Impact of different triathlon races on the systemic cytokine profile and metabolic parameters in healthy individuals: a systematic review | BMC Sports sciences, medicine and rehabilitation

In the current systematic review, we aimed to evaluate the impact of different triathlon events on the profile of cytokines (pro- and anti-inflammatory) and metabolic markers in triathletes. First, we verified the increase in pro-inflammatory cytokines including IL-1, IL-2, IL-6, IL-8, IL-12p40, INF-γ, MCP-1, TNF-α in PBMC, serum and plasma levels after different triathlon competitions. Second, in most studies we observed an increase in the production of anti-inflammatory cytokines (IL-4 and IL-10) at serum and plasma levels after the sprint, long-distance and Ironman races. Third, among metabolic factors, we observed an increase in the concentration of blood and plasma markers of muscle damage (CK, LDH and Myostatin), muscle fatigue (FFA and lactate), physiological stress (Cortisol) and inflammatory phase. (CRP) in athletes from different triathlon events.

Cytokines are signaling proteins produced by immune and non-immune cells that have cell signaling functions, positive and/or negative regulation of various genes and their transcription factors, and even stimulate or restrain inflammation promoted by various stimuli, including bacteria and viruses. [32, 33]. Alves et al. 2022, through a systematic review with meta-analysis, demonstrated that exposure to high running volume (exercise time, duration and distance covered) is associated with a higher concentration of pro-inflammatory cytokines, including IL-1β, IL-8 and TNF -α. Furthermore, serum levels of IL-1ra and IL-10 increased as a result of long-term aerobic exercise [6]. However, the authors only considered long-distance modalities (half marathon, marathon and ultramarathon), with the exception of triathlon. Similarly, from the data in the current systematic table it was concluded that several triathlon races promoted an increase in PMBC, serum or plasma concentration of pro-inflammatory cytokines. [6, 9].

High concentrations of pro-inflammatory cytokines are observed at the end of triathlon races and can be explained by the volume of the race, including the intensity of the exercise. In contrast, they had no association with the triathlon distance. This result confirms studies on endurance athletes. Studies have observed leukocytosis and high serum levels of pro-inflammatory cytokines after marathon races [34,35,36]. The metabolic activity and damage observed in muscle cells as a result of long-distance races, such as triathlon, appear to serve as important catalysts for the migration of some leukocytes, along with the release of cytokines. In addition, there are neuroendocrinological and metabolic multifactorial mechanisms that involve extreme stimuli and underlying consequences. Strenuous physical exertion such as triathlon increases immunosuppression [9, 37, 38]. The possible relationship between exercise and UTRI can be explained and modeled by a “J” curve, which can occur during competitions as well as during training, usually caused by rhinovirus, adenovirus and parainfluenza virus [19, 39, 40]. Furthermore, this profile of disease involvement may affect health and performance-related physical fitness components such as maximum oxygen volume, respiratory coefficient, and lactate threshold. [35, 36].

The anti-inflammatory response was assessed by serum levels of IL10 and IL4. Studies have shown that strenuous exercise can increase IL-10 levels, allowing it to return to basal levels during the rest period [41]. Furthermore, Santos et al. (2019) have shown that the magnitude of plasma IL-10 increases is related to the duration of exercise [42]. Furthermore, there is evidence that increases in serum levels of IL-10 are correlated with low levels of chronic, low-grade inflammation and tissue health. [7]. Huang et al. An increase in plasma IL-4 was found in 2019. Nevertheless, Suzuki et al., 2006 did not see any difference between serum levels before and after IL-4. According to our findings, there is no significant improvement in IL-4 due to the different aerobic exercise protocols [43]. Furthermore, the low serum IL-4 levels observed at the end of triathlon races can be explained by the strong inhibitory effects of IL-10 and IL-6 observed after long-distance triathlon races. These jointly contribute to the prevention of excessive systemic inflammation [44].

Long-term training protocols such as triathlons are known to cause changes in other biomarkers (gene expression and protein levels) [40]. A significant expansion of EGF and VEGF levels in many hematopoietic, endothelial and smooth muscle cells of the vasculature towards epithelial cells was observed. [40]. Furthermore, evidence has shown that aerobic exercise should activate the production and release of EGF and VEGF due to physiological adaptation to exercise, such as angiogenesis, indicating that EGF and VEGF are important biomarkers of aerobic exercise. [45]. At the same time, the studies noted that plasma CK levels increased after the race. As observed in a randomized double-blind crossover study by Galan et al. In 2018, CK serum levels improved after treadmill running to exhaustion [46].

Furthermore, Danielsson et al., 2017 reported an increase in CK levels after an Ironman distance triathlon, which is associated with masculinity [8]. Subsequently, it was known to improve FFA and LDH levels in sprint, Ironman and long-distance triathlons. Finally, cortisol levels were increased during triathlon protocols. It is known that the physiological demands of long-distance running such as triathlon should cause an increase in FFA, LDH, cortisol and lactate levels due to adaptation to the extensive energy expenditure of long-distance training protocols. [47,48,49]. Finally, according to previous evidence, an increased Myostatin level was reported in the aftermath of the Sprint and Iron Man Triathlon. Ben-Zaken et al., 2017 found that Myostatin expression was associated with a favorable outcome in long-distance running performance [50].

Because chronic systemic inflammation can be considered a factor affecting the performance of triathlon athletes, recommendations for managing the pillars of improving physical capacity (nutrient availability, sleep behavior, strength training) are important to modulate the immune response. In addition, it reduces both physical and physiological problems and accelerates the recovery and rehabilitation process after injuries. In this regard, individuals who practice triathlon can benefit from the immunomodulatory effects of a strength training strategy in combination with training for the sport. [51, 52]. Furthermore, adequate nutrient availability is known to benefit immune function, including cell-mediated immunity and a balanced inflammatory response. Finally, studies have shown that good sleep behaviors could be a complementary approach to reducing chronic inflammation [53, 54].

Strengths and limitations

The current systematic review presents important limitations that should be taken into account when generalizing the findings. First, we considered different distances of the triathlon race, which means that the generalization of the findings must be specific. The limitations of this systematic review mainly related to the methods of the studies. For example, the lack of control over the covariates (such as age, nutritional status, sleep quality, etc.) may be an important source of bias among the included studies. Another important point is the characteristics of the recorded sample. Because we only described gender and distance of participation, additional information such as level of competition and training characteristics may be useful in future research.

Therefore, the heterogeneity in the quality of reference sources is the strength of this review, as it observed efforts of serum levels of the inflammatory cytokine, as well as biomarkers associated with performance in different triathlon races. On the other hand, it must be emphasized that the studies did not randomize their populations, a procedure recognized by PRISMA. Some studies have not examined all outcomes considered relevant in this scenario. However, we find our work equally relevant because it systematically summarizes the available evidence for future research.