Effect of Thymus Extract Supplementation on Endurance Exercise Performance, Liver and Oxidative Enzymes in Rats

The levels of Weight, lipid profile of groups and the levels of serum liver enzymes are shown in Table 1. It shows that there was no significant difference in liver enzymes between the two groups. This result is also seen in the weight (P < 0.05). It was also observed that thyme supplementation reduce some lipid indices compared to the control group.
The activity of citrate synthase and cytochrome c oxidase as well as endurance running performance was measured in rats supplemented or non-supplemented with thyme extract. As shown in Fig. 1a, the activity of complex IV significantly diminished (around 30 %) in soleus muscle of rats treated with thymus extract relative to the non-supplemented group (t10.012= 4.62, P=0.001). Approximately, the same pattern of significant variation (24% decrease) was found when we compared the activity of citrate synthase between two groups, Fig. 1b (t8.11= 4.23, P=0.003).
In addition, the exhaustive running time of the rats in supplemented group was significantly prolonged (over three times longer) as compared to the non-supplemented group, Fig. 2 (t18= 4.05, P=0.001).
The present study has revealed that thyme supplementation significantly reduces the activity of citrate synthase (35%) and cytochrome c oxidase (24%), which are important for improving muscular endurance [27], compared to the non-supplemented group (respectively, t14=0.0004 and t14=0.001). These findings suggest that thyme supplementation may restrict the inducing effect of exercise training on citrate synthase and cytochrome c oxidase activity and mitochondrial biogenesis (Figs. 1, 2). This finding was consistent with earlier ones where different kinds of antioxidant supplements have been utilized, in which PGC-1α, NRF-1, Tfam and cytochrome c have been revealed to be down-regulated [28], [29].
Regarding down-regulation of mitochondrial enzyme activities in the thyme extract supplemented group, as mentioned in some of the previous studies, it is possible that chronic intake of such a powerful antioxidant like thymus extract may hamper, and even inhibit the improving effect of exercise on physiological adaptations like mitochondrial biogenesis due to reduced reactive oxygen species (ROS). Based on the evidence currently available, it seems that appropriate ROS levels act as a biological messenger, initiating cellular signaling cascades via the process of electron transfer to promote adaptive responses within the body [30]. In fact, levels of exercise producing oxidative stress can result in the activities of enzymes and the activation of MAP kinases (p38 and ERK1/2), NF-κB, protein kinase B (Akt), mammalian target of rapamycin (mTOR), p70 ribosomal S6 kinase (p70s6K) pathway [31]. It is possible that the dosage of thyme extract used in the present study was high and alleviated the favorable effects of moderate exercise-induced ROS in producing beneficial cellular adaptation.
In contrast to the activity of mitochondrial oxidative enzymes, our findings indicate that running performance of thyme extract supplemented group was significantly better than non-supplemented group, more than five times longer (t18= 4.05, P=0.001). This challenging result suggests that thyme supplementation could elevate the exercise tolerance. Although there are studies examining the effects of some compounds with polyphenolic structure on the endurance performance, no such study has been conducted with any total plant extract [23], [32], [33]. The effect of thyme extract on endurance performance was investigated for the first time here and positive results were obtained. Such an improvement in performance seems to be related to cardiovascular, chemo-preventive, immunological, or other adaptations [31] rather than mitochondrial biogenesis (due to lower mitochondrial enzymes activities in supplemented group). In this regard, some researchers have also shown that PGC-1α KO mice have regular voluntary running activity in spite of diminished basal mitochondrial respiratory activity [34] indicating that biogenesis may be helpful but not necessary in promoting endurance running performance. These contrasting findings could be related to compensatory accommodations in various tissues because of reduced mitochondrial enzyme activities.
Furthermore, thyme supplementation has led to decreased hematocrit and mean corpuscular volume, both important in determining blood viscosity (not reported). It is well accepted that low blood viscosity has direct correlation with cardiac output, oxygen delivery to the tissue, maximum oxygen consumption, lipid oxidation during exercise, and endurance performance [35]. In this way, thyme extract supplementation can help athletes to improve their endurance capacity. On the other hand, in vitro studies have shown flavonoids are adenosine A1 receptor antagonist (such as caffeine, methyl xanthine, theophylline) which could be partly responsible for their stimulating and ergogenic effects [36]. A1 receptors have an inhibitory effect on most of the body tissues. In the brain for example, these receptors reduce metabolic activity and induce lethargy and sluggishness by inhibiting cholinergic receptors, decreasing the release of synaptic vesicles in the presynaptic and stabilizing magnesium on N -methyl-D-aspartate receptor. In the heart tissue, A1 and A2 adenosine receptors regulate myocardial oxygen consumption and coronary blood flow. Stimulating A1 receptor via decreasing the ability to conduct electrical impulses and repression of pacemaker cells ultimately declines heart rate and function. So, we can say that adenosine A1 receptors have an inhibitory effect on sympathetic tone and thyme extract may inhibit these receptors by increasing sympathetic activity [37] and may lead to improved exercise performance. However, for further clarification precise mechanisms by which thyme extract improves endurance exercise performance needs to be investigated at length.
The liver is an important member of the natural maintenance of blood glucose and maintenance of blood sugar during exercise is important. An increase in liver enzymes, including alanine amino transferase (ALT), which is commonly associated with liver damage and inflammation [8]–[10], causes liver inflammation, which over time, disturbs the normal balance of metabolism. Intense exercise may act as a stress and increase liver enzymes and ALT, AST may increase during exercise so thyme supplement reduces this effect. The consumption of thyme does not allow these enzymes to increase. (Table 1).
In addition, during the last weeks of present study, some rats of both groups (mostly non-supplemented group) did not cooperate in the training after the early stages of practice and were reluctant to continue for the rest of the training period even at low-intensities. This reluctance may be a sign of over-training in non-supplemented rats, which could be the evidence of increased oxidative stress. This condition was much lower in the thyme supplemented group. Taking into consideration that the above mentioned assumption is correct, it is possible that supplementation with thyme; as a strong antioxidant; helps in the antioxidant system of the rat body to prevent the harmful effects of too much oxidative pressure and subsequent over-training. It is necessary to prove by measuring indices likes levels of lactate, ammonia, urea, creatine kinase, the ratio of testosterone to cortisol and lipid peroxidation in basal resting condition[38] during the similar training periods and before beginning intense exercises.