Dietary thiols in exercise: oxidative stress defence, exercise performance, and adaptation

It is well-established that muscular work is associated with oxidative stress and that prolonged or high-intensity exercise results in oxidative damage to macromolecules in both blood and skeletal muscle [1‐3]. This exercise-induced oxidative damage can impair physical performance via various mechanisms relating to compromised myocellular structure and function. Furthermore, chronic oxidative stress in athletes, often brought about by overtraining, has been linked to chronic fatigue [4], longer term performance decrements [5], muscle atrophy [6], and illness [7]. Exogenous and endogenous antioxidants reduce oxidative stress, thus it is not surprising that dietary antioxidants are a popular supplement for athletes in an attempt to enhance recovery, optimise performance, and reduce the oxidant load to maintain long-term health.

The two groups of biologically active molecules that fall under the umbrella term ‘free radicals’ include reactive oxygen species (ROS) and reactive nitrogen species (RNS). During exercise, oxygen consumption can increase as much as 100-fold [8] and is associated with rapid increases in ROS production and accumulation. This constant production of free radicals by skeletal muscle requires the buffering capacity of an endogenous defense system, and a multitude of mechanisms have evolved to detect and respond to elevated oxidant production. The action of these systems determines the overall endogenous antioxidant capacity, and if this capacity is exceeded, oxidative stress ensues, potentially resulting in detrimental oxidation of cell membranes, functional proteins, and DNA [9‐11].

Paradoxically, the redox activities of ROS and RNS play critical roles in cell signaling and exercise adaptation, a phenomenon summarized by the concept of hormesis, where low levels of stress promote adaptation to and thus protection from, subsequent stress [12]. Various exercise studies have observed improved endogenous defence in rodents following training [13‐17], along with reduced ROS production at the same absolute exercise workload during following training [18]. However, at very high concentrations, ROS can move beyond being advantageous, and have detrimental effects on performance. During heavy endurance training, endogenous antioxidant capacity cannot meet the increasingly high ROS generation, resulting in a state of oxidative stress and subsequent cellular damage [19]. Dietary supplementation with antioxidants in this situation may be beneficial; however, the exact ‘balance’ of (exogenous) antioxidant supplementation to ROS levels that may enhance performance without blunting adaptive pathways is currently unknown. Commonly supplemented antioxidants include the well-known vitamins such as C and E, and in recent times, research concerning the effects of these on exercise performance in humans has grown considerably, producing equivocal results [20‐24]. While acute administration in humans may potentially enhance performance [25], the majority of studies suggest no benefit [23, 26‐30]. Furthermore, there is concern that chronic supplementation may blunt the free radical-driven adaptive effects of training by interfering with the essential endogenous antioxidant response to ROS [31, 32]. More recently, the thiol donors’ glutathione, cysteine, and taurine, have generated interest for the improvement of exercise recovery and performance, with their potential to boost endogenous antioxidant defence in a way that generalized antioxidants cannot. However, the use of these is limited by poor supplement bioavailability and several side effects [33, 34]. This has created a niche for the advent of palatable, digestible, and tolerated dietary thiol sources. In this review we focus on the emergence of thiols as antioxidant supplements for the endurance athlete, the current research surrounding their ergogenic potential and benefits, and recent efforts to produce a high-cysteine food source.

While research into the effect of common dietary antioxidants on acutely reducing free-radical load is well documented in humans, evidence of any exercise performance or adaptation benefits are equivocal [20‐23, 30, 31, 35‐38]. Few studies have observed performance benefits as a result of supplementing with specific dietary antioxidants [37] and many well controlled studies in this area have found no performance benefit [21‐23, 39‐42]. It has been suggested that antioxidant supplementation may only improve performance when endogenous levels are already depleted, and after reaching normal concentrations, no further benefit is seen [35]. Similarly, antioxidant supplementation does not appear to benefit recovery time following acute exercise [43‐46], with only a limited number of studies showing minor recovery benefits [30, 47]. On the other hand, there is an increasing body of work that has shown that ingestion of specific foods, high in antioxidant compounds can accelerate recovery [44, 48, 49]. Both cherries [44, 48] and blueberries [49] seem to expedite the recovery of muscular force following strenuous eccentric work. However, whether this is due to the antioxidant capabilities of the foods, or the polyphenolics is a point of contention. Our previous work [49] seems to infer the latter.

A number of published studies to date indicate that supplementation with common dietary antioxidants may actually be detrimental to athletic performance by blunting the adaptive effect of exercise training [31, 50‐52]. These adaptations, including upregulated endogenous defense, muscle protein synthesis, and mitochondrial biogenesis, are important for improved exercise capacity and recovery. Dietary antioxidants such as vitamins C and E are generally required in very small amounts, working with endogenous antioxidants to maintain or re-establish redox homeostasis [53]. However, these general antioxidants non-specifically scavenge all free radicals, regardless of their source; hence increased intake may affect various pathways including cellular signaling pathways that are important for exercise adaptation. In contrast, endogenous antioxidant systems are a complex and compartmentalized network that involve controlled and localized production of specific ROS [54]. These ROS are uniquely balanced by endogenous antioxidants to levels where cellular signaling can still be carried out. In athletes, disruption of this system by over-supplementation with generalized exogenous antioxidants can result in blunted signaling pathways [31, 38, 50]. This in turn can compromise recovery and attenuate exercise adaptations, negatively affecting performance.

In addition to their effect on adaptive pathways, at high levels, exogenous antioxidants can promote oxidation, contributing to acute decrements in exercise performance. For example, vitamin C can react with metal ions released from exercise-induced tissue damage, giving rise to harmful hydroxyl radicals [55], and vitamin E can become a free radical itself when it reacts with a free radical [56], damaging lipid membranes if it is not converted back to its reduced form. Following acute eccentric induced muscle injury in young men, Childs et al. [57] found increased oxidative stress and tissue damage supplementation with vitamin C and a cysteine derivative n-acytylcysteine (NAC), compared to a placebo. Similarly, in ultra-endurance (Ironman™) competitors, vitamin C or E supplementation prior to racing significantly elevated markers of oxidative stress and showed significant decreases in antioxidant enzyme activity post- race, compared with no supplementation [58]. For a more comprehensive review on the various dietary antioxidants that can form a pro-oxidant state, please refer to Yordi et al. [59].

In contrast to exogenous antioxidants, high levels of endogenous antioxidants do not appear to contribute to oxidative stress. Rather, it is depleted levels that can give rise to an oxidant state [60]. It could therefore be suggested that consuming dietary sources of endogenous antioxidant precursors, such as thiol donors, may enhance exercise performance and recovery without the negative effects associated with exogenous antioxidant supplementation.

Despite their potential benefits, the use of supplementary thiol donors to improve endogenous antioxidant status may not necessarily be the ideal way to enhance performance and recovery. As previously mentioned, high intakes of NAC in healthy individuals may have pro-oxidant effects [107‐109], and both oral [110] and systemically administered [83] NAC can be poorly tolerated. Similarly, while methionine supplementation has been shown to increase GSH levels in those with sub-optimal levels [115, 116], excess intakes in healthy individuals may increase homocysteine levels which has been shown to contribute to cardiovascular disease and several mental health disorders [117, 118]. Therefore, alternative dietary sources of thiols may provide a safer option for athletes and others.

Endurance athletes are susceptible to cellular damage initiated by excessive levels of aerobic exercise-produced ROS. Whilst ROS can contribute to the onset of fatigue, there is increasing evidence that they play a crucial role in exercise adaptations. The use of general antioxidant supplements such as vitamin C and E in athletes is common; however, their ability to enhance performance and facilitate recovery is controversial, with many studies suggesting a blunting of training adaptations with chronic supplementation. However, as the up-regulation of endogenous antioxidant systems are brought about by exercise training, athletes may benefit from increasing these systems through dietary thiol donors. While the thiol donors methionine and NAC may increase endogenous antioxidants and antioxidant enzymes, there can be adverse effects associated with their use. Thus, the discovery for the use of hydrolysed keratin to potentially enhance endogenous GSH and taurine may have important implications for athletes hoping to enhance performance and recovery without blunting training adaptation.