Bacteroidetes

Your gut hosts trillions of microbes, and a large share of them belong to one group called Bacteroidetes (a major bacterial phylum, meaning a broad family of related bacteria). The amount of these bacteria in your stool shifts with age, diet, pregnancy, illness, and medication, and researchers have connected those shifts to conditions ranging from fatty liver disease to inflammatory bowel disease.

This is a research marker, not a settled clinical test. There is no agreed-upon target level, and what counts as a healthy reading varies widely between people. What it can offer is a snapshot of how your gut community looks right now, and a baseline you can compare against later if you start making intentional changes to your diet, exercise, or supplements.

What This Test Actually Measures

The lab uses polymerase chain reaction (PCR), a method that copies and counts specific stretches of bacterial DNA in your stool sample. Most assays target a fingerprint gene called 16S rRNA, then express your Bacteroidetes level as a share of total gut bacteria rather than an absolute count. Some labs measure the whole Bacteroidetes group, while others use a closely related pair, Bacteroides and Prevotella, as a stand-in.

Reported Bacteroidetes shares in healthy adults span a huge range across studies, depending on the population sampled and the lab methods used. That variability is wider than the difference between many disease and healthy groups, which is one reason a single Bacteroidetes number cannot be read like cholesterol or blood sugar.

Fatty Liver Disease

Stool studies have produced mixed findings on Bacteroidetes in nonalcoholic fatty liver disease and nonalcoholic steatohepatitis (NASH), the more serious form of fatty liver disease where the liver shows scarring and inflammation. In one study, adults with confirmed NASH had a smaller share of Bacteroidetes in their stool than healthy controls, and the link held even after adjusting for body mass index and fat intake. Other studies have reported the opposite pattern, with higher Bacteroidetes in people with fatty liver disease. The direction of the association is not settled.

The picture also shifts in liver transplant recipients. In people who developed recurrent fatty liver disease after transplant, higher Bacteroidetes was tied to worse liver activity scores. Because the same group of bacteria has been reported moving in opposite directions across studies and clinical settings, your result needs to be read in the context of your full health picture.

Inflammatory Bowel Disease

In people with inflammatory bowel disease (IBD), which covers Crohn's disease and ulcerative colitis, stool PCR studies have generally found lower phylum-level Bacteroidetes along with reductions in two other major bacterial groups. The picture at the genus level is more mixed. A recent systematic review reported that the Bacteroides genus is decreased in Crohn's disease but may actually be increased in ulcerative colitis. A separate ulcerative colitis study using a different PCR method found Bacteroidetes was sharply reduced compared with healthy adults, and the test was suggested as a faster, cheaper way to check gut balance before stool transplant procedures.

Type 1 Diabetes

In adults with type 1 diabetes, stool PCR found higher Bacteroidetes and lower Firmicutes (another major bacterial group) compared with healthy controls. Researchers think this shift may relate to changes in immune signaling in the gut, though no one has shown that changing your Bacteroidetes level treats or prevents the disease.

Parkinson's Disease

Parkinson's disease patients also show higher Bacteroides and a lower Firmicutes-to-Bacteroidetes ratio compared with controls in stool PCR work. This finding fits a broader pattern in Parkinson's research suggesting that gut microbiome shifts may show up alongside the disease, though the evidence is not entirely uniform across studies, and the cause and effect direction is still being worked out.

Obesity and Body Weight

Bacteroidetes is most often discussed in the context of obesity, usually as part of the Firmicutes-to-Bacteroidetes ratio. Some studies find lower Bacteroidetes in obese adults and children. Others find the opposite, with higher Bacteroidetes in overweight and obese groups. A large review concluded the ratio is not a reliable marker of body weight, with between-study variation often larger than the difference between lean and obese groups within any single study.

Why the Findings Point Different Directions

This is not a straightforward higher-is-bad or lower-is-bad marker. Different diseases have been linked to opposite shifts in Bacteroidetes, and even within a single condition the direction of change is not always consistent across studies. Lower levels have been reported in some inflammatory bowel disease and NASH cohorts, while higher levels show up in type 1 diabetes, Parkinson's, and post-transplant fatty liver disease. What this likely means is that Bacteroidetes is a sensitive indicator of gut community state, not a single switch for health. Your result needs to be read alongside your other health information, not in isolation.

Why a Single Reading Is Not Enough

Stool microbiome composition changes day to day. In one detailed study tracking participants over multiple days, individual gut bacterial makeup varied with stool moisture, gut transit time, and stool acidity. A single Bacteroidetes share captures one moment in a system that is constantly moving.

Tracking matters more than the first number. Get a baseline, then retest in 3 to 6 months if you are making real changes to diet, taking a new supplement, starting probiotics, or recovering from antibiotics. Annual checks make sense if you are using this marker as part of broader microbiome tracking. The goal is to see your own trajectory, since the size of typical between-person differences makes individual change more meaningful than any one snapshot.

When Results Can Be Misleading

Several factors can shift a single reading without telling you anything meaningful about your health. Pay attention to these before acting on a result.

• Recent diet: dietary changes can produce large temporary microbial shifts within days. A sudden switch to high protein, high fiber, or a restrictive diet in the days before testing can move your number without reflecting your usual gut state.
• Stool consistency and transit time: looser stools are linked to higher Bacteroidetes shares and more Bacteroides. A bout of diarrhea or laxative use can push the number up without indicating a true biological change.
• Recent surgery or major illness: post-surgery stool samples show meaningful shifts in bacterial groups compared with pre-surgery samples. A reading taken in the weeks after a hospital stay may not reflect your stable baseline.
• Lab method and sample handling: Bacteroidetes shares can differ based on how the stool is preserved, how DNA is extracted, and which primers the lab uses. If you want to compare readings over time, use the same lab and method.

Decision Pathway for Unexpected Results

Because there is no standard cutoff, a single Bacteroidetes share rarely points to a specific diagnosis on its own. The right next step depends on what you saw, what changed, and what other information you have.

• Big shift from your own baseline: retest in 4 to 8 weeks under similar conditions before drawing conclusions. Day-to-day microbiome variation alone can move the number.
• Gut symptoms alongside the result: consider companion testing for inflammation in the gut wall, such as fecal calprotectin, plus a multiplex stool pathogen panel to rule out infection. Talk with a gastroenterologist about whether a fuller workup makes sense.
• Concerning liver, blood sugar, or weight context: pair this with a liver enzyme panel, fasting insulin, and HbA1c (a marker of average blood sugar) before reading the microbiome result as a driver. The microbiome result becomes more useful when interpreted alongside metabolic markers.
• Result looks fine but symptoms persist: a normal Bacteroidetes share does not rule out gut problems. Comprehensive stool panels that include inflammation, digestion, and pathogen testing add information this marker alone cannot.