Hyodeoxycholic Acid

If you have ever wondered whether your gut bacteria are quietly helping or hurting your metabolic health, this is one of the few molecules that gives you a direct readout. HDCA (hyodeoxycholic acid) is a bile acid your body can only make with the help of specific gut microbes, which means your level reflects the conversation happening between your liver, your intestine, and the bacteria living inside it. This test measures HDCA in stool.

Research links this molecule to fatty liver, gut barrier health, inflammation, and atherosclerosis. It is still a research-grade marker without standardized clinical cutpoints, but tracking it gives you an exploratory window into a system that standard liver and lipid panels do not see.

What HDCA Actually Is

Bile acids are detergent-like molecules your liver makes from cholesterol to help you digest fats. After they reach the intestine, gut bacteria modify them into different forms (called secondary bile acids). HDCA is one of those secondary forms, produced when microbes act on bile acids reaching the lower intestine. Your liver and kidneys then further process it. The amount that ends up in stool reflects how active those microbial conversions are.

What makes HDCA unusual is how your body handles it. In a study using volunteers with surgically placed bile-collection tubes, orally administered HDCA was well absorbed and then heavily packaged with a sugar molecule (a process called glucuronidation) before being excreted. Between 30% and 84% left the body through urine, a pattern described as unique among bile acids.

Fatty Liver Disease (NAFLD)

NAFLD (nonalcoholic fatty liver disease) is one of the most studied connections. In a study of 230 children with NAFLD, HDCA-family bile acids in blood were lower in those with the disease, and lower levels tracked with more severe disease. The proposed mechanism involves activating two metabolic switches in the liver that turn fat into fuel rather than letting it accumulate. Stool HDCA reflects the upstream microbial step that produces these bile acids in the first place.

What this means for you: if you have any signs of fatty liver, such as elevated ALT (alanine aminotransferase, a liver enzyme) or imaging that suggests fat in the liver, a low stool HDCA may add context. It suggests your gut microbes may not be producing enough of the bile acids that help your liver burn fat.

Inflammatory Bowel Disease

In inflammatory bowel disease, HDCA tells two different stories depending on where you measure it. In a study of 668 people with Crohn's disease and ulcerative colitis, serum HDCA (a related but different measurement) was elevated in both conditions, reflecting the body's attempt to detoxify a more toxic bile acid (lithocholic acid) when the gut barrier is compromised.

In a separate study of 79 people with active ulcerative colitis, HDCA in stool moved in the opposite direction. Stool HDCA was negatively correlated with calprotectin (a stool marker of gut inflammation), meaning the lower the stool HDCA, the more active the disease. This isn't a contradiction. Serum and stool capture different aspects of the same biology: serum reflects what is leaking into circulation and being detoxified, while stool reflects what microbes are still able to produce locally. The test you are looking at measures the stool side.

Colorectal Adenoma and Cancer Risk

In a study of 46 people, those with advanced colorectal adenomas (precancerous growths) had higher levels of HDCA-family bile acids in fluid sampled from the lower intestine. A separate study of 36 people with colorectal cancer plus type 2 diabetes found altered relationships between HDCA and gut bacteria associated with that high-risk combination. The pattern is exploratory, but it suggests HDCA-family bile acids may behave differently in the bowel of people developing colorectal disease.

Heart Disease and Atherosclerosis

People with atherosclerosis (plaque buildup in arteries) had reduced serum HDCA in research linking the bile acid to cardiovascular biology. The mechanistic work behind this finding was done in animal models, where HDCA reduced plaque burden by reshaping immune cell signaling, so the human evidence here is associational rather than causal. Whether stool HDCA tracks the same direction as serum HDCA in this context has not been directly established.

Cognitive Decline

In a study of 2,251 people, serum HDCA and its glycine-conjugated form were among five gut-derived metabolites that helped distinguish dementia from cognitively unimpaired people, with discrimination accuracy of about 0.72 (where 1.0 would be perfect and 0.5 would be no better than chance). This is exploratory work in serum rather than stool, but it adds HDCA to a list of gut metabolites being studied for their potential connection to brain aging.

Reconciling the Mixed Signals

HDCA is not a clean "high is bad" or "low is bad" marker. In fatty liver, atherosclerosis, and active ulcerative colitis stool, lower levels track with worse disease. In serum during inflammatory bowel disease and in the bowel during precancerous adenoma development, higher levels appear. The unifying framework is that HDCA reflects the state of gut-liver-microbiome communication, and that conversation can break down in different directions depending on the disease. Treat your number as one piece of a metabolic and inflammatory picture, not as a verdict.

Reference Ranges

HDCA is a research-grade marker. There are no standardized clinical reference intervals, no consensus guidelines, and no agreed-upon cutpoints for diagnosis. Different labs use different methods (most commonly methods that measure exact molecule concentrations from a stool sample), and absolute numbers can vary substantially between assays. The practical implication is that your single value carries less meaning than your trend over time within the same lab.

Treat any number you receive as orientation, not a verdict. The most useful interpretation comes from comparing your value to your own previous values, measured by the same method, and from looking at HDCA alongside other markers of gut, liver, and metabolic health.

Why One Reading Is Not Enough

Bile acids respond to what you ate, when you ate, what your gut microbes did overnight, and how your liver is processing things on a given day. A single HDCA value is a snapshot of an active conversation, not a fixed property of your biology. The trend matters far more than any single number.

For tracking, get a baseline measurement, then retest in 3 to 6 months if you are making targeted changes (diet shifts, treating a fatty liver diagnosis, addressing gut symptoms). After that, annual testing is reasonable for most people who want to monitor this system. If you are using HDCA to track response to a specific intervention, retest at the timepoint when the intervention's effect would be expected to show up.

What to Do With an Abnormal Result

Because HDCA does not have clinical cutpoints, an unusual result is best interpreted as a prompt to look at the systems it reflects, not as a diagnosis. If your HDCA looks low and you have other signs of fatty liver (elevated ALT, AST, GGT, or imaging findings), that pattern is worth investigating with a hepatologist or your primary care doctor. If your HDCA looks unusual alongside gut symptoms or markers of inflammation like fecal calprotectin, a gastroenterologist is the right person to involve.

Useful companion tests to order alongside HDCA include a full bile acid profile (to see HDCA in context with other bile acids), liver function tests, hs-CRP (high-sensitivity C-reactive protein, a marker of low-grade inflammation), and if gut symptoms are present, calprotectin and a stool microbiome analysis. The combination is more informative than HDCA alone.

When Results Can Be Misleading

• Recent dietary changes: bile acid composition in stool reflects what you ate in the days before collection. A sample taken during an unusual eating pattern (very high fat, low fiber, or after travel) may not reflect your usual biology.
• Recent antibiotic use: HDCA depends on specific gut bacteria. A recent course of antibiotics can suppress the microbes that produce it, lowering your reading without reflecting your usual biology.
• Acute illness or recent surgery: any condition that alters gut transit, bile flow, or systemic inflammation can shift bile acid profiles in the short term.
• Cholestyramine or other bile acid sequestrants: these medications bind bile acids in the gut and can dramatically alter the bile acid pattern measured in stool, without indicating any underlying disease.