Underweight and Unbalanced: Gut Microbial Diversity in Underweight Japanese Women

Researchers link BMI and differences in gut microbiota in young, underweight Japanese women

November 1, 2025: Scientists have identified connections between low body weight and altered gut bacteria in young Japanese women. Researchers Dr. Katsumi Iizuka and Dr. Hiroaki Masuyama discovered that underweight women exhibited reduced gut microbial diversity and higher proportions of inflammation-associated bacteria. Their findings highlight the importance of gut health in weight management.

Low body weight among young women is associated with health risks including irregular menstruation, fertility challenges, compromised immunity, and reduced bone density. Japan has experienced a sustained increase in underweight women aged 20 to 39 over recent decades. This trend coincides with dietary shifts, eating disorders like anorexia nervosa, and potential gut microbiota imbalances. While previous studies noted decreased gut diversity in both underweight and overweight individuals, variations specific to young Japanese women remained unexplored.

To address this, researchers from Fujita Health University and Symbiosis Solutions Inc. investigated connections between body weight, diet, and gut microbiota in underweight Japanese women. Published in Nutrients on October 17, 2025, the study compared 40 underweight women (BMI

Surprisingly, dietary habits didn’t significantly differ between groups. However, gut analysis revealed underweight participants had lower microbial diversity and elevated levels of Bacteroides, Bifidobacterium, and Erysipelatoclostridium—bacteria linked to inflammation. In contrast, normal-weight women showed higher abundances of beneficial bacteria like Prevotella and Dorea. “Underweight women displayed reduced gut diversity and microbial profiles suggesting inflammatory tendencies, independent of diet,” noted Prof. Iizuka.

These results imply gut health may be as crucial as caloric intake for weight management. “Beyond macronutrient balance, consuming fiber-rich and fermented foods could improve weight by modifying gut bacteria,” explained Iizuka. When considering long-term applications, he added, “Combining dietary data with microbiota analysis might help evaluate nutritional absorption and metabolic efficiency after eating.”

As interest in gut health grows globally, this research offers new pathways for personalized nutrition approaches tailored to underweight women in Japan.