I've posted in the past on a two year followup of a single-person self-experiment, a successful attempt to favorably adjust the balance of populations in the aging gut microbiome via a single treatment with flagellin immunization. This approach was intended to motivate the immune system into more aggressively destroying problematic microbes that tend to grow in number with age. As assessed using the Viome service, the intervention appeared to produce a sizable, lasting benefit to the quality of the gut microbiome in a basically healthy 50-ish individual. You can look back at those posts for the details of the protocol and further references regarding this use of flagellin.
As is the case for flagellin immunization, there is animal data to show that fecal microbiota transplantation from a young individual to an old individual rejuvenates the gut microbiome. There is a lot more of this animal data for fecal microbiota transplant, however, produced in varied species, and demonstrating that the improvement lasts for a lengthy period of time, and even results in improved health and extended life span.
Thus the same 50-ish individual mentioned above later undertook a fecal microbiota transplant, using donor material provided by a healthy, athletic 20-year-old volunteer, and further assessed the effects of this intervention on the gut microbiome. Stool samples were tested beforehand, one month afterwards, and six months afterwards. All of the measurements were again made using the Viome service. In the charts below, marks indicate that the flagellin immunization was conducted in 05/2020, and fecal microbiota transplant at the end of 08/2022. The dates marked on the horizontal axis are the dates of Viome testing.
In summary, while the flagellin intervention greatly reduced microbial diversity in the process of greatly improving other metrics, that diversity was restored by fecal microbiota transplant from a young individual. That restoration appears lasting as of the six month mark. The transplant produced small gains in some of the other metrics assessed by Viome, though not to the same degree as the flagellin immunization. That may well be because much of the potential scope for improvement was already achieved. Interestingly, neither intervention did much for Metabolic Fitness. According to Viome, half of the population falls into the 22-28 range for Metabolic Fitness (on a scale of 0 to 100!) which makes one wonder a little regarding the algorithm used in the construction of this value.
The protocol for conducting a fecal microbiota transplant at home is almost too simple to talk about, but there are a few points that are worthy of thought. The mechanics of it are straightforward. A fresh stool sample is provided by the donor, and that material is mixed with water. A few fluid ounces of the result are delivered as an enema. The recipient then lies in a suitably sloped position, abdomen higher than chest, for 30 minutes or so, in order to encourage the enema fluid to flow as deeply as possible into the intestine. Repeat this process for two to three times a few days to a week apart.
As to the points worth of thought: when fecal microbiota transplantation is conducted in the clinic as a treatment for C. difficile infection, colonic cleansing and colonoscopy equipment may be used, but more importantly, donor stool samples are screened for potentially pathogenic microbes that an older individual may respond poorly to. This screening is wise for an older recipient, as the aged immune system is far less competent than a young immune system, and that is an important factor when it comes to suppressing undesirable microbial species in the gut. What is innocuous to a young person may be much less innocuous to an old person. Rather than going through the process of finding a willing, healthy volunteer, and rolling the dice on potential issues, one can use services that will sell screened and characterized stool samples from young individuals, such as Human Microbes. This is recommended.