High bilirubin from fasting or hard training

High bilirubin means there is more red blood cell breakdown product in your blood than expected, usually above 17 to 21 µmol/l. In a healthy athlete, three harmless causes often arrive at once.

And one of them is something the lab asked you to do.

Fasting raises your bilirubin. That has been described since the 1970s (PMID 937224). You fast twelve hours because your glucose and lipids need it, and in the meantime you push this value up.

What is bilirubin?

Bilirubin is the waste product formed when your body breaks down old red blood cells. The haemoglobin in them is converted to bilirubin, your liver attaches a sugar group so it becomes soluble, and it then leaves your body through your bile and stool.

That intermediate step in your liver is where it gets interesting.

Labs therefore often report two numbers: total bilirubin and direct bilirubin. The difference between them is the indirect fraction, which your liver has not processed yet.

Why does fasting raise your bilirubin?

Because fasting temporarily slows processing in your liver and changes uptake from your gut. The result is a rise in the indirect, unconjugated fraction. The effect is large enough that doctors once used it deliberately as a test: restrict food and see whether the value jumps.

Research from 1976 showed significant rises on a low-calorie diet (PMID 937224), and later work confirmed the pattern with reduced caloric intake (PMID 7156881).

Say you eat your last meal at 20:00, draw the next morning at 08:30, and your bilirubin reads 26 µmol/l while your liver is otherwise entirely normal. Those 14 hours without food are a real candidate for the difference.

Draw another time two hours after breakfast and the same person can land neatly inside the range.

What is Gilbert syndrome?

Gilbert is a benign, inherited variant in which the liver enzyme that conjugates bilirubin is less active. It occurs in roughly 5 to 10 percent of people, causes no symptoms and needs no treatment. Many people discover it by accident during a blood test.

The signature is a mildly raised total bilirubin with a normal direct bilirubin and otherwise normal liver values.

What makes Gilbert notable here: the value rises specifically with fasting, illness, sleep deprivation and hard exertion. Exactly the conditions under which an athlete schedules a blood test.

The top two rows together are by far the most common in healthy athletes. The bottom two belong with a doctor.

Does hard training raise your bilirubin?

Yes, through red blood cell breakdown. At every footstrike during running, red cells in the soles of your feet are mechanically damaged, and the released haemoglobin is converted to bilirubin. Research comparing running and cycling at equal exertion found a fourfold greater rise in free plasma haemoglobin after running (PMID 12391035).

Cyclists and swimmers see this effect markedly less than runners.

If you also see a low haptoglobin or a raised LDH, that fits the same mechanism. How your marrow responds is covered in high reticulocyte count.

How do you know whether it is your liver?

By looking at the split and at your liver enzymes, not at the total alone. If the rise sits almost entirely in the indirect fraction and your ALT, AST and gamma-GT are normal, that points away from liver damage and towards fasting, Gilbert or breakdown.

If the direct fraction rises with it, that is a different story.

That pattern belongs with your GP, alongside your symptoms. The rest of the liver values are covered in liver values explained and in elevated liver values.

When should you discuss a raised value with your GP?

With yellow eyes or skin, dark urine, pale stool, itching, pain in the upper right of your abdomen, or a raised direct bilirubin. Also if your total stays clearly raised on a non-fasted repeat with no training beforehand.

Thuisarts describes an abnormal blood result as a reason to look further rather than a diagnosis, and the NHG guidelines approach liver values through the same pattern thinking. RIVM points out that reference values differ per laboratory in blood testing, which matters for bilirubin because the upper limit sits between 17 and 21 µmol/l depending on the lab.

Take your whole result, including the direct bilirubin. Without that split your GP is missing half the information.

Where do you start?

Check whether only your total is raised with a normal direct bilirubin, and whether your liver enzymes moved with it. If they did not, schedule a repeat that is not fasted and with no hard training in the preceding 48 hours. If the value stays the same, Gilbert is a real and entirely harmless explanation to discuss with your GP.

Our complete blood count test gives you your result with an assessment by a BIG-registered doctor.

How to plan a draw that your preparation does not distort is in fasted blood draws for athletes. The broader context is in the complete biomarker guide for athletes. And if dark urine comes with it, read dark urine after training.

References

1. Effect of changes in dietary components on the serum bilirubin in Gilbert's syndrome. American Journal of Clinical Nutrition. 1976. PMID 937224.
2. Serum bile acids in Gilbert's syndrome before and after reduced caloric intake. Scandinavian Journal of Gastroenterology. 1982. PMID 7156881.
3. Telford RD, Sly GJ, Hahn AG, et al. Footstrike is the major cause of hemolysis during running. Journal of Applied Physiology. 2003. PMID 12391035.
4. RIVM. Blood testing and reference values. Accessed 2026.
5. NHG. Dutch College of General Practitioners, laboratory diagnostics in primary care. Accessed 2026.
6. Thuisarts. My blood result is abnormal. Accessed 2026.

Disclaimer

Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. For treatment decisions, discuss your results with your GP.