*2.5. Other Promising Markers of AKI (YKL-40, MCP-1 and TNF-alfa, Trefoil Factor 3 (TTF3), Calbindin)*

There are over 100 biomarkers of AKI [61]. The urinary biomarkers which have been assessed in numerous studies are: chitinase 3-like protein 1 (YKL-40), MCP-1, TNF-alfa, osteopontin, DKK-1, micro RNAs, hemojuvelin, clusterin, CYR-61, cytochrome-C, epidermal growth factor, malondialdehyde, calprotectin, urine AGT angiotensinogen, matrix metalloproteinase 9, urine cysteine-rich 61, Na+/H<sup>+</sup> exchanger isoform 3 protein, netrin-1, fetuin-A and trefoil factor 3 (TFF3) [5,8,9]. Most of these markers were not studied after exercise.

In one of the most interesting and largest studies concerning AKI biomarkers after exercise, Mansour et al. found increases in several urinary markers: a 4.5-fold increase in TNFα, a 6.69-fold increase in MCP-1 and an 8.99-fold increase in YKL-40 after a marathon [46]. Sugama et al. found increased uMCP-1 after a duathlon, but this change was not significant when normalized to uCr [59]. Semen et al. found a significant uTNFα increase after a half marathon [50]. Calbindin and TTF3 were studied in one study, and both of these markers increased after HIIT training [48]. The TTF3 family is a group of small molecule polypeptides, and uTTF3 was significantly reduced following renal tubular damage [9]; therefore, the increase in uTTF3 after exercise was surprising.

## **3. Discussion**

Studies concerning new markers of AKI after physical exercise combine two different entities. Markers of AKI were introduced to diagnose the early phase of kidney injury in critically ill patients with sepsis or shock [5,6,8,9], but in these studies, they were measured in healthy subjects during physical activity, like walking, running or cycling. The increase in new AKI biomarkers was anticipated in these studies, because even much less sensitive markers, like serum creatinine and urinalysis, show changes after exercise [3,23].

Although many markers of AKI were described, only a minority were studied after exercise. Some markers are classified as injury markers (e.g., uKIM-1, uTNFalfa, uIL-6, uIL-8, uIL-18, uNGAL) and others as repair biomarkers (e.g., uYKL-40, uMCP-1) [46]. Another classification is based on the site of injury. There are markers of tubular (e.g., NGAL, IL-18, L-FABP, KIM-1, IGFBP-7) and glomerular (e.g., matrix metallopeptidase 9 [MMP-9]) injury [9]. The authors of this review used the classification proposed by Oh et al. [5] (Table 1), which has practical implications.

## *3.1. Limitations of the Studies Presented*

There are several limitations to the studies presented in this review. Twenty-seven studies were analyzed. All but one were published during the last 10 years and 15/27 in the last 5 years. The number of subjects studied ranged from 9 to 167, although most of the studies had a small number of participants: in 16 studies, fewer than 30 participants were investigated and only in 3 studies were there more than 100 participants studied. In 12 studies, only males were analyzed, and only in 7 studies were both sexes represented to the same or very similar degree. In three studies, all concerning marathons, more females were analyzed. The mean age of the participants differed greatly, ranging from 20 to 60 years. There were many different study designs proposed by the researchers, although the most common was a marathon (distance 42,195 m), which was used in 5 studies (Figure 1, Table 4).

**Figure 1.** Number of studies in different types of exercise. Abbreviations: u—urinary, s—serum, p—plasma, Cyst-C—cystatin C, NGAL—neutrophil gelatinase-associated lipocalin, KIM-1—kidney injury molecule-1, L-FABP—liver-type fatty-acid-binding protein, IL—interleukin, IGFBP-7—insulin-like growth factor binding protein 7, TIMP-2—tissue inhibitor of metalloproteinases-2.


**Table 4.** Studies on changes in new AKI markers after single exercise in healthy subjects—ordered according to the year of publication.


#### **Table 4.** *Cont.*

**Abbreviations:** u—urinary, s—serum, p—plasma, Cyst-C—cystatin C, NGAL—neutrophil gelatinase-associated lipocalin, KIM-1—kidney injury molecule-1, L-FABP—liver-type fatty-acid-binding protein, Il—interleukin, TTF3—trefoil factor-3, TNFα—tumor necrosis factor α, YKL-40—chitinase 3-like protein 1, MCP-1—monocyte chemoattractant protein-1, IGFBP-7—insulin-like growth factor binding protein 7, TIMP-2—tissue inhibitor of metalloproteinases-2, Cr—creatinine, Osm—osmolality, u.f.—urine flow, uMarker/uCyst-C—urinary marker normalized to cystatin C, uMarker/uCr—urinary marker normalized to creatinine, uMarker/uOsm—urinary marker normalized to osmolality, uMarker/u.f.—urinary marker normalized to urine flow, HIIRT—high-intensity interval resistance training, VO2max—maximal oxygen consumption, HRmax—maximal heart rate, RH—relative humidity, monomeric and oligomeric flavanols (MOF-VVPP).

The diagnosis of AKI was never confirmed by kidney biopsy, which is completely understandable. The studies were performed in relatively small groups and no cases of AKI requiring HD or rhabdomyolysis were observed, because both of these severe complications are extremely rare after exercise. There is insufficient data to describe changes in AKI biomarkers in such severe complications of exercise.

The follow-up was defined in a different way in the studies presented, and in some, there was no follow-up at all.

Suggestions: Studies in larger groups, preferably multicenter, are needed. It could be reasonable to study all markers after a marathon, which is the classical distance, relatively long and intensive. Indeed, it is the most commonly studied type of exercise. More studies in females are also required. Studies with a precisely defined follow-up with several time points, as well as observational case studies on changes in AKI biomarkers in subjects with severe complications could be very interesting.
