in producing and moving fat became more active.
These changes were reversible. When wheat flour was removed from the mice's diet, their body weight and metabolic problems improved quickly. In mice, at least, moving away from a wheat-heavy diet toward a more balanced one helped bring weight back under control.
Whether the same holds for people is still an open question. "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. His team also plans to study how whole grains, fiber-rich foods, food processing and meal timing shape the body's response to carbohydrates. The long-term goal, he said, is a scientific basis for balancing taste and health in nutrition advice, food education and food development.
The findings were published in the journal Molecular Nutrition & Food Research.