Acute metabolic responses to a 24-h ultra-marathon race in male amateur runners

The extreme caloric demand during ultra-endurance competition requires an adequate supply of metabolic fuels (Kreider 1991; Zaryski et al. 2005). Because carbohydrate stores in the body are limited, the long-duration muscle activity raises the need for carbohydrates not only as an energy source for muscles, but also for their role both in the rapid metabolizing of fats for energy and in providing glucose for normal functioning of the central nervous. Therefore, the adequate supply of carbohydrates is considered crucial for endurance athletes. Since the runners in the present study had free access to food during the entire event, no significant changes in blood glucose concentration were found, which implies adequate carbohydrate supply during the competition. On the other hand, it could be easily predicted that the ultra-endurance race will have a major impact on fat metabolism, and in particular will enhance fat oxidation (Helge et al. 2007; Jeukendrup et al. 1998). Since the athletes were not fed with large amount of lipids, our findings of a more than 50% decrease in serum triacylglycerols, associated with significant, almost threefold increases in circulating FFA, glycerol and β-hydroxybutyrate, reflect the use of fatty acids as fuels. These results support the view that marathon runners compared with normal individuals have a higher fat turnover rate at submaximal exercise intensities (Sjödin and Svedenhag 1985). It is worth to note that the decrease in TG was associated with significant decline in total and LDL cholesterol and a significant rise in HDL cholesterol. Moreover, highly significant negative correlations between the distance covered during the race and several lipid profile indices (serum TG, cholesterol total, LDL and HDL) strongly support the view of the beneficial effect of endurance effort on serum lipid profile. It should be stressed that not only concentrations of TG, total cholesterol and its fractions as the independent risk factors, but also the common cholesterol ratios (TC/HDL, LDL/HDL, and TG/HDL), which are considered better predictors of future coronary heart disease (Natarajan et al. 2003; Kinosian et al. 1994; Grover et al. 1999) were already relatively low at the start of the race, and reached the lowest levels at the finish. As expected, total and LDL-cholesterol were significantly positively related with age in the participating runners. Comparable changes in the lipid profile after prolonged exercise, suggesting reduction in cardiovascular disease (CVD) risk, were also observed by other authors (Gastmann et al. 1998; Ginsburg et al. 1996). We are aware that there are more accurate methods to estimate the risk of CVD and atherosclerosis, e.g. determining lipoprotein(a) level or particle size distribution in LDL (Superko 1996). However, a surrogate method for evaluation of the atherogenicity of plasma lipoproteins based on the assessment of the AIP calculated as log(TG/HDL-C) (with TG and HDL-C expressed in molar concentrations) may also be useful (Dobiášová and Frohlich 2001). The most recent findings from studies in a cohort of adult subjects on the negative correlation between AIP and particle sizes of HDL and LDL support the view that AIP is a reliable predictor of the cardiovascular risk (Dobiášová et al. 2011). It should be stressed, therefore, that negative AIP values, which have been found in a half of the runners at the start of the race and in all of them—at the finish, may indicate a lowered risk of CVD. Taking into account that changes in concentration of small dense LDLs are significantly correlated with changes in TG (Baumstark et al. 1993), it may be postulated that the reduction in pro-atherogenic small dense LDLs induced by prolonged endurance effort would be larger in subjects with larger reductions in serum TG.

Another important aspect of metabolic response to ultra-endurance efforts is the role of IL-6. The concentration of this cytokine correlated positively with serum FFA (R = 0.47, P < 0.005), glycerol (R = 0.42, P < 0.005) and HDL-C (R = 0.63, P < 0.005) levels, and negatively—with serum TG (R = −0.46, P < 0.05). These findings strongly support the view that IL-6 enhances lipid turnover by stimulating lipolysis and fat oxidation (Bruce and Dyck 2004; Petersen and Pedersen 2005), thus it may be considered as one possible mediator of the beneficial effects of physical activity on fatty acid metabolism. Data from the literature provide also a strong evidence that IL-6 is involved in mediating glucose homeostasis during exercise (Petersen and Pedersen 2005), notably that activation of the IL-6 gene during exercise may be sensitive to muscle glycogen content. Low muscle glycogen substantially enhances IL-6 gene transcription and IL-6 protein production in skeletal muscles, whereas carbohydrate ingestion during exercise attenuates increases in plasma IL-6 (Keller et al. 2001; Li and Gleeson 2005). With regard to the results of the present study, one should notice that free access to carbohydrate-rich food during the race did not preclude the possibility that muscle glycogen stores in our runners were depleted, which could activate IL-6 expression. Sustained supply of exogenous sugars allowed the competitors to complete the race with no impairment of glucose homeostasis, although well maintained glucose concentration could blunt the response of serum IL-6 (Li and Gleeson 2005).

It is well known that a continuous long-lasting run elicits a ventilatory response. The most distinctive feature of the response is a tachypnea associated with variable degrees of hypocapnia and respiratory alkalosis (Hanson et al. 1982). The hypocapnia develops when a strong respiratory stimulus causes the lungs to remove more carbon dioxide than is produced in metabolically active tissues. Indeed, in the present study a variable degree of hypocapnia and respiratory alkalosis did develop during the race. Alkalosis at the completion of the race was evident in nine subjects, showing above-normal pH values (7.46–7.51) and pCO₂ less than 35 mmHg. This was associated with a tendency towards a lowered free ionized calcium concentration. Hyperventilation-induced decreases in plasma concentrations of CO₂, [H⁺] and available calcium, acting as potent vasodilators during exercise, may limit O₂ delivery to locomotor skeletal muscle by causing vascular constriction and reducing blood flow (Chin et al. 2007). Of note, we have found in this study that serum IL-6 level correlated negatively with capillary blood pCO₂ (R = −0.51, P < 0.001), while it correlated positively with capillary blood pH (R = 0.53, P < 0.0005). These findings may imply that the respiratory-induced hypocapnic alkalosis might, at least to some extent, modulate in vivo production of this cytokine. To our knowledge, there is but a single study providing evidence that CO₂ concentrations modulated cytokine levels in endotoxin-stimulated human whole blood cell cultures (Kimura et al. 2008). Notably, those authors found that hypocapnic alkalosis stimulated IL-6 production, whereas the opposite effect, i.e. a reduced IL-6 concentration was observed after hypercapnic acidosis. Given that no detectable amounts of this cytokine were found by those authors in blood samples without endotoxin stimulation, one may presume that the elevated serum IL-6 evidenced during and after the 24 h race in all athletes in our study was, at least partly, due to exercise-induced muscle damage (Suzuki et al. 2006; Nieman 1997) and/or ensuing endotoxemia resulting from increased intestinal permeability (Jeukendrup et al. 2000). This presumption is strongly supported by positive correlations between serum IL-6 level and serum activities of CK (R = 0.65, P < 10⁻⁴), AST (R = 0.77, P < 10⁻⁴) and ALT (R = 0.79, P < 10⁻⁴) that are considered indirect markers of work-induced muscle damage. Abnormally high serum activities of these enzymes are indicative of their leakage from skeletal muscle or other tissues into the bloodstream due to mechanical damage or increased membrane permeability.

Recent research demonstrated that plasma IL-6 increases exponentially with exercise intensity and duration, and the mass of muscle recruited (Pedersen and Febbraio 2005, 2008). However, a fairly stable serum IL-6 concentration evidenced in the present study, as well as a significant correlation between IL-6 and distance covered (R = 0.68, P < 10⁻⁵), seem to support most recent finding of Wallberg et al. (2011) that IL-6 does not increase after 12 h of exercise, which suggests that the main determinant of the IL-6 response is the intensity of exercise.

The results of our study demonstrate that running a 24 h ultra-marathon resulted in highly significant increases in serum activities of CK, AST and ALT, emerging in the second half of the race (between 12 and 24 h). Substantial increases in serum activities of AST and ALT following prolonged exercise, and fairly stable serum GGT are indicative of significant skeletal muscle and minor hepatic damage (Noakes 1987;Whitfield 2001; Rosales et al. 2008). In our study, the highest increase was recorded (Table 5) for the mean serum activity of CK that is widely accepted as an indirect marker of muscle damage (Noakes et al. 1983; Kim et al. 2007, 2009; Miles et al. 2008; Brancaccio et al. 2010). However, there were remarkable inter-individual differences in the relative magnitude of the increase, amounting to 27-fold difference between the highest and the lowest response. The reason for this variability is not clear; however, one may suspect that the extreme increases in serum CK activity may be related to the development of exertional rhabdomyolysis (Skenderi et al. 2006).

The body’s response to tissue damage involves mobilization of immune cells and their migration towards sites of the injury (Tidball 2005; Kim et al. 2007; Bessa et al. 2008; Nieman 1997). In the present study, the immune response to muscle damage was characterized by a pronounced, almost twofold increase in total WBC count at the completion of the marathon distance, which persisted until the end of the race. The major components were neutrophils and monocytes, which reached counts almost three times those pre-race. It is known that increases in leukocyte counts during sustained moderate exercise, even in thermally comfortable environment (which was the case in the present study), are related mainly to elevations in plasma catecholamines, which induce a demargination of leukocytes (Brenner et al. 1998). It is well established that the release of leukocytes, and particularly neutrophils and monocytes, is stimulated by inflammatory mediators, such as IL-6 (Suzuki et al. 2003; Nieman 1997). This assumption is corroborated by our finding that IL-6 responses correlated significantly with ultra-endurance exercise-induced increases in total leukocyte (R = 0.56, P < 10⁻⁴), absolute neutrophil (R = 0.57, P < 10⁻⁴) and monocyte (R = 0.71, P < 10⁻⁷) counts, which seems to support the previous findings that IL-6 may mediate recruitment and activation of neutrophils and monocytes in response to exhaustive exercise (Suzuki et al. 2003). However, it is well documented that IL-6 is produced in skeletal muscles also in non-inflammatory conditions, and that muscle contraction per se is a major stimulus for de novo synthesis of IL-6 in myocytes (Hiscock et al. 2004; Pedersen and Febbraio 2008).

One important feature of IL-6 is that it induces the production of hepatocyte-derived CRP (Fischer 2006) that is involved in the induction of anti-inflammatory cytokines in circulating monocytes, in suppression of the synthesis of proinflammatory cytokines in tissue macrophages (Pue et al. 1996) and is responsible for the recognition and removal of damaged cells (Plaisance and Grandjean 2006). In our study, plasma hsCRP concentration during the first 42.195 km distance was fairly stable. Changes in plasma hsCRP content occurred later, i.e. after finishing the marathon distance, when it rose in an exponential pattern to reach, at the finish of a 24 h race, the level more than 20 times higher than the pre-race value. Significant correlation between plasma CRP and IL-6 found in our study (R = 0.51, P < 0.0005) supports the presumption of the role of IL-6 as a primary inducer of hepatic production of acute phase proteins (Fischer 2006).

In summary, the main metabolic responses to the 24-h ultra-marathon race were: (i) a marked shift in substrate utilization towards fat oxidation to maintain blood glucose homeostasis, and (ii) favorable changes in serum lipid profile. The ultra-endurance run led to a substantial skeletal muscle damage and an acute inflammatory response evidenced by a wide range of changes in muscle injury-related indices and dramatic elevations in serum interleukin-6 and hsCRP. These effects were more pronounced during the second half of the race, and the primary determinants of these changes were both the duration and intensity of the exercise. The predominant ventilatory response to the ultra-endurance effort of the 24 h race was a tachypneic, respiratory alkalosis associated with hypocapnia and hyperkalemia, but not hyponatremia. Some results of this study suggest that the elevation in serum IL-6, which likely is due to increased production, might be related to hypocapnia that develops during prolonged endurance exercise. However, the correlation between these phenomena may as well be an indirect, non-causal association related to the effects of prolonged muscle effort on both pCO₂ and IL-6.