1. Introduction
Endurance performance during high intensive exercises is mainly determined by the capacity of the aerobic metabolism [
1]. It generally induces muscle fatigue, defined as the reversible decline in skeletal muscle contractile function [
2]. Fatigue is multifactorial and is often associated with many physiological parameters including reduced neural input and disruptive metabolic changes in skeletal muscles such as lactic acidosis and the production of oxidative free radicals [
2]. Moreover, it could lead to oxidative stress as a result of an imbalance between reactive oxygen species (ROS) production and intrinsic antioxidant defense [
3].
To alleviate oxidative stress, some ergogenic strategies have been tested. Numerous studies have reported that different types of supplementation such as polyphenols were of interest to protect against these mechanisms [
4,
5,
6]. Indeed, although some studies demonstrated no or harmful effects [
7,
8], most studies observed the positive effects of antioxidants on oxidative stress or performance [
9,
10,
11]. More particularly, polyphenols, have great antioxidant capabilities and protective effects [
12,
13]. In addition, polyphenols increase the synthesis and bioavailability of nitric oxide (NO) [
14,
15] which is well known as the most important mediator of vasodilation [
16]. NO also plays an important role in many functions such as blood flow, mitochondrial respiration and platelet function [
17]. As a consequence, the beneficial effects of NO have been demonstrated on muscle strength [
18,
19], but also during recovery following intensive efforts [
20].
To date, most of the studies exploring the effects of polyphenols investigated several days or weeks of supplementation on vascular, blood parameters (blood pressure, NO concentration, oxidative stress markers) or endurance performance [
4,
21]. For instance, Trinity et al. [
22] observed no alteration in the cycling time-trial after seven days of polyphenol dietary supplementation. However, conflicting results are often obtained [
19]. According to a recent review [
23], depending on the type of polyphenols, chronic consumption has potentially detrimental to promisingly beneficial effects. Only few studies have investigated the effects of a single intake on immediate performance and recovery capacity [
5,
10,
24,
25,
26]. Therefore, the present work aimed to study the effects of an acute intake of a specific profile of polyphenols from grape and apple on physical performance. More specifically, performance in the present study referred to a high-intensity cycling exercise until exhaustion revealing the capacity to maintain a constant strong effort hereafter named endurance. We therefore hypothesized that an acute supplementation of polyphenols would increase the time to exhaustion during a high-intensity cycling exercise.
4. Discussion
The main aim of the present study was to investigate the effects of an acute intake of a specific formulation of polyphenols from apple and grape on endurance capacity and recovery. The present study demonstrated that the intake of polyphenols prior to an endurance exercise increased the time to exhaustion and lengthened the time to onset of maximal perceived exertion as compared with the placebo. Taken together, these results suggested an increased endurance capacity with the acute intake of polyphenols.
The mean endurance test duration was ~25.5 min and ~28.0 min with the placebo and polyphenols, respectively. Such a duration may appear low when considering the power output (70% of the maximal aerobic power). This short duration could firstly be attributed to the initial test that used 1 min increments which could overestimate the maximal aerobic power. Secondly, volunteers were physically active and not specifically trained for cycling. The inherent variability of this population and of the measurements was counterbalanced by the large sample size tested here and the randomized, crossover design. Indeed, as indicated in a recent review [
11], most studies considered small samples. The 48 participants of this randomized, crossover, double-blind study therefore strengthen our conclusions.
The increased time to exhaustion is concordant with some previously published studies [
25]. In this last study, the authors registered significant increases in the running time to exhaustion with the acute intake of polyphenols as compared to the placebo [
25]. In addition to some increases in the endurance aerobic performance, other authors revealed some improvements in anaerobic power with caffeine-based products [
10]. However, the few studies that investigated the effects of the intake of acute polyphenols are often conflicting. Other authors [
22,
26] did not register any effect of polyphenols on the cycling time-trial performance in elite or well-trained cyclists. Such discrepancies could be ascribed to the training status (elite vs. amateur athletes), nature and duration of exercise or to the type of polyphenols used [
23]. For example, Jowko et al. [
24] concluded that an acute intake of green tea polyphenols was not efficient to attenuate exercise-induced oxidative stress while Morillas-Ruiz [
5] detected some protective effects of polyphenols (mostly from fruits) against exercise-induced oxidative stress. Also polyphenols from cranberries and grapes increased artery flow-mediated dilation [
26].
It is important to note that the increased time to exhaustion with polyphenols, observed here, is obtained with a delayed fatigability (as witnessed by the late rate of perceived exertion), but with similar physiological responses compared to the placebo. Indeed, the heart rate, blood pressure, VO
2 and ventilation rate are similar between conditions. These results are concordant with previous findings [
10].
Some studies attributed antioxidants’ and more particularly polyphenols’ effects to enhanced blood flow [
26]. Although not tested here, several mechanisms might explain the present increase in endurance through an action on NO. Indeed, polyphenols (notably from grape sources) have great antioxidant capabilities [
12,
13] and increase the synthesis and bioavailability of NO [
14,
15], thus having the potential to delay fatigue. Additionally, polyphenols such as green tea or grape have been associated to improved endothelial function [
31]. Based on in vivo and ex vivo preclinical unpublished observations with Vinitrox™, we could speculate that performance benefits might be due to the modulation of NO-dependent vasodilation with NO synthesis increase and protection. Previous studies demonstrated that this antioxidant effect might also contribute to improving NO effects by two main actions. Firstly, its lifespan protects against O
2− and, secondly, it prevents eNOS uncoupling, leading notably to a lower flow-mediated dilation of arterioles [
32,
33].
Previous studies demonstrated that the primary mechanism of NO is the increase in muscle perfusion through a direct vasodilator action on vascular smooth muscle cells and an inhibition of adrenergic vasoconstriction [
34,
35]. This muscular hyperemia might induce an increased oxygen supply to muscle cells, as well as higher nutrient supply and metabolite product removal, which would allow an enhanced aerobic metabolism [
25]. Another mechanism may lie in the action of NO on the glucose metabolism of the muscle, in particular through an increased muscle uptake. In addition, the endogenous production of NO on the sarcoplasmic reticulum Ca
2+ is likely to improve muscle contractile performance [
36,
37].
The second aim of the present experiment was to assess the effects of polyphenol supplementation on exercise recovery. Indeed, an NO production increase is supposed to enhance oxygen and nutrient delivery to active muscles, thus improving tolerance to physical exercise and recovery mechanisms [
16]. Contrarily, our results revealed a significant lengthening of the time of half-recovery of the VO
2 under the polyphenol condition. Therefore, the lengthening of the recovery that could reflect the existence of an oxygen debt might be surprising when considering NO effects. However, it is in agreement with the longer duration of the endurance tests. Indeed, the duration of the O
2 debt has been shown to be directly related to the exercise duration [
38]. The longer recovery could therefore be primarily attributed to the longer-endurance exercise (obtained with polyphenols ingestion) rather than physiological mechanisms related to polyphenol ingestion and potential vasodilation effects. However, additional measurements are necessary to verify this speculative statement. Our result might be of great interest, particularly for people wishing to lose weight since the lengthening of the O
2 debt is associated with increased energy expenditure, and more specifically lipids oxidation [
38,
39].
Lastly, the absence of any muscle pain two days after exercise in both conditions indicates that this acute polyphenol supplementation allowed participants to perform longer exercises without further adverse effects. This is of particular interest for athletes training regularly since one of the limiting factors of training is often muscular pain resulting from effort.
In conclusion, the present randomized, crossover, double-blind and controlled study demonstrated that the acute supplementation of polyphenols in healthy, physically active males allowed significant increases in endurance performance (hereby the capacity to maintain a strong effort) with greater energy expenditure as demonstrated by the lengthening of time to exhaustion and time to maximal perceived exertion. Also, the main cardiovascular and respiratory measured parameters showed no significant differences between conditions. A similar observation was obtained on muscle pain two days after exercise. These results indicate that performance improvements, as a result of acute polyphenol intake, have been obtained under safe conditions and without additional pain. In contrast with some previous studies, the present conclusions were made on an almost large sample size and reinforce the positive effects of polyphenols on cycling endurance. An interesting perspective of this work would be to control diet during the duration of the experiment and to quantify specific biomarkers in order to better understand the mechanisms behind the present results.