This post was updated Dec. 1 at 12:24 a.m.

A UCLA Health study found that certain types of bacteria in the gut microbiome could increase the risk of HIV.

While there are other studies which have found gut microbiome changes in those diagnosed with HIV, the majority of such studies do not follow participants over time, said Dr. Jennifer Fulcher, first author of the September study. This could prevent researchers from accounting for factors outside of HIV that could affect the microbiome, she added.

The gut microbiome is a group of microorganisms, such as bacteria and fungi, within the human digestive system, according to the Harvard School of Public Health. The makeup of the microbiome is affected by both environmental and genetic factors, and microbiome imbalance has been linked to many human diseases, such as Parkinson’s Disease, according to a study from Nature Scientific Reports.

The UCLA researchers used samples other studies had gathered over time from men who have sex with men, according to the study. In the United States, this population is the most affected by HIV, according to the CDC.

Fulcher, who is an assistant professor in the Division of Infectious Diseases of the Department of Medicine at the David Geffen School of Medicine, said the study found there was a difference in the pre-infection microbiomes of the experimental group that later acquired HIV and the control group, which did not.

This included changes in amounts of certain bacterial species, such as those from the Bacteroides group, according to the study. Many bacteria from this group are linked to parts of the immune system that prevent inflammation, which means decreased levels of these microorganisms could lead to chronic gut inflammation and increased HIV susceptibility according to the study.

However, the researchers did not find much difference before and after infection in the microbiomes of those who acquired HIV, Fulcher said. She added that this could be due to the fact that the samples they used were only collected up to a year after infection, which is early when considering the lifelong course of HIV infection.

It is difficult to acquire data about how HIV affects bodily systems in the long term because those diagnosed with HIV usually start treatment immediately, said Theodoros Kelesidis, an associate professor in the infectious diseases division at the School of Medicine.

Another major limitation of the study was the number of participants, which is a common issue in the field of HIV research, said Grace Aldrovandi, co-author of the study and chief of the Division of Infectious Diseases at UCLA Mattel Children’s Hospital. She added that the study only collected data about men, though women can also get HIV, and certain samples came from people who used drugs that could have also affected their microbiome composition.

However, the participants in the study were racially diverse, Fulcher said, adding that many of the participants identified as racial and ethnic minorities, which are vulnerable populations in regard to HIV infection.

Studies regarding HIV can overrepresent certain populations since vulnerable groups may have less access to care and research facilities, Kelesidis said.

More data is needed to understand the connection between the microbiome and HIV, Fulcher said. Because the study results may not be generalizable to all people, Fulcher said she would like to see research on samples from people of multiple geographic locations and sexes.

Aldrovandi said she hopes to see new studies that propose and test interventions using the results from their study.

“I don’t think we’re at that point (of creating prevention measures) now with the data that we have, but it’s the first step of a long journey or hopefully not too long a journey,” Aldrovandi said.