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Instalab ResearchBilirubin is the yellow pigment your body makes when it breaks down heme, mainly from old red blood cells. Your liver processes it, and it gets excreted in bile. For a long time, scientists thought of it as metabolic trash, but newer research shows it plays an active role as a hormone-like molecule and antioxidant.
Normal total bilirubin in adults falls roughly between 0.2 and 1.0 mg/dL (or 3.4 to 17 μmol/L), with most of it in the unconjugated form. When researchers talk about "low bilirubin," they typically mean values in the lowest quartile or third of that normal range, not zero. So even if your number is technically "in range," it could still be on the lower side compared to the population.
This is where the research is most consistent. Multiple population studies have found an inverse relationship between bilirubin levels and cardiovascular disease. People with lower bilirubin within the normal range experienced more coronary artery disease events and cardiovascular deaths. One large study of statin-treated patients found that cardiovascular events tracked inversely with bilirubin levels, meaning those at the bottom of the range fared worse.
Research in chronic kidney disease patients has also suggested bilirubin acts as a cardiovascular protector, with higher levels linked to better outcomes.
However, there's an important caveat. At least one study using a genetic approach (which can help separate true cause-and-effect from mere correlation) found that bilirubin may not directly protect against inflammatory or other diseases. The beneficial associations seen in clinical studies could partly be due to confounding, meaning other factors that happen to travel alongside higher bilirubin. This is a meaningful piece of the puzzle and a reason to interpret the heart disease link with some caution.
Several studies have linked lower bilirubin to higher rates of metabolic syndrome (a cluster of conditions including high blood sugar, excess belly fat, abnormal cholesterol, and high blood pressure) and greater insulin resistance.
This connection shows up in both children and adults. A study of children and adolescents found that those with lower serum bilirubin had higher insulin resistance and more metabolic syndrome. A long-term cohort study tracking adults over 16 years found the link was especially strong in women.
Cross-sectional data from the UK Biobank, one of the world's largest health databases, found that higher bilirubin was associated with lower BMI, better lipid profiles, and lower blood pressure.
Yes, at least in some populations. An analysis combining clinical trial data and real-world evidence found that lower bilirubin was associated with faster decline in kidney filtration rate (eGFR, the standard measure of how well your kidneys clean your blood). A separate study in patients with type 2 diabetes found that lower bilirubin correlated with more albuminuria, which is protein leaking into the urine and an early sign of kidney damage.
If you already have diabetes, hypertension, or other kidney risk factors, low bilirubin might be one more signal that closer monitoring of kidney health is worthwhile.
Low bilirubin has appeared as a risk marker in some cohort studies looking at cancer and certain neurodegenerative diseases. But the evidence here is thinner and less consistent than the cardiovascular and metabolic data. The research positions bilirubin more as a potential marker of these risks rather than a proven cause.
It's worth noting that this is not a case where "more is always better." Dangerously high bilirubin causes jaundice and can be a sign of liver disease, bile duct blockage, or excessive red blood cell destruction. In newborns, very high levels can cause a serious brain condition called kernicterus. So bilirubin operates on a spectrum where both extremes carry risk, but for different reasons.
Bluntly: there is no evidence-based treatment aimed at raising bilirubin, and current clinical guidelines do not recommend one. Guidelines for managing abnormal liver blood tests focus overwhelmingly on investigating elevated bilirubin, not low levels. If your bilirubin is low but the rest of your liver panel (ALT, AST, alkaline phosphatase, GGT, albumin) and blood counts are normal, that is generally considered a benign finding.
A few things worth confirming with your clinician:
The biggest gap in the research is causation. Most of the studies linking low bilirubin to disease are observational. They can show that low bilirubin and heart disease tend to appear together, but they can't prove that low bilirubin caused the heart disease. The genetic study that questioned a direct protective role of bilirubin is a good reminder that correlation in large populations doesn't always mean what it appears to mean.
There are also no clinical trials testing whether raising bilirubin (even if we could reliably do it) would prevent disease. Until that kind of evidence exists, low bilirubin is best understood as a subtle risk marker, not a diagnosis or a call to action on its own.
If your bilirubin came back on the low side and everything else looks normal, you probably don't need to lose sleep over it. But you can use it as useful context. Think of it as one small data point among many. If you also have other risk factors for heart disease, metabolic syndrome, or kidney problems, it may be worth mentioning to your doctor as part of the bigger picture, especially to discuss whether closer monitoring makes sense for you.