The bacteria that live in the human gut are a chatty bunch. Speaking a language of small molecules, they communicate with each other, prime the immune system, and influence behavior. In recent years, scientists have found evidence that changes in the composition of the gut microbiome also contribute to obesity.

Scientists have identified several microbial metabolites involved in metabolic disorders such as fatty liver disease, insulin-resistance, and cardiovascular disease. However, whether microbe-derived metabolites directly cause obesity or other metabolic diseases has remained controversial.

“The mitochondria in our cells are endosymbionts. They’re prokaryotic, and there is this whole area of quorum sensing where bacteria can communicate amongst themselves,” said Andrew Neish a pathologist who studies host-microbiome interactions at Emory University. “It just brought up this idea that potentially there was this interaction between the microbiota and mitochondria.”

In a new Nature Metabolism study, Neish, co-senior author Dean Jones who studies oxidative stress and metabolomics at Emory University, and their teams reported that the microbial metabolite δ-valerobetaine (VB) inhibited mitochondrial fatty acid oxidation in cells and led to diet-induced weight gain in germ-free mice, which are typically resistant to diet-induced obesity (1). These findings provide a new mechanistic link between the microbiome and obesity and present VB as a potential drug target.

Over the past twenty years, evidence that the gut microbiome plays a role in obesity has grown. Germ-free mice, which are mice born without gut microbiomes, remain thin even when fed a Western diet, which is high in sugar and fat (2). When scientists transplanted the microbes from the guts of mice with a conventional microbiome into germ-free mice, the formerly germ-free mice gained weight and developed insulin-resistance, despite eating less (3). Additionally, mice with a genetic mutation that causes obesity have a decreased proportion of Bacteroidetes bacterial species and an increased proportion of Firmicutes species in their guts compared to healthy mice (4).

Some of these findings in mice have translated to humans, with some studies reporting a lower proportion of Bacteroidetes species in the guts of obese people compared to those of healthy individuals. However, other studies have reported the opposite or no correlation (5). 

Recent research in mice revealed that gut microbial metabolites regulate the expression of a family of microRNAs in white adipose tissue, which in turn regulate insulin-sensitivity and energy expenditure (6). These microRNAs are also dysregulated in obese humans, providing greater support for a gut microbiome contribution to obesity.

To identify a link between the gut microbiome and mitochondria, Neish, Jones, and their teams exposed germ-free mice to the bedding of mice with a normal microbiome, allowing the germ-free mice to acquire a conventional microbiome.

By assessing the metabolomes of mitochondria from mouse liver cells from either the germ-free mice or mice that acquired the conventional microbiome, the researchers identified VB as the most differentially regulated metabolite. VB was highly abundant in conventionalized mice, but completely missing from the germ-free mice. The researchers demonstrated that VB did not come from food and could only have originated from the gut microbiome.

The researchers found that VB affects mitochondrial metabolism by depleting carnitine levels in cells. During fasting, cells use carnitine to shuttle long-chain fatty acids into the mitochondria so that they can be converted into energy. In both cells and mice treated with VB under fasting conditions, carnitine levels dropped, and mice accumulated more lipids in their livers, hearts, and brains, indicating that VB prevented cells from performing fatty acid oxidation.

When the researchers fed germ-free mice a Western diet and treated them with VB, they saw that the previously obesity-resistant mice had a greater than 80% increase in weight compared to germ-free mice fed only a Western diet.

For Kristina Martinez-Guryn, a metabolism and microbiome researcher at Midwestern University who was not involved in the study, the VB-dependent weight gain in the germ-free mice was “the proof in the pudding” of the microbial-metabolism connection.

“In this field, we have wondered for a long time why germ-free mice are resistant to diet induced obesity,” she said. “We always wonder and say, ‘oh, it's probably metabolites that the microbes are producing. They're communicating with the host to drive these things.’ So it's really nice to see a solid example of that in this study.”

The research team also performed retrospective analyses on different sets of human clinical data to look for associations between VB levels and the microbiome in humans. They reported that people with an obese body mass index had significantly more VB circulating in their blood than non-obese individuals did.

Overall, the researchers “did a number of different things here to support that this microbe-derived metabolite is having an effect on metabolism through decreasing fat oxidation,” Martinez-Guryn said. “I was really excited about this paper.”

She also cautioned that the clinical association data of VB and the microbiomes of obese people did not seem as strong as their analyses of VB in mice and cells. But, she added, “humans are very complex. There are a lot of things that are going to likely affect the VB levels that they measured… There are probably many other factors going on that would lead to that being more of a moderate observation.”

Because of the clear influence diet has on VB’s interaction with metabolism, Neish, Jones, and their teams are performing follow up experiments to study this process further. They are feeding mice different diets, manipulating the microbiome, and investigating the effects of VB on different tissues in the body.

According to Martinez-Guryn, understanding the complex interactions between diet, the gut microbiome, and metabolism will be important for developing obesity therapeutics.

“Moving forward, it's not to say that VB is bad in somebody who's eating a normal healthy diet, but if somebody is eating a Western diet, which a lot of us do… then it becomes a problem based on what they saw,” she said.

Neish and Jones agreed. They are eager to investigate if and how VB levels can be manipulated to help treat fatty liver disease and obesity.

“For better or worse, this is a molecule that we've coevolved with and we potentially can learn to exploit for the better,” Neish said.

References

1. Liu, K.H. et al. Microbial metabolite delta-valerobetaine is a diet-dependent obesogen. Nat Metab  3, 1694-1705 (2021).
2. Bäckhed, F. et al. Mechanisms underlying the resistance to diet-induced obesity in germ-free mice. Proc Natl Acad Sci  104, 979-984 (2007).
3. Bäckhed, F. et al. The gut microbiota as an environmental factor that regulates fat storage. Proc Natl Acad Sci  101, 15718–23 (2004).
4. Turnbaugh, P. et al. An obesity-associated gut microbiome with increased capacity for energy harvest. Nature  444, 1027-1031 (2006).
5. John, G.K. & Mullin, G.E. The Gut Microbiome and Obesity. Curr Oncol Rep  18, 1-7 (2016).
6. Virtue, A.T. et al. The gut microbiota regulates white adipose tissue inflammation and obesity via a family of microRNAs. Sci Transl Med  11, eaav1892 (2019).