What does the evidence actually show about the gut microbiome and obesity?
The most consistent finding across multiple studies is that people with obesity tend to have lower gut microbiome diversity — meaning fewer different types of bacteria living in their gut. A 2021 systematic review and meta-analysis of 32 studies found that while many individual studies reported lower diversity in obese versus non-obese adults, the overall statistical difference was not significant when the data was pooled together [1]. This highlights that the connection is not universal and depends heavily on the individual.
The idea that obesity is simply linked to a higher ratio of Firmicutes to Bacteroidetes bacteria is an oversimplification. The same 2021 meta-analysis found that while some studies showed more Firmicutes and fewer Bacteroidetes in obese people, the results were highly inconsistent across studies [1]. In fact, a 2021 study of 21 obese and 21 control subjects found the opposite — a significantly lower Firmicutes/Bacteroidetes ratio in the obese group [3]. This contradiction shows that the relationship is not as straightforward as once thought.
Does the connection between gut bacteria and obesity differ for different people?
Yes, the connection varies significantly based on factors like age, metabolic health, and even menopausal status. A 2021 study of obese children found that those who were 'metabolically unhealthy' had significantly lower bacterial diversity compared to healthy-weight children, while 'metabolically healthy' obese children did not [2]. This suggests that the gut microbiome's role may be more closely tied to metabolic complications of obesity than to body weight itself.
In older adults, the type of fat matters. A 2024 study of postmenopausal women found that those with high levels of visceral fat (the dangerous fat around internal organs) had a distinct gut microbiome profile — higher levels of Proteobacteria and a lower Firmicutes/Bacteroidetes ratio — compared to women with low visceral fat [6]. This study also linked these bacterial changes to higher levels of inflammatory markers in the blood, suggesting a pathway through which gut bacteria might contribute to obesity-related inflammation.
Is the connection causal — does the gut microbiome actually cause obesity?
The evidence for a direct causal role of the gut microbiome in human obesity is weak and contested. A 2023 critical review argues that early, highly publicized mouse studies overstated the effect, and that subsequent human trials — including randomized controlled trials of fecal microbiota transfer — have failed to show that altering the microbiome leads to weight loss [4]. This review directly challenges the idea that the microbiome is a major driver of body weight in humans.
However, newer genetic analysis techniques suggest a causal link may exist for specific bacteria. A 2023 Mendelian randomization study — a method that uses genetic data to infer cause-and-effect — found that the genus Akkermansia had a protective effect against childhood obesity, with each increase in its abundance linked to a 43% lower risk of obesity [5]. This is the strongest evidence to date for a specific bacterial genus playing a causal role, but it does not mean the entire microbiome causes obesity.
About These Sources
This answer is built on 6 peer-reviewed studies — published from 2021 to 2024, 1 from 2024 or later, 3 in Q1 journals, collectively cited 656 times — selected as the most relevant from 7 studies that passed quality screening, drawn from 66 papers retrieved from a database of over 500 million.
Sources used in this answer
Gut Microbiome Composition in Obese and Non-Obese Persons: A Systematic Review and Meta-Analysis
In a systematic review and meta-analysis of 32 cross-sectional studies, the authors found no significant overall difference in gut microbiome diversity between obese and non-obese adults, and the commonly reported higher Firmicutes/Bacteroidetes ratio in obesity was not consistently observed across studies.
The role of the gut microbiota on the metabolic status of obese children
In a study of 86 Chinese children, those with metabolically unhealthy obesity had significantly reduced gut bacterial diversity compared to healthy-weight controls, while metabolically healthy obese children did not, suggesting the microbiome is linked to metabolic health, not just weight.
Characteristics of gut microbiota in people with obesity
In a study of 21 obese and 21 control adults, the obese group had significantly lower gut microbiome diversity and a decreased Firmicutes/Bacteroidetes ratio, contradicting the common hypothesis of an increased ratio in obesity.
Questioning the foundations of the gut microbiota and obesity
This critical review argues that early mouse studies overestimated the role of the gut microbiome in obesity, and that subsequent human trials, including randomized controlled trials of fecal microbiota transfer, have failed to show a significant effect on body weight.
Mendelian randomization analyses support causal relationship between gut microbiota and childhood obesity
Using Mendelian randomization, this study found that the bacterial genus Akkermansia has a causal protective effect against childhood obesity, with each increase in its abundance associated with a 43% lower risk of obesity.
Visceral adiposity in postmenopausal women is associated with a pro-inflammatory gut microbiome and immunogenic metabolic endotoxemia.
In a study of 50 postmenopausal women, those with high visceral fat had increased levels of Proteobacteria and a lower Firmicutes/Bacteroidetes ratio, along with higher markers of inflammation, linking the gut microbiome to obesity-related metabolic endotoxemia.