Low body weight in young women has been linked to a range of health concerns, including disrupted menstrual cycles, infertility, weakened immune function, and a long-term decline in bone density. Japan has seen a rising trend in the proportion of underweight women between the ages of 20 and 39, with little to no change over the past two decades. The persistence of this trend raises concerns over the long-term health implications, especially as lean body weight has been correlated with changing dietary habits, diseases like anorexia nervosa, and even imbalances in gut microbiota. While previous research has reported lower alpha gut microbiota diversity in the underweight and overweight categories compared to those with normal weight, the differences in gut microbiota diversity in young, underweight Japanese women remain unclear.
To explore this further, a team of researchers from Japan, led by Dr. Katsumi Iizuka, a Professor at the Department of Clinical Nutrition, Fujita Health University, Japan, and Dr. Hiroaki Masuyama, Symbiosis Solutions Inc., set out to investigate the link between body type, dietary patterns and gut microbiota diversity in young, underweight Japanese women. They focused on understanding whether the gut microbiota diversity differed significantly between women who were underweight and those with normal body mass index (BMI). Their research was published in Volume 17, Issue 20 of the journal Nutrients on October 17, 2025. Prof. Iizuka says, "The differences in the diversity of the gut microbiota of young Japanese women by body weight have not been investigated. If differences in the gut microbiota between thin young women and women of normal weight become clear, I believe that improving eating habits could lead to improvements in the gut microbiota and consequently, body weight."
The study observed and compared 40 underweight women with 40 age-matched women, between 20 and 39. The underweight group included women who were referred to the Nutrition Clinic at Fujita Health University with a BMI less than 17.5 at their first consultation, while the control group was composed of women with normal BMI (18.5 ≤ BMI < 25) and were selected from a database by Symbiosis Solutions Co., Ltd. (Tokyo, Japan). Stool samples were collected using standardized kits, while food frequency questionnaires were used to assess dietary patterns.
Interestingly, the researchers found no significant differences in dietary patterns between the two groups. However, an analysis of the gut microbiota showed that underweight women had a significantly lower microbiota diversity and an increased presence of species, such as Bacteroides, Bifidobacterium, and Erysipelatoclostridium, which are linked to inflammation. On the other hand, the normal-weight group had a healthier abundance of bacteria like Prevotella and Dorea. Prof. Iizuka mentions, "Among Japanese women aged 20–39, those in the underweight group showed a greater decrease in gut microbiota diversity compared to the normal-weight group, rather than differences in dietary patterns." He adds, "Underweight women demonstrated reduced gut microbiota diversity and enrichment of taxa associated with inflammatory tendencies."
These findings suggest that gut health is as important as caloric intake when it comes to addressing low body weight. Prof. Iizuka explains, "Not only the amount of energy or the ratio of protein, fat, carbohydrate, but actively consuming dietary fiber and fermented foods may lead to weight improvement in underweight individuals by causing changes in the gut microbiota."
When asked about the long-term implications of the study, he said, "By combining the dietary survey data with gut microbiota analysis, it may become possible to evaluate not only the nutrients and energy contained in food, but also the quality of digestion and metabolism within the intestines after eating."
With gut health gaining global attention, the findings of this study offer new insights into how the gut microbiota could aid in more personalized nutritional care for underweight Japanese women.
