Finding red blood in the urine or foamy urine can be alarming for some. The presence of blood and protein in urine is considered a warning sign by many—and rightly so, as serious conditions can indeed be the cause. However, if the symptoms occur reproducibly after physical exertion and disappear within a few days, there is often no cause for concern. In these cases, it is usually exercise-induced hematuria or exercise-induced proteinuria, both of which are classified as largely harmless phenomena.
What happens during exercise-induced hematuria?
In exercise-induced hematuria, blood can be detected in the urine after prolonged physical exertion such as jogging, cycling, or swimming—or after contact sports. If the condition is pronounced enough, the red discoloration of the urine can be seen with the naked eye. A mild form, on the other hand, is only detectable through a positive test strip result for hemoglobin. Studies on endurance athletes suggest that after a marathon, at least microscopic blood can be found in the urine of one in five runners.
The mechanisms underlying this phenomenon can be divided into traumatic and atraumatic forms, but are not yet fully understood in every detail. In traumatic mechanisms, it is assumed that minor injuries to the kidney or urinary tract occur during physical contact in contact sports, causing microbleeding into the urine. Even with an empty bladder, repeated friction between the posterior bladder wall and the bladder floor can occur during running—known as “foot-strike hematuria.” Whether small traumatic events caused by repetitive running motion or the bicycle saddle also play a role in endurance sports such as running or cycling remains a matter of debate.
More commonly, an atraumatic mechanism is likely at play. During exertion, blood flow to the kidney is reduced in favor of working muscles; the resulting reduced perfusion and increased permeability of the glomerular membrane allow small amounts of red blood cells to pass into the urine. An additional peculiarity concerns the test strip itself: the muscle protein myoglobin is chemically very similar to hemoglobin found in blood, reacts identically on the test strip, and is also reddish. Since small muscle traumas regularly occur during exercise, releasing myoglobin, a positive “blood” field on the test strip can also be triggered by myoglobin—without red blood cells actually being present in the urine. True hematuria can therefore only be confirmed microscopically by detecting red blood cells in the urine sediment.
What is behind exercise-induced proteinuria?
In exercise-induced proteinuria, proteins can be detected in the urine after exercise. The cause is a temporarily reduced filtration function of the kidney for proteins: the protein filter becomes more permeable under physical exertion. The mechanisms of this change are multifaceted. Contributing factors include the developing metabolic acidosis, sympathetic-associated adrenaline release, and changes in pulse and blood pressure that affect hemodynamics and thus the pressure conditions at the protein filter. After a marathon, temporary proteinuria can be detected in up to 30% of participants, which disappears within one to two days.
When medical evaluation is advisable
Harmless does not mean ignore. Exercise-induced hematuria or proteinuria should be investigated more closely if blood or protein is detectable in the urine for longer than 72 hours after the end of exertion, if hematuria occurs without relation to exercise, or if additional symptoms such as fever, flank pain, burning urination, or significant weight loss occur. Age over approximately 45 years, smoking, occupational exposure to aromatic amines, and anticoagulant medication also shift the assessment toward the need for evaluation.
A distinct warning sign is dark brown to cola-colored urine after very intense exertion, combined with muscle pain and weakness. This may indicate rhabdomyolysis—muscle breakdown in which large amounts of myoglobin are released. In this case, prompt medical consultation is indicated.
Sources
AWMF – S1 Guideline “Non-visible Hematuria” (Registration number 053-028): register.awmf.org/de/leitlinien/detail/053-028
AWMF / DGSP – S2k Guideline “Sports Medicine Preventive Examination” (Registration number 066-002): register.awmf.org/de/leitlinien/detail/066-002
German Society of Urology (DGU) – S1 Guideline Non-visible Hematuria: urologenportal.de
Bennett CM, Fagan E, Chaharbakhshi E et al. – “Haematuria in Sport: A Review,” Sports Medicine & Health Science 2018: pubmed.ncbi.nlm.nih.gov/29500137
Lippi G, Sanchis-Gomar F – “Exertional hematuria: definition, epidemiology, diagnostic and clinical considerations,” Clinical Chemistry and Laboratory Medicine 2019: pubmed.ncbi.nlm.nih.gov/31188754
European Association of Urology (EAU) – Guidelines Overview: uroweb.org/guidelines
The content of this article is for general information purposes only and has been prepared with the utmost care based on recognized medical sources. It does not constitute a healing claim, diagnosis, or therapy recommendation and in no case replaces a personal consultation with a qualified physician. In case of doubt or persistent symptoms, please seek medical advice immediately. Medipee assumes no liability for decisions you make based on the information provided here.
