The gut microbiome is the population of microorganisms present in the gut, largely consisting of bacteria as well as other microbes like viruses, fungi, and protozoa. Research has revealed that the microbiome plays a critical role in digestion, as well as other aspects of health and disease, including cognitive function and immune system responses. Furthermore, it can be impacted by various factors like diet, lifestyle, and weight.

A recent study published in the Open Access journal Nutrients by researchers from the Fujita Health University in Japan found that women aged 29-39 who were underweight had significantly lower gut microbiota diversity compared to healthy weight control groups.

The study highlights the importance of maintaining a healthy weight, particularly for women, to support the gut microbiome as well key biological processes that are impacted by drastic weight changes.

The importance of maintaining a diverse gut microbiome

A diverse gut microbiome is composed of a wide variety of microbe species and is important for gut health. Past research confirms that a diverse composition promotes health and improves well-being.

Microbiome diversity is associated with specific lifestyles and regularly maintaining a diverse diet – including a high intake of fruits and vegetables. This can be referred to as dietary patterns which vary between ages, gender, and lifestyles.

An individual’s dietary pattern also has immense health impacts, often determining risk of disease such as heart disease and obesity. For example, excess intake of foods that are ultra processed has been shown to heighten systemic inflammation via changes to the gut microbiome. This in turn increases the risk of developing inflammatory and chronic diseases.

Additionally, research has linked the gut microbiome to changes in mental health and cognitive function, with factors such as stress impacting the susceptibility to pathogenic bacteria. On the other hand, dysbiosis, a change in the gut microbiome’s diversity and composition, can alter the hypothalamic-pituitary-adrenal axis –the brain’s central coordinator that mediates the body’s stress responses.

Measuring microbiome diversity

Specifically, gut microbiome alpha-diversity is associated with health and a reduction in acute and chronic diseases. Alpha diversity refers to a measure of species richness and diversity within a single sample, and is sometimes used as a predictor of health and disease states. This differs from beta-diversity, which is a measure of diversity within two different areas or people..

Despite the link between alpha-diversity and healthiness, alternative research suggests that this measure is not a complete indicator of how healthy the gut is, and further information is required to elucidate the healthiness of a microbiome aside from microbe diversity.

Nonetheless, alpha diversity can be an important indicator of the state of the gut microbiome. Furthermore, the measure can be used to understand how certain factors, such as weight, can impact gut diversity, and its associated role in determining important biological processes and responses.

Essential body fat is important for hormone health

Adipose tissue, known as body fat, is crucial for hormone health, particularly in women, who require a minimum of 10-13% of essential fat, according to the American Council on Exercise. An ideal body fat percentage above the essential range is dependent on age, sex and levels of activity, and is often determined by an individual’s specific biological blueprint.

Excess body fat, seen in the context of chronic conditions like obesity, is linked to a high risk of developing conditions like breast cancer in women. This is because fat cells produce oestrogen, which, when in excess, increases the risk of breast cancer.

On the other hand, insufficient body fat can also cause significant health complications. An underweight state in women can lead to amenorrhea (loss of menstrual period), vitamin deficiencies, immune dysfunction, future decline in bone density, and infertility. Low body weight can also impact the gut microbiome, its composition and associated systemic responses.

Around 20% of women in Japan are considered underweight – a figure notably higher than that observed in Western countries.

The research published in Nutrients evaluated the gut microbiome of underweight women in Japan and compared it to healthy weight women with a standard body mass index (BMI) to understand how low body weight impacted microbiome composition. They also assessed the dietary patterns of the women to see if it varied according to body type.

Prof. Izuka, corresponding author of the study explains the aim of the research:

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.

Study design

The researchers compared 40 underweight women with 40 women with standard BMIs, both groups between ages 20-39. They analysed the dietary habits, diversity, and patterns of the women, which were evaluated using relevant food questionnaires.

The food frequency questionnaire is one of the most common food questionnaires used in Japan to evaluate nutrient intake and frequency of food consumption based on 10 different types of food groups. This includes meat, fish, eggs, dairy products, soybeans, green & yellow vegetables, seaweed, fruit, tubers, and fats & oils.

The gut microbiome composition of the women was analysed from collected stool samples provided by the participants.

Alpha diversity is reduced in the gut microbiome of underweight Japanese women

Interestingly, the results showed that the dietary patterns of underweight women did not differ from those of women with a standard body weight. However, the alpha diversity measures were significantly lower in underweight women compared to women of healthy weight.h.

Prof. Izuka summarises these findings:

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. Underweight women demonstrated reduced gut microbiota diversity and enrichment of taxa associated with inflammatory tendencies.

The results showed specific changes in the gut microbiome in women with low-body weight. This includes significant increases in bacterial species such as Bacteroides, Erysipelatoclostridium, and Enterocloster, as well as a decrease in Dorea.

Importantly, they also found that intake of dietary fibre amongst Japanese women was considerably low. This could be because of the overall rise in meat consumption and decreased intake of grains such as rice and barely. Dietary fibre is vital to maintain a healthy gut microbiome and foster a population of ‘good’ bacteria.

Future work

The gut microbiome is a powerful ecosystem that is impacted by many factors, including weight and diet. The discussed research highlights the importance of body weight in mediating the alpha- and beta-diversity of the gut microbiome and subsequently nurturing a healthy gut.

Furthermore, the study reinforces that people are not consuming enough fibre. Insufficient fibre intake has been linked to a rise in diseases such as colorectal cancer. It is also vital to maintain a healthy gut microbiome.

Prof, Izuka explains the potential application of the research:

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.

The study could be valuable to develop clinical interventions for people struggling with low body weight, by assessing the impact of body weight on the microbiome.

To read more research on the role of food and diet in health, access the journal Nutrients or access the full MDPI journal list.