Fasting drives up bilirubin because it simultaneously pushes multiple metabolic levers in the same direction: the liver produces more bilirubin, the blood has a harder time delivering it to the liver for processing, and the gut reabsorbs more of it back into circulation. No single mechanism explains the full effect. Instead, at least three distinct pathways converge, which is why even a relatively short fast of 24 to 48 hours can visibly raise bilirubin levels on a blood test and, in some people, cause noticeable yellowing of the eyes.
The Liver Ramps Up Bilirubin Production
Bilirubin is a breakdown product of heme, the iron-carrying molecule inside red blood cells and certain liver enzymes. An enzyme called heme oxygenase is responsible for cracking open heme and releasing bilirubin. During fasting, the activity of this enzyme in the liver increases sharply. Research in the 1970s showed that fasting for up to 72 hours produced a threefold increase in hepatic heme oxygenase activity, and that this stimulation appeared to be driven by glucagon released in response to low blood sugar.1PubMed Central. Metabolic regulation of heme catabolism and bilirubin production. I. Hormonal control of hepatic heme oxygenase activity More heme oxygenase means more heme gets dismantled, which means more bilirubin is dumped into the bloodstream. This is essentially the production side of the equation: even if everything downstream worked perfectly, you would still see higher bilirubin simply because the body is making more of it when you stop eating.
The hormonal trigger matters here. Fasting lowers blood glucose, which triggers glucagon release from the pancreas. Glucagon, in turn, raises cyclic AMP levels in liver cells, and that intracellular signal drives heme oxygenase activity upward. Epinephrine (adrenaline) can do the same thing through a similar pathway, which may explain why stress and fasting together can push bilirubin even higher than fasting alone.
Free Fatty Acids Get in the Way
When you fast, your body switches to burning fat for energy. Fat cells release free fatty acids (FFAs) into the blood, and those FFAs create problems for bilirubin handling in two ways.
First, bilirubin normally travels through the bloodstream bound to the protein albumin. Free fatty acids also bind to albumin, and when the ratio of FFAs to albumin climbs high enough, the fatty acids start competing with bilirubin for the same binding spot. Research on premature infants, who are especially sensitive to unbound bilirubin, found that when the FFA-to-albumin ratio reaches about 4:1, free fatty acids displace bilirubin from albumin’s primary binding site.2PubMed Central. Effect of Free Fatty Acids on Bilirubin–Albumin Binding Affinity and Unbound Bilirubin in Premature Infants Displaced bilirubin floats free in the blood, which both raises measured bilirubin levels and, in vulnerable populations like newborns, increases the risk that bilirubin will cross into tissues where it can cause harm.
Second, bilirubin and free fatty acids share a common transport system for entering liver cells. A carrier protein called bilitranslocase helps move both molecules from the blood into the liver for processing. When FFAs flood this transporter during fasting, they can crowd out bilirubin, slowing the liver’s ability to pull bilirubin out of the blood.3PubMed. Serum free fatty acids and bilirubin concentration during fasting in patients with Gilbert’s syndrome and normal controls This bottleneck has been especially well documented in Gilbert’s syndrome, where the transporter is already compromised genetically. But even in people without Gilbert’s, the competition between fatty acids and bilirubin for liver uptake contributes to fasting-related bilirubin rises.
The Gut Recycles Bilirubin Back Into the Blood
Under normal circumstances, the liver processes bilirubin and sends it into the bile, which flows into the intestine. Gut bacteria convert it into other pigments, and most of it leaves the body in stool (which is why stool is brown). But some bilirubin can be reabsorbed from the intestine and sent back to the liver through a loop called enterohepatic circulation.
Fasting slows intestinal motility. When the gut moves sluggishly, bilirubin sits in the intestine longer, giving it more time to be reabsorbed. A study in rats demonstrated that fasting decreased intestinal motility and the elimination of bile pigments, allowing enhanced enterohepatic circulation and an increased reflux of bilirubin back into the blood. The researchers concluded this was a major factor in fasting-induced hyperbilirubinemia.4PubMed. Fasting-related hyperbilirubinemia in rats: the effect of decreased intestinal motility In practical terms, the body keeps recycling the same bilirubin instead of excreting it, which inflates blood levels even beyond what increased production alone would cause.
Why Eating Again Brings Bilirubin Back Down
One of the more striking findings about fasting hyperbilirubinemia is how specifically it can be reversed. A classic study tested what happens when you give fasting subjects different nutrients: glucose, saline, amino acids, fat, and a non-caloric sugar substitute (mannitol). Only oral glucose reversed the rise in bilirubin. Amino acids, fat, saline, and mannitol all failed.5PubMed. Effects of caloric and noncaloric materials in fasting hyperbilirubinemia
This glucose specificity makes sense given the hormonal mechanism. Glucose intake suppresses glucagon release and raises insulin levels, which would be expected to dial back the glucagon-driven increase in heme oxygenase activity. Glucose also promotes glycogen storage in the liver, helping restore the supply of UDP-glucuronic acid, the molecule the liver uses to conjugate bilirubin and make it water-soluble for excretion. When that substrate runs low during fasting, conjugation slows down and unconjugated bilirubin accumulates. Re-feeding with carbohydrates replenishes the supply.
This finding has practical relevance for anyone doing intermittent fasting or extended fasts. If you get blood work done during a fast and your bilirubin comes back elevated, eating a carbohydrate-containing meal before a retest would likely bring the number back to baseline. Many clinicians are aware of this, but it can still catch patients off guard when they see “high bilirubin” flagged on a lab report.
The Rise Is Bigger in Men Than in Women
Not everyone responds to fasting with the same degree of bilirubin increase. A retrospective study of healthy adults participating in phase I clinical trials found a statistically significant increase in total serum bilirubin levels in fasting males compared with non-fasting males, with longer fasting times correlating with higher bilirubin levels. However, the same study found no correlation between fasting and bilirubin levels in clinically normal females.6PubMed. Fasting increases serum bilirubin levels in clinically normal, healthy males but not females: a retrospective study from phase I clinical trial participants
The reasons for this sex difference are not entirely settled, but a few factors probably contribute. Men generally have higher baseline bilirubin levels than women, partly because they have higher red blood cell mass (more heme to break down). Estrogen may also influence bilirubin metabolism in ways that buffer women against the fasting effect, though the specifics are still being worked out. A separate study looking at young women confirmed that fasting duration was one of the main non-genetic determinants of bilirubin levels, alongside hemoglobin and body mass index, suggesting the effect exists in women but is subtler.7The American Journal of the Medical Sciences. Determinants of Bilirubin Levels in a Young Female Population
For lab interpretation, this means a mildly elevated bilirubin in a fasting man is less alarming than the same number in a fasting woman, where it may warrant a closer look at other causes.
Gilbert’s Syndrome and the Fasting Test
Gilbert’s syndrome is a common, harmless inherited condition affecting roughly 5 to 10 percent of the population. People with it have a reduced ability to conjugate bilirubin in the liver, so their baseline bilirubin runs a bit high. The fasting effect is exaggerated in these individuals, and clinicians have historically used this as a diagnostic tool.
In one of the earliest formal studies of this approach, reducing caloric intake to 400 calories a day for 72 hours produced a significant increase in bilirubin in both Gilbert’s patients and healthy controls, but the rise was much steeper in Gilbert’s patients. Even a 24-hour version of the restricted diet could distinguish Gilbert’s syndrome from other causes of unconjugated hyperbilirubinemia.8PubMed Central. Diagnosis of Gilbert’s syndrome: role of reduced caloric intake test A follow-up study compared caloric restriction with a nicotinic acid (niacin) provocation test and found both approaches produced significantly higher bilirubin levels in Gilbert’s patients than in controls.9PubMed. Comparison of nicotinic acid- and caloric restriction-induced hyperbilirubinaemia in the diagnosis of Gilbert’s syndrome
The caloric restriction test is less commonly used today because genetic testing can identify the relevant variant in the UGT1A1 gene more directly. But the principle behind it still matters: if you have Gilbert’s syndrome and you fast, your bilirubin will spike more dramatically than in someone without the condition. This can produce visible jaundice (yellow eyes, sometimes yellow skin) during periods of illness, stress, or deliberate fasting, which can be alarming if you don’t know about your diagnosis. If you have been told your bilirubin runs high and fasting makes your eyes visibly yellow, Gilbert’s syndrome is the most likely explanation, and it is benign.
Drugs That Amplify the Fasting Effect
Certain medications can pile onto the fasting-induced bilirubin rise and push levels higher than either factor alone would cause. The best-known example is atazanavir, a protease inhibitor used in HIV treatment that raises bilirubin by inhibiting the UGT1A1 enzyme, the same enzyme that is genetically under-active in Gilbert’s syndrome. A pilot study found that patients on atazanavir who fasted for a short period saw their mean unconjugated bilirubin climb from about 2.3 mg/dl before the fast to about 3.8 mg/dl afterward, a statistically significant increase.10PubMed Central. Short Communication: Fasting Increases Serum Concentrations of Bilirubin in Patients Receiving Atazanavir: Results from a Pilot Study
This interaction is clinically relevant because visible jaundice is one of the reasons patients stop taking atazanavir. If those patients also happen to be fasting for religious, medical, or personal reasons, the combined effect can push bilirubin high enough to cause noticeable yellowing, even though the underlying cause is not liver disease. Clinicians managing patients on atazanavir or similar drugs (including some cancer medications processed through the same pathway) often advise patients to avoid prolonged fasting or to be aware that bilirubin readings taken while fasting will be artificially inflated.
What Happens to Bilirubin on a Ketogenic or Very-Low-Calorie Diet
Given that fasting raises bilirubin through multiple pathways, you might expect that a ketogenic diet, which keeps caloric intake up but severely restricts carbohydrates, would do the same. The picture here is more nuanced. Ketogenic diets produce high levels of circulating free fatty acids, which could theoretically compete with bilirubin for albumin binding and liver uptake. But they also provide calories, which keeps insulin active and limits the glucagon-driven increase in heme oxygenase.
A study examining liver function tests during the initiation of a ketogenic diet in pediatric patients found that while certain liver markers did change, serum bilirubin remained normal.11PubMed Central. Acute Changes in Liver Function Tests During Initiation of Ketogenic Diet This suggests that caloric adequacy may be more protective than carbohydrate intake specifically, at least over the short term. The classic finding that only glucose reverses fasting hyperbilirubinemia does not necessarily mean that a zero-carbohydrate diet permanently elevates bilirubin; it may mean that the initial rise during a fast is best corrected by glucose, while longer-term metabolic adaptation on a ketogenic diet uses other compensatory mechanisms.
Time of Day Matters Too
Bilirubin does not stay flat across the day even under normal eating conditions. It follows a circadian rhythm, and that rhythm interacts with fasting in ways that can muddy the interpretation of lab results. A study of healthy men found that bilirubin peaked during the nighttime hours under normal sleep conditions, with the average peak occurring around 10:30 in the morning and the intraindividual variation across the day ranging from about 13 to 43 percent.12PubMed. Circadian variability of bilirubin in healthy men during normal sleep and after an acute shift of sleep That is a huge natural swing. If you combine the normal circadian peak with a fasting state (say, blood drawn first thing in the morning after an overnight fast), you may catch bilirubin at its daily high and its fasting high simultaneously, producing a reading that looks concerning but is actually just two normal physiological rhythms stacked on top of each other.
This is worth keeping in mind when interpreting routine blood panels. The “fasting blood draw” that most labs request is designed to stabilize glucose and lipid readings, but it introduces a confound for bilirubin by definition. A mildly elevated bilirubin on a morning fasting panel does not automatically mean something is wrong.
When Fasting Bilirubin Should Prompt Further Investigation
Most of the time, a modestly elevated bilirubin in a fasting person is a physiological artifact that resolves with eating. But there are situations where it warrants attention. Unconjugated hyperbilirubinemia in adolescents and young adults needs to be distinguished from other causes, most commonly Gilbert’s syndrome versus increased bilirubin production from mild hemolytic disorders like beta-thalassemia trait, with the rare Crigler-Najjar syndromes as important additional considerations.13Journal of Clinical Medicine and Regenerative Medicine. Clinical Differentiation of Gilbert Syndrome, Crigler-Najjar Syndrome and β-Thalassemia Trait
The key distinction is between bilirubin that responds to fasting and feeding (which points toward benign metabolic causes) and bilirubin that stays elevated regardless of nutritional state (which may suggest ongoing red blood cell destruction or a more serious conjugation defect). A few clues help sort this out:
- Pattern with meals: If bilirubin drops reliably after eating carbohydrates and rises with fasting, Gilbert’s syndrome or simple fasting physiology is the most likely explanation.
- Hemoglobin and reticulocyte count: If those are abnormal alongside elevated bilirubin, a hemolytic process (where red blood cells are being destroyed faster than usual) deserves investigation.
- Conjugated vs. unconjugated: Fasting preferentially raises unconjugated bilirubin. If the conjugated (direct) fraction is the one that is high, the problem is more likely in the liver or bile ducts, not in fasting physiology.
- Age of onset: Gilbert’s syndrome typically becomes apparent around puberty. New-onset unconjugated hyperbilirubinemia in a middle-aged adult who has not changed their eating habits deserves a different workup.
The Phenomenon Across Species
Fasting hyperbilirubinemia is not unique to humans. Horses are especially prone to it, and the mechanisms appear to differ somewhat from what happens in people. In horses, increased bilirubin production does not seem to be the primary driver. Instead, the rise appears to come mainly from decreased hepatic uptake of bilirubin, likely because horses and ponies experience dramatic spikes in blood lipids during fasting, and those lipids compete with bilirubin for carrier-mediated transport into the liver. Depletion of the substrates needed for bilirubin conjugation may also play a role in equine fasting hyperbilirubinemia.14PubMed. Equine fasting hyperbilirubinemia Veterinarians routinely see elevated bilirubin in horses that have gone off feed due to illness, transport, or surgery, and the clinical challenge mirrors the human one: deciding whether the bilirubin rise reflects fasting physiology or genuine liver disease. The fact that different mammalian species emphasize different pathways in the same overall phenomenon suggests that fasting hyperbilirubinemia is an ancient metabolic feature, not a quirk of human biochemistry.