In this study, for the first time, the effect of two types of dietary regimen during exercise on AKI outcomes was investigated. Following eight weeks of moderate-intensity endurance exercise along the use of different diets, the following results were obtained:
1) Following AKI, the serum urea and creatinine, and urinary albumin levels increased in the exercised and non-exercised groups, and this increase was less in the exercised rats than in non-exercised ones. Serum urea levels were lower in the exercised group with TR than the exercised group with CR and just exercised group, while urinary albumin was lower in exercised groups with CR and TR diets than just exercised groups. 2) GFR levels decreased after AKI in exercised and non-exercised groups, and this decrease was less in the exercised rats than non-exercised ones. It was also less in the exercised rats on TR diet than in exercised rats on CR diet. 3) The relative weight of the kidney increased after injury in the exercised and non-exercised groups, and this increase was less in the exercised rats than non-exercised ones. 4) The MDA levels increased and TAC levels decreased following injury in the exercised and non-exercised groups. These changes were less in the exercised groups than the non-exercised groups. They were also less in exercised rats with prescribed regimens than rats that only performed exercise. Also, the level of TAC reduction was lower in the exercised group with TR than in exercised group with CR. 5) Increase in inflammatory factor (TGF-β1) levels following injury in exercised and non-exercised groups showed that the increase was less in the exercised group than the non-exercised group and also, it was less in the exercised group with CR compared to just exercise group and exercised group with TR. 6) Decrease in SIRT1 levels following AKI in exercised and non-exercised groups revealed that the decrease in exercised group was less than the non-exercised group. This decrease was also less in the exercised group with TR after injury than in just exercised group.
Rhabdomyolysis, the leading cause of AKI, is common in athletes and can occur 1 to 10 hours after exercise and resolve after 2 to 10 days (38). Therefore, the effectiveness of exercise as a treatment depends on its duration, intensity and type.
In this study, the effect of TR and CR diets during exercise was investigated. The beneficial effects of these diets depend on many factors including age, physical activity and disease status (39). The TR diet is a form of Islamic fasting (40). A CR diet however, in which daily calorie intake is limited, can also be an intervention for athletes who want to control their body weight and increase their physical function and energy (26). CR in athletes reduces the risk of metabolic disease and mortality (39).
Increased serum urea and creatinine levels following AKI in the present study have also been shown in other AKI models, such as the use of cisplatin (41). Lower increase in the serum urea and creatinine levels in the exercised group compared to non-exercised group has been confirmed in the study of Weslei et al (2019) (42). De lima et al (2019) showed that regular exercise with moderate intensity for four weeks before AKI causes a decrease in serum urea and creatinine levels, and tubular injury (42). In this study, it was shown that TR diet combined with exercise resulted in a lower increase in serum urea and creatinine levels following AKI. Prevention of excessive increase in serum urea and creatinine levels following AKI by the use of TR diet has also been reported (25), which its mechanism of action is still unknown. Therefore, exercise prevents excessive increase in serum urea and creatinine levels after AKI, and when combined with TR diet, its effect is greater. A study showed that TR diet in athletes did not change the renal function indexes including serum urea and creatinine levels (43). The difference in the results could be due to differences in the studied species, and intensity and duration of exercise.
The increase in urinary albumin levels after AKI in the present study has also been confirmed in the study of Palm et al (2004) (44). One possible cause of the increase in urinary albumin is increased kidney injury and tubular damage. Consistent with our result that revealed less increase in urinary albumin after AKI in the group with previous exercise, it has been shown that regular exercise for 10 weeks reduces urinary albumin levels in diabetic rats (45). Also, endurance exercise with moderate intensity for four weeks before induction of diabetes decreases urinary albumin levels after injury (46). There are also contradictory results regarding the effects of exercise on proteinuria, and evidence suggests that the intensity of exercise is very important in this area. Some studies have shown that high-intensity exercise increases albuminuria in laboratory animals and humans, while moderate-intensity and regular exercise prevents albuminuria / proteinuria in STZ-induced diabetic rats (47–49). Although exercise prevents increase in urinary albumin levels following AKI, when combined with TR and CR diets, its effect is greater. According to the searching that we carried out, no study has been conducted to investigate the effects of these two diets on the reduction of proteinuria to this date.
In the present study, similar to another study (50), GFR decreased following AKI, and this decrease was less in exercised group compared to non-exercised group. It has been shown that performing eight weeks of moderate-intensity endurance exercise before induction of diabetes improves GFR after injury and prevents a large decrease in GFR (54). Toyama et al. (2010) reported that exercise for 12 weeks improves GFR in patients with CKD (55). Although the mechanism by which moderate-intensity exercise improves renal function is not well understood, there is evidence to suggest that better metabolic control, reduced oxidative stress, and increased nitric oxide (NO) production may play a role in this protective process (51, 52). Decreased GFR before injury was observed in the exercised rats on CR diet in the present study. In one study, the CR diet alone reduced GFR by reducing tubular hypertrophy (56). It is possible that the reduction of tubular hypertrophy in the CR diet group caused the decrease in GFR was not compensated in the CR diet group with exercise. The decrease in GFR in the TR diet group during exercise was less than the CR diet group during exercise, which is probably due to compensatory tubular hypertrophy both before and after injury.
Amaral et al., similar to the present study, reported that the relative weight of kidney increased after kidney injury, and eight weeks of moderate-intensity endurance exercise before induction of diabetes prevented the increase in relative weight of the kidney after injury (18). The reason for the increase in relative kidney weight in the TR diet group during exercise, before and after AKI in this study, may be due to the increase in compensatory hypertrophy, which is in contrast with the CR diet group during exercise. In a study, CR diet prevented a relative increase in heart weight associated with aging (51). Therefore, CR with exercise has a more effect on preventing the increase in compensatory renal hypertrophy after AKI.
Therefore, it can be concluded that in the present study, exercise probably decreased renal injury by reducing oxidative stress (49), which is the cause of tubular damage, and by increasing renal NO (48) and proliferation of kidney tissue cells (52) prevented GFR reduction. Also, compensatory hypertrophy in TR group led to less reduction in GFR, increased relative kidney weight, and improved renal function, which could be observed by smaller increase in serum urea and creatinine, and urinary albumin levels. In the TR diet group with exercise, these effects were reinforced and played a more effective role in improving kidney function.
The increase in lipid peroxidation and the decrease in antioxidant defense after AKI in this study are also confirmed by Ibrahim et al (2008), (53). Evidence suggests that exercise before AKI prevents the subsequent increase in oxidative stress (58, 60). Exercise is said to strike a balance between the oxidant and antioxidant systems (54). The findings of Húngaro et al (2020) study are contradictory to the results of our study, as they reported that moderate-intensity endurance exercise for four weeks before AKI could not prevent oxidative stress (55). The reason for the difference between this study and our study could be the difference in the duration of exercise and the type of animal used in these studies. Walsh et al (2014), similar to the results of present study, reported that dietary restriction prevents the progression of oxidative stress after AKI (56). The protective mechanism of this regimen is still unknown, but it has been shown that this protective effect is probably exerted by an increase in antioxidant factors (64) that prevent DNA oxidative damage induced by kidney injury (57). One study even suggested that a reduction in oxidative stress by CR diet may be due to weight loss. Another study found that exercise did not prevent oxidative stress in brain tissue, but the CR diet prevented the production of ROS products and established a balance between oxidant and antioxidant systems (58). The difference could be due to differences in the type, intensity and duration of exercise, as well as the studied tissue and the study conditions. Wycherley et al (2008) showed that CR with exercise reduced serum MDA, possibly due to weight loss. Reduction of oxidative stress may also be due to reduced insulin resistance and blood pressure following CR diet (59).
Although in this study weight gain was observed during exercise with TR diet, kidney weight loss could have been the cause of reduced oxidative stress in the CR diet. TR has been shown to prevent mitochondrial fragmentation associated with severe mitochondrial dysfunction, overproduction of free radicals, and worsening of AKI. At present, the molecular mechanisms of this effect are unclear, but it is possible that the nuclear factor-erythroid factor 2-related factor 2 (NRF2) and peroxisome proliferator-activated receptor-γ coactivator 1-α (PGC1α), are both oxidation-sensitive transcriptional regulators, which are activated by nutrient restriction, affect mitochondrial homeostasis and play a role in the beneficial effects of TR (60–62). It seems that the reduction of oxidative stress both before and after AKI in the TR group during exercise may be due to the effect of TR on preventing mitochondrial damage following regulating oxidation-sensitive transcriptional factors, which requires further research.
In the present study, similar to another study, AKI increased the level of TGF-β1 in kidney tissue (63). It has been shown that regular moderate-intensity exercise before induction of diabetes partially reduces the progression of renal fibrosis by significantly reducing advanced glycation end products (AGE), which consequently reduces the production of TGF-β1 in mesangial, fibroblasts and tubular cells (64). The results of Húngaro et al (2020) study contradict the findings of present study, as they reported that moderate-intensity endurance exercise for four weeks before AKI could not prevent inflammation (55). A reason for the increased expression of TGF-β1 is the increased expression of fibronectin and type four collagen following kidney damage, which in turn causes the accumulation of extracellular matrix and the progression of fibrosis. Exercise before induction of injury, reduces the expression of fibronectin and type four collagen in kidney tissue and prevents the progression of fibrosis by reducing TGF-β1 production after AKI (65–67). These effects of exercise in the present study might have caused the level of TGF-β1 after AKI in the exercised group to be less than in the non-exercised group.
In other studies, similar to the present study, it has been reported that dietary restriction prevents the progression of renal fibrosis by reducing TGF-β production after AKI (25, 68). Liu et al (2020) found that CR diet reduces TGF-β1 and ultimately fibrosis in aging-related kidney disorders (69). The CR-induced protection against fibrosis could be due to reduced oxidative stress, which reduces mitogen-activated protein kinase (MAPK) activity, activator protein-1 (AP-1) regulation, and TGFβ1 expression and signaling (70). Given the effect of CR diet during exercise on oxidative stress in the present study, it is likely that CR during exercise reduces TGFβ1 by reducing oxidative stress.
A decrease in SIRT1 levels in kidney tissue following AKI was observed in this study similar to the study of Zhong et al (2018), (27). Decreased SIRT1 expression is associated with increased nuclear factor kappa B (NF-κB) acetylation. Exercise suppresses NF- κB activity and inflammation by increasing SIRT1 expression in the kidney. In addition, exercise induces mitochondrial complex expression and the release of antioxidant enzymes by improving the activity of SIRT1 enzyme in the kidney (71). Prevention of SIRT1 reduction after AKI by previous exercise may have contributed to the reduction of oxidative stress and TGFβ1 after AKI in the exercised group. Marton and his colleagues showed that in metabolic disorders caused by aging, exercise could not prevent the reduction of SIRT1 in the cerebellum (72), which is contrary to our results. This difference could be due to the age of the animal, duration of exercise and the type of tissue studied. In the present study, an increase in SIRT1 after AKI was observed in the TR group during exercise. In another study, the TR diet restored the circadian rhythm of SIRT1 expression in the liver following metabolic disorders (73).
Therefore, in AKI, due to kidney injury, there is an increase in oxidative stress and inflammation, and also a decrease in renal function indexes and SIRT1 levels, which are less in the presence of exercise. However, with the application of CR and TR diets, especially TR, these changes are reduced in the presence of exercise. Applying the TR regimen before injury prevents structural damage to the kidney and renal function reduction in kidney injury (74). These effects are associated with the response to prevent proliferation of epithelial cells of damaged tubules and suppression of extracellular signal-regulated kinases1 / 2 (ERK1/2) activation in ischemic kidney. The TR diet reduces damage to epithelial cell of proximal tubule, death of tubular cell and activation of ERK1/2 in response to stress (75). The CR improves AKI by increasing autophagy and dealing with reduced renal expression of endothelial nitric oxide synthase (eNOS), and PGC-1a caused by kidney damage. It also reduces acute tubular necrosis, and prevents reduction of renal function during injury (76). But more research is needed in the future to discover the molecular mechanisms of this effects.