As exercise results in proteinuria, the question arises if exercise may lead to acute or chronic kidney injury. Investigation of urine samples from marathon runners showed that 40 % had elevated markers for acute kidney injury after the race. Acute kidney injury was determined by serum creatinine and urinary markers such as cystatin C, neutrophil gelatinase-associated lipocalin, and kidney injury molecule 1 supported the diagnosis. Cardiovascular magnetic resonance imaging results led to the suggestion that athletes were not volume-depleted, which could have triggered false conclusions. Eventually, all markers returned to normal baseline values after 24 h; however, repetitive long-distance running exercise has to be investigated concerning potential long-term alterations of kidney functions [19]. Urinary markers for renal damage were analyzed from patients with chronic kidney disease after a 20-min treadmill walk at 40–60 % exercise intensity. L-type fatty acid-binding protein (a new marker of tubular function in chronic kidney disease and acute kidney injury) as well as the common markers urinary albumin, N-acetyl-beta-D-glucosaminidase, and α1-microglobulin was not significantly influenced after exercise [11]. Junglee et al. [15] recently showed that muscle-damaging exercise prior to exercise in the heat can be seen as a risk factor for acute kidney disease.

Exercise of 15-min duration to maximal heart rate or exhaustion was shown to result in albuminuria. Within 24 h, measures returned to baseline in all subjects. It was therefore concluded that urinary parameters connected to disease or protein excretion should not be measured within 24 h after exercise [10].

In contrast to other body fluids, fewer studies were performed on the urinary proteome. Besides the fact that plasma or serum is the most common matrix in clinical studies, urine has a high salt concentration and is relatively dilute regarding proteins (~150 mg of protein/day) in general but still retains the issue of highly abundant albumin. On the other hand, urine can be sampled noninvasively and is available in large volumes of 1–2 l/day. Gel-based and gel-free proteomics was used to map and categorize urinary proteins [2, 25, 34]. Adachi et al. [2] reported on an unexpectedly high percentage of membrane proteins. Compared to the entity of Gene Ontology entries, extracellular lysosomal and plasma membrane proteins were enriched and the latter were proposed to originate from renally eliminated exosomes. Such exosomes were subsequently investigated as potential source of new biomarkers [12].

Studies of untargeted analyses of urinary proteins or a certain fraction are rare. As summarized in the previous sections, exercise may change the urinary proteome differently depending on exercise intensity but also on temperature, hydration status as well as physiological condition. Besides parameters that result in variation of the proteome, Gür et al. described that the amount of protein excreted does not depend on age, duration of running or training, or athletic background of athletes as shown by examinations of participants of a half marathon [7]. The same exercise intensity may lead to different changes such as muscle damage or hematuria in some but not all athletes leading, e.g., to the excretion of myoglobin or hemoglobin and their fragments, which are usually not detected in urine. Therefore, even when measuring protein amounts and analyzing same amounts of proteins, substantial inter- and intra individual qualitative differences can occur. In addition to differences in renal filtration and reabsorption, proteins may not only originate from the kidney filtrate but could be post-renal, e.g., from parts of ureter, bladder, or urethra including proteins from the inner membranes as well as bacterial contamination through infection or other external contamination.

A pilot study using two-dimensional gel electrophoresis for comparison of the urinary proteome of elite athletes performing different types of exercise (endurance exercise, strength sport, and team sport) showed considerable differences within as well as between the groups [17]. This study was performed in a doping control context addressing the question how much the urinary protein varies and if that may influence the detection of, especially peptide and protein based, prohibited substances such as erythropoietin, insulin, or chorionic gonadotrophin. The idea was that in addition to the existing blood passport of athletes, a urinary passport or protein map may allow indication of, e.g., gene doping. In addition, from a sport physiological point of view, the urinary proteome could provide indications as to the nutritional and training status of an athlete. That way, muscle damage may be identified by myoglobin and its fragments in urine or erythrocytosis by the detection of hemoglobin or other erythrocyte-derived proteins. These parameters may be used for training control and modulation.

Protein maps of strength sport athletes, endurance sport athletes, and team sport athletes (10 each) showed that respective 2D patterns were too different within as well as between groups to allow a software-based comparison. Therefore, visual inspection was performed for evaluation of relevant differences. Proteinuria (>15 mg protein/mmol creatinine) was found in 2/10 strength sport athletes, 5/10 team sport samples and 10/10 endurance sport samples and 0/10 samples from the control group. Endurance and team sport samples showed comparable protein patterns with protein spots considered as ‘elevated’ that contained transferrin, albumin, prostaglandin-H2 D-isomerase, immunoglobulin kappa chain and alpha-2-glycoprotein 1, gelsolin isoform b fragment, CD 201 antigen, kininogen 1, and clusterin isoform 1. In comparison, strength sport samples showed a higher amount of low molecular weight proteins (elevated spots contained transthyretin, CD 59 antigen, GM 2 ganglioside activator, and apolipoprotein A) including also fragments from high molecular weight proteins (albumin, transferrin, hemopexin, or IgG fragments) [17]. In this case, lactate cannot be the reason for increased protein excretion as the exercise time is too short for a sufficient production of lactate. Adrenergic activity is also discussed to be connected to proteinuria and results in higher blood pressure. As a matter of fact, blood pressure is extremely increased during weight lifting to values up to 370/360 mm Hg [22] and may be the reason for proteinuria in this case.

Within the same study, stability of urine samples was tested and it was found that the protein pattern did not change within four weeks of storage at 4 °C.

In a consecutive project, marathon runners who participated in the same marathon competition were investigated. As control groups, competitive athletes at rest with a mean endurance sport measure of 13–20 h/week (triathlon, biking, and running) were used. In addition, a control group of healthy volunteers performing occasional exercise (5 h/week) was acquired. No differences were found between the two control groups. Nine out of ten marathon runners had protein/creatinine ratios of >15 mg/mmol (15–73 mg/mmol). A relative decrease in acidic proteins was observed after exercise, which may be interesting for variations in EPO levels in doping control. Manual evaluation of gels showed six spots to be clearly elevated in marathon runners compared to healthy volunteers. Orosomucoid may be elevated because of increased glomerular filtration. If it can be shown that there is an increase in plasma as well, it could indicate an unspecific immune response due to muscle lesions. Another spot observed in marathon runners but not in controls contained hemopexin and may indicate hemolysis. The same phenomenon applies to a spot containing carbonic anhydrase I, which is responsible for the rehydration of carbon dioxide to bicarbonate within erythrocytes. Zinc α-2-glycoprotein 1, which is assumed to have a stimulatory effect on lipolysis, may be increased in plasma as well because of an increased energy consumption and demand. The increase of transferrin, an iron transport protein, may be due to glomerular filtration changes or to increased transferrin synthesis as athletes often have reduced iron levels [18]. For confirmation of the reason for higher concentration of these proteins in urine, plasma samples should be collected in addition in future studies.

In addition, it was investigated how the protein patterns change from rest to post-exercise and back in one volunteer in a yet unpublished study. The protein patterns changed after exercise and, within a few hours, returned back to the pattern observed prior to the intervention. Besides, it was found that high altitude (3,500 m, isobaric, 1 h) does not have an influence on the protein pattern or protein amount. That is in contrast to residence in high altitude, where increased protein excretion is reported [32]. Exercise in altitude resulted in qualitatively similar protein patterns. Nevertheless, oxygen saturation was lower at high altitude, and power on a bicycle ergometer was lower.