Bread and Rice May Cause Weight Gain by Slowing Metabolism, Not Just Adding Calories, Study Finds

A new study has thrown fresh light on why carbohydrate-heavy diets may contribute to weight gain — and the answer may have less to do with overeating than previously assumed.

Researchers at Osaka Metropolitan University's Graduate School of Human Life and Ecology found that mice strongly preferred foods like bread, rice, and wheat flour over their standard diet, and that this preference triggered metabolic changes leading to weight and fat gain — even without a significant increase in total calorie intake.

The findings were published in the journal Molecular Nutrition and Food Research.

Why Carbohydrates Were Put Under the Microscope

While high-fat diets have long been the focus of obesity research, carbohydrates such as bread, rice, and noodles are consumed daily by billions of people around the world. Their specific role in metabolism and weight gain has, until now, been comparatively underexplored.

The research team, led by Professor Shigenobu Matsumura, set out to understand whether mice preferred carbohydrate-rich foods over standard chow, and what metabolic consequences followed from that preference. 

Also Read: Fun facts about bread

What the Study Found

Mice were divided into several feeding groups and offered combinations of standard chow, bread, wheat flour, rice flour, and high-fat diets. The results were striking.

The animals strongly favoured the carbohydrate-rich options and stopped eating their regular chow almost entirely. Despite this, their total calorie intake did not increase significantly — yet both body weight and fat mass went up.

Mice given rice flour gained weight in a similar pattern to those given wheat flour, suggesting the effect is not specific to wheat but linked more broadly to carbohydrate preference and its metabolic consequences.

Interestingly, mice on a high-fat diet combined with wheat flour gained less weight than those on a high-fat diet combined with standard chow.

Slower Energy Burning, Not Overeating

Using respiratory gas analysis to measure energy use, the team found that the weight gain was driven not by consuming more calories but by a reduction in energy expenditure — the body simply burning less energy.

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

When wheat flour was removed from the diet, both body weight and the associated metabolic abnormalities improved relatively quickly — pointing to a potentially reversible effect with dietary adjustment.

What This Could Mean for Human Diets

Professor Matsumura was careful to note that the findings are based on animal models and that the next step is to examine how these metabolic patterns apply to actual human dietary habits.

The research team also plans to investigate how factors such as whole grains, dietary fibre, food processing methods, and the timing of carbohydrate consumption affect metabolic responses — with the ultimate goal of helping people find a scientifically grounded balance between taste and health in everyday diets.