Bread and Carbohydrates May Drive Weight Gain, Mouse Study Finds

New research involving mice indicates that eating bread may increase body weight and fat mass, even when calorie consumption remains broadly unchanged.

Led by researchers at Osaka Metropolitan University in Japan, the study draws attention to the potential for carbohydrates to promote weight gain, alongside excess dietary fat - the factor most commonly emphasised in dietary advice.

Nutrition experts have previously discussed the links between bread, carbohydrates and weight gain. However, detailed evidence on this relationship has been limited, particularly regarding wheat flour and the metabolic processes that may be involved.

Bread, carbohydrates and weight gain

The researchers found that greater wheat bread consumption was linked to lower energy expenditure. This appeared to shift metabolism towards prioritising fat storage, despite similar calorie levels across the diets.

"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," says nutritionist Shigenobu Matsumura of Osaka Metropolitan University.

For the experiments, laboratory mice could choose between their usual healthy cereal-based chow and simple bread, baked wheat flour or baked rice flour. The animals were monitored for changes in body weight and for the way their bodies used energy both at rest and during activity.

Blood samples enabled the researchers to measure hormone, blood glucose and metabolite levels. After the experiments, tissue tests were also used to evaluate gene expression in the liver.

The results showed that the mice readily replaced their usual diet with carbohydrate-rich snacks. This switch resulted in weight gain and increased fat tissue, especially among male mice.

Metabolic effects of wheat flour

Additional analysis and follow-up experiments indicated that the two principal changes were caused by the foods themselves, rather than by overeating or reduced exercise. Mice on the wheat flour diet burned fewer calories overall, while genes that convert carbohydrates into fat became active.

After nine weeks, the mice fed wheat flour also had higher insulin and leptin levels. These hormones are involved in metabolism and appetite control. Their blood glucose and triglyceride levels, however, remained comparable with those in the control group.

The study did not examine how these hormonal changes may contribute to fat accumulation. It does, though, align with the carbohydrate-insulin theory of obesity, which associates insulin with fat storage and reduced energy expenditure.

A further test involving the wheat flour group found that restoring the chow diet halted weight gain and reversed the metabolic changes.

"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," says Matsumura.

These findings add to evidence that the foods we consume can alter how the body processes food and burns the calories within it. In the case of bread, the research suggests that it may slow the body's metabolic engine.

Limits of the mouse study

One restriction of the research is that it used mice rather than human participants. Comparable processes may be occurring in people, but this remains uncertain and will need to be explored in future work.

The team also plans to test a wider range of foods to establish precisely which properties of bread produce this response.

As with any dietary study, these results should not be viewed in isolation. Many other influences can affect how metabolism responds to food and drink, including age and hormone-related changes.

Further studies should clarify the part wheat and bread can play in the diet, as well as why the simple "calories in, calories out" principle is not always straightforward.

"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," says 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."

The research has been published in Molecular Nutrition & Food Research.