Is Your Gut Bacteria the Reason You Can’t Lose Weight?

An Arivale Hot Topic

Jennifer Lovejoy, Arivale Chief Science Officer, PhD
Jennifer Lovejoy
Arivale Chief Science Officer, PhD

Some people with obesity are resistant to weight loss, even when they appear to be compliant with diet and exercise regimens, and the reason for this weight-loss resistance is unclear. While genetics likely play a big role, increased attention is being turned to the gut microbiome as a factor in why some people may have a harder time losing weight.

There are good reasons to think the gut microbiome may be involved in obesity.

While much of the research has been conducted in mice, there are studies in humans that confirm an association between our microbes and weight. For example, some studies have shown consistent differences in the composition of the gut microbiome between lean and obese populations, although this hasn’t been found in every study1. And, the microbiomes of obese mice and humans appear to be more efficient at “harvesting” the calories from food consumed2. Finally, it’s been shown the gut microbiome is altered after weight-loss surgery, and these changes may be part of the reason why such surgeries are so effective at producing large and sustained weight losses3.

The Study

In a new study from the Mayo Clinic that was covered by Newsweek, researchers studied 26 overweight or obese middle-aged adults who were participating in a lifestyle-based obesity treatment program.

Intensive lifestyle intervention is the recommended first approach to treating people with obesity and is generally effective at reducing body weight by the recommended 5 to 10 percent. The Mayo Clinic’s 12-month intensive lifestyle intervention includes dietary, physical activity, and behavioral approaches delivered via group sessions.

For this study, researchers only looked at people who had completed the first three months of the program. The group was divided into the nine participants who had lost at least 5 percent of their initial weight (“responders”) and those who hadn’t (“weight loss resistant”). Gut microbiome patterns between the two groups were then compared.

Microbial composition analysis identified two differences between the responders and weight loss resistant before the lifestyle intervention was started. A group of bacteria called Phascolarctobacterium was significantly increased in the successful losers, while a bacterial group called Dialister was significantly increased in the weight loss resistant participants.

Somewhat paradoxically, Phascolarctobacterium has been previously associated with increased obesity and weight gain in rats on a high-fat diet, whereas in this study it seemed to be associated with weight loss.

Dialister is a pathogenic bacteria mainly associated with dental infections and gingivitis, although one previous study found Dialister was increased in non-obese, healthy adults relative to obese adults with cardiovascular risk factors.

The researchers also indirectly attributed the functions of gut microbes using previously validated methods and identified six genes encoding carbohydrate-active enzymes and binding proteins that predicted weight loss resistance.

Based on these findings, the researchers concluded that differences in the gut microbiome may contribute to differences in success rates in behavioral weight-loss programs.

Arivale’s Take

Although this study was small, it adds to a body of literature suggesting there is a relationship between the gut microbiome and obesity. In particular, it sheds some light on the as-yet unanswered question of why some individuals are unable to lose much weight despite diligently following diet and exercise regimens.

Nonetheless, the coverage in Newsweek is a great example of a headline (“Can’t Lose Weight? Your Gut Bacteria Could Be to Blame, According to Study”) dramatically overstating the actual findings of the research study. The study didn’t test – or conclude – that the differences in gut microbiome were the cause of the weight loss differences. Unfortunately, research is still too early to be able to confidently describe the role of the gut microbiome in obesity.

In the Newsweek article, one study author is quoted as stating the “potential use is to develop personalized strategies for weight loss…”. We agree there is great “potential.” Unfortunately, we still don’t know enough about causality, nor how to directly manipulate specific bacterial groups even if research did prove they were causal in weight gain. So, we’re still far from being able to personalize strategies for weight loss based on the microbiome.

There were several limitations to the study. In addition to the small size, which is mentioned in the Newsweek article, the authors note the considerable dietary variety inherent in the Mayo Clinic’s obesity treatment program, which could have confounded the microbiome results, since diet is such a strong driver of microbiome composition. Additionally, the method used to determine microbiome composition (16S rRNA sequencing) does not allow for direct assessment of bacterial function, so future studies using other techniques, larger cohorts, and longer follow up will be important for drawing conclusions about the gut microbiome and weight loss resistance.


  1. Castaner O, Goday A, Park YM, Lee SH, Magkos F, Show STE, Schroder H. (2018). “The Gut Microbiome Profile in Obesity: A Systematic Review.” International Journal of Endocrinology. 2018 Mar 22;2018:4095789. doi: 10.1155/2018/4095789.
  2. Jumpertz R, Son Le D, Turnbaugh PJ, Trinidad C, Bogardus C, Gordon JI, Krakoff J. (2011). “Energy-balance studies reveal associations between gut microbes, caloric load, and nutrient absorption in humans.” The American Journal of Clinical Nutrition. 2011 Jul; 94(1): 58–65.
  3. Tremaroli V, Karlsson F, Werking M, Stahlman M, Kovatcheva-Datchary P, Olbers T, Fandriks L, Le Roux CW, Nielsen J, Backhed F. (2015). “Roux-en-Y Gastric Bypass and Vertical Banded Gastroplasty Induce Long-Term Changes on the Human Gut Microbiome Contributing to Fat Mass Regulation.” Cell Metabolism. 2015 Aug 4; 22(2): 228–238.

[Arivale Hot Topics address health stories currently in the news. The Arivale Clinical Team’s commentary on these news articles is not a review of the scientific evidence, nor an endorsement of a specific study, and is not meant as official medical opinion.]