Contribution
An update to earlier reviews that adds recently studied microbial metabolites (succinate, branched-chain amino acids, bacterial proteins) to the established short-chain fatty acid and bile acid routes. It proposes manipulating the microbiota as a therapy for appetite disorders.
Key insight
The microbiome can reach appetite by producing signals the host already uses. These include metabolites that trigger gut satiety hormones, and proteins that mimic host appetite peptides: “ClpB, the best studied bacterial protein, can act as a mimetic of alpha-melanocyte-stimulating hormone (α-MSH) to result in similar anorexigenic effects”.
Assumptions
- Rodent-to-human transfer. It is assumed rather than shown. The authors write: “To date, our understanding of the gut microbiota roles in modulating appetite is mainly based on in vitro studies and rodent models.”
- A mutualist frame. The abstract speaks of microbiota that “develop the intricate interactions to manipulate the metabolic requirements and hedonic feelings”. No account of microbial fitness interests is given.
Key results
- Humans: associations. Anorexia nervosa patients show lower microbial diversity, and bulimia patients higher bacterial ClpB.
- Humans: interventions. “acute colonic propionate delivery decreases food intake and stimulates the secretion of PYY and GLP-1, while long-term colonic propionate delivery has little effects on PYY and GLP-1 release in humans”. In one inulin-propionate study, “the reward processing and hedonic response rather than the secretion of PYY and GLP-1 contribute to the inulin-propionate-caused energy intake reduction”. Prebiotic trials in adults and children conflict.
- Transplant. “Fecal microbiota transplantation (FMT) from a healthy individual to an AN patient has led to weight gain”. This is a single case report.
- The authors’ own verdict. “it will be essential to conduct well-designed clinical trials or assemble clinical data, in order to fill the large gaps between clinical and experimental knowledge”. And: “Although there is as yet no evidence that such treatments would be safe and efficient for feeding-related diseases”.
Claims
| id | claim | strength | support |
|---|---|---|---|
| C1 | Microbial SCFAs, bile acids and indoles raise gut satiety hormones and reduce intake | moderate in rodents | mouse mechanism studies; human propionate effect acute only |
| C2 | Bacterial proteins mimic host appetite peptides (ClpB and α-MSH) | moderate | in vitro and rodent; a human correlate in bulimia |
| C3 | Eating disorders co-occur with altered microbiota | moderate as association | human cross-sectional |
| C4 | Microbial therapies can treat appetite disorders | asserted (proof of concept) | one FMT case report, mixed trials |
| C5 | The field rests mainly on in vitro and rodent evidence | strong (authors’ own statement) | Conclusions |
Connections
- The owner’s example (CLAIM-166, CASE-047). It supports the existence of routes from gut microbes to hunger, and causal effects in rodents. It does not show that the human microbiome drives everyday hunger. In humans, the cleanest effect (propionate) is a delivered metabolite, not a microbial community.
- Interoception (THEORY-045). Felt hunger is the experience of a regulatory state, and these are routes into that state from a lower-level collective. That bearing is the record’s.