Bilirubin: what the blood test shows and why it matters for athletes

Bilirubin appears on almost every liver function test, and a slightly high result is one of the most common reasons an otherwise healthy athlete gets called back by the surgery. Our editorial team explains what the marker measures, how training and genetics affect it, and which findings deserve attention.

Where bilirubin comes from

Bilirubin is a yellow pigment produced when the body recycles haemoglobin from old red blood cells. Red cells live for roughly 120 days; when they are retired, mainly in the spleen, the haem part of haemoglobin is broken down first to biliverdin and then to bilirubin. This form, known as unconjugated or indirect bilirubin, does not dissolve well in water and travels in the blood bound to albumin.

The liver takes up this unconjugated bilirubin and attaches glucuronic acid to it using an enzyme called UGT1A1. The resulting conjugated, or direct, bilirubin is water-soluble and is excreted into bile, then into the gut. Bacteria there convert it into compounds that give stools their brown colour and, after partial reabsorption, give urine its pale yellow tint.

Because bilirubin passes through several stages, a raised level can point to a problem at any of them: too many red cells being destroyed, a liver that is slow to process the pigment, or a blockage that stops bile from draining. That is why laboratories often report total bilirubin and, when needed, split it into direct and indirect fractions.

In the UK, bilirubin is typically reported in micromoles per litre (µmol/L) as part of a standard liver function test panel, alongside ALT, ALP, albumin and sometimes GGT. Reference ranges differ slightly between NHS trusts, but an upper limit in the region of 20 µmol/L is common for adults.

What the test can reveal

A total bilirubin result within range is reassuring but not the whole story; the pattern with other liver markers matters more than one number. A doctor will look at whether bilirubin is raised alone or together with enzymes that indicate liver cell damage or bile duct problems.

The British Society of Gastroenterology guidance on abnormal liver blood tests, published by Newsome and colleagues, emphasises that isolated mild rises in bilirubin are common and usually benign, while combinations with other abnormal markers deserve a structured work-up.

Visible jaundice, a yellow tinge to the whites of the eyes or the skin, usually appears only when bilirubin climbs well above the reference range. By the time someone looks yellow, the cause should always be investigated promptly.

Gilbert's syndrome: the common harmless finding

The most frequent reason for a mildly raised bilirubin in a fit young adult is Gilbert's syndrome. It is an inherited variation in the UGT1A1 gene, described in detail by Bosma and colleagues, that reduces the liver's ability to conjugate bilirubin. Estimates suggest it affects several per cent of the population.

People with Gilbert's syndrome have an intermittently raised indirect bilirubin, while their liver enzymes, albumin and blood count are normal. Levels tend to rise with fasting, dehydration, intercurrent illness, lack of sleep and, notably for athletes, heavy exertion.

The condition is not a disease in the usual sense and needs no treatment. Some researchers, including Vítek, have even explored whether mildly higher bilirubin, an antioxidant, is associated with lower cardiovascular risk, although that question remains under study.

The practical relevance for athletes is avoiding unnecessary worry and repeated tests. Once a GP has confirmed the pattern, it is worth keeping a copy of the result so that future readings, for example at a sports medical, can be interpreted in context.

Why training can change your result

Exercise affects liver blood tests more than many people realise. Pettersson and colleagues showed that a single bout of heavy weight training in healthy men could raise markers such as AST, ALT and CK for days, largely due to muscle rather than liver damage. A doctor unaware of recent training could misread the results.

Bilirubin itself can rise after prolonged endurance events. Foot-strike haemolysis in runners, mechanical destruction of red cells in the soles of the feet, releases haemoglobin and increases the bilirubin load. Dehydration and a low calorie intake on long training days add to the effect.

For this reason, sports doctors often recommend a short break from very hard sessions before routine blood tests. The exact length depends on the individual, but avoiding a marathon, heavy leg session or competition in the preceding couple of days is sensible.

• Tell the clinician about recent hard training, races or weight cuts.
• Drink normally and avoid a prolonged fast unless the test requires it.
• Mention all supplements and medicines, including herbal products.
• Note any recent illness, as viral infections can affect liver tests.

Supplements, drugs and red flags

Several substances used in gyms can affect bilirubin. Oral anabolic steroids, particularly 17-alpha-alkylated compounds, are known to cause cholestasis, which raises conjugated bilirubin and can lead to jaundice. Some bodybuilding supplements, including products contaminated with undeclared steroids and certain herbal extracts, have been linked to drug-induced liver injury.

The ACG clinical guideline by Kwo and colleagues lists anabolic steroids and herbal and dietary supplements among recognised causes of abnormal liver chemistries. Taking a careful history of everything a person consumes is therefore an essential part of the assessment.

Red flags that require prompt medical attention include yellow skin or eyes, dark urine, pale stools, itching all over the body, abdominal pain under the right ribs, unexplained weight loss and fever. Do not wait for a routine appointment if these appear.

Prescription medicines and anabolic agents should never be stopped, started or adjusted without medical supervision. If you have used performance-enhancing drugs, telling your doctor honestly is the safest course; NHS clinicians are there to treat you, not to judge you.

Editorial conclusions

Bilirubin is a by-product of red cell recycling that the liver processes and clears in bile. A single number means little without the rest of the liver panel and a full blood count.

In fit young people, a mild isolated rise is most often due to Gilbert's syndrome, harmless and worth documenting once it is confirmed.

Hard training, dehydration and fasting can push results up, so plan routine tests around rest days. Rises together with other abnormal markers, or any visible jaundice, need proper investigation.

You may also find our articles on ALT and AST in athletes, the full blood count for active people and liver safety with supplements useful.

References

1. Fevery J. Bilirubin in clinical practice: a review. Liver Int. 2008;28(5):592–605.
2. Bosma PJ, Chowdhury JR, Bakker C, et al. The genetic basis of the reduced expression of bilirubin UDP-glucuronosyltransferase 1 in Gilbert's syndrome. N Engl J Med. 1995;333(18):1171–1175.
3. Newsome PN, Cramb R, Davison SM, et al. Guidelines on the management of abnormal liver blood tests. Gut. 2018;67(1):6–19.
4. Kwo PY, Cohen SM, Lim JK. ACG clinical guideline: evaluation of abnormal liver chemistries. Am J Gastroenterol. 2017;112(1):18–35.
5. Pettersson J, Hindorf U, Persson P, et al. Muscular exercise can cause highly pathological liver function tests in healthy men. Br J Clin Pharmacol. 2008;65(2):253–259.
6. Vítek L. The role of bilirubin in diabetes, metabolic syndrome, and cardiovascular diseases. Front Pharmacol. 2012;3:55.