The primary aim of the study was to investigate the effect of blueberry consumption on markers of EIMD and inflammation after strenuous eccentric exercise. By employing a single-leg model, we were able to minimize confounders such as training status, health status, genetics, and lifestyle-relate factors. Further, by closely controlling diet and exercise prior to and during the experimental period, we were able to implement a feeding strategy to successfully explore the effectiveness of New Zealand blueberry consumption on muscle function recovery following strenuous eccentric repetitive quadriceps exercise.
The main findings reveal that consumption of blended New Zealand blueberries at specific times pre and post eccentric muscle damaging exercise accelerates the recovery of muscle peak isometric strength and facilitated a decline in eccentric exercise-induced oxidative stress. The eccentric muscle damaging exercise applied in this study has previously been employed by this group [28
] and was designed to assess the effectiveness of dietary intervention on the ensuing recovery events. The greatest loss in peak and average torque/tension was seen 12 hours following the 300 maximal eccentric contractions of the quadriceps muscle, indicating muscle damage had been achieved. Indeed, the significant decrease in muscle strength (isometric, concentric and eccentric) observed in both blueberry and control beverage conditions demonstrated that pre-consumption of the blueberry beverage had no treatment effect on the ability of the 300 repetitive eccentric quadriceps muscle contractions to cause the damage and weakness which is expected after a physical effort of this nature. Importantly, in relation to recovery from the 300 eccentric contractions, a significant time-treatment interaction effect on peak isometric tension was observed. Although time-dependent improvement in peak concentric and eccentric muscle strength in the blueberry beverage condition were also evident, they were not statistically significant. While only a small number of subjects were employed in this study, the results support the trend that the consumption fruit, like New Zealand blueberries may expedite recovery in muscle function. For example, similar nutritional interventions trials involving cherry juice [30
] or pomegranate-derived ellagitanins [31
] have showed an improvement in isometric muscle strength following an eccentric muscle damaging exercise.
The data also indicate that ingestion of a blueberry beverage had no effect on perceived muscle soreness. These observations are similar to other reported in other intervention studies involving fruit [30
] where an improvement in muscle function, but not pain was reported. In contrast, using a plant phytochemical-protein supplement combination “BounceBack” an improvement in delayed onset muscle soreness was observed independent of exercise-induced inflammation; however, no muscle function performance was reported [32
Blueberry fruit demonstrate a high antioxidant capacity [14
]. The source of this antioxidant capacity is thought to be attributed to the wide range of anthocyanins contained in this fruit and since the vitamin C levels within blueberries are relative low compared to other fruit - the contribution of vitamin C to antioxidant capability is likely to be minor (Table ). In this study, the effect of vitamin C is also minimized by the addition of a vitamin C fortified apple juice to both the control and blueberry beverages. This resulted in an overall similar antioxidant capacity as determined by ORAC, which further supports the minor contribution of vitamin C. Furthermore our addition of banana to both treatment beverages, which replaced milk (shown to reduce the antioxidant capability of blueberries [21
] and dextrose to the control beverage (equivalent to the sugar content found in the blueberry smoothie) ensured that the nutritional and antioxidant capability difference between the control and the blueberry beverage was primarily due to the polyphenolic compounds-derived from the blueberries. Consuming blueberry fruit to enhance plasma antioxidant capacity may be dependent upon what the fruit is consumed with. Serfini et al.
] showed that consumption of 200
g fresh blueberries (the same amount used in this study per serving) in healthy humans caused a transient increase in plasma antioxidant capacity, which was dramatically reduced when the fruit was consumed in conjunction with protein, i.e. a blueberry/milk smoothie. In contrast, Dunlap et al.
] showed no change in plasma antioxidant capacity after two months of feeding blueberries in dogs on a normal healthy diet, whereas Kay and Holub [34
] found that humans fed a high fat diet with blueberry fruit had a higher serum antioxidant capacity compared to a control group. In this study, the blueberry beverage was made in the absence of milk and the subjects’ food intake was controlled by provision during the first 24 hours of participation. During the three recovery days, subjects were provided breakfast in the mornings and during the first trial kept a food diary of other food intake (four meals plus snacks). These meals were then replicated during the second trial. Subsequent analysis of food diaries revealed that subjects maintained a similar diet pattern and limited their intake of antioxidant-rich foods as requested. We are therefore confident that the elevation in plasma antioxidant capacity observed following 60 hours of recovery was as a result of the blueberry beverage consumption. It is possible that some sugars in fruit could mediate a control of oxidative stress and the benefits observed in our study. Lotito & Frei reported that phytochemical-rich foods containing some sugars e.g. fructose increased plasma uric acid in human volunteers and contributed to plasma antioxidant status [35
]. Dextrose however, was unlikely to be responsible for any effects here as it was utilized in our placebo (equivalent to the sugar content found in the blueberry smoothie) and showed no effects on plasma antioxidant status or control of exercise-induced oxidative stress as reported previously [36
The 300 repetitive eccentric muscle contractions caused an increase in oxidative stress (ROS-generating potential, protein carbonyls) and inflammatory (CK, IL-6) markers following the eccentric exercise in both experimental conditions. The elevation in these parameters indicates that the strenuous exercise employed in this study is capable of inducing muscle damage (the increase in CK coincided with loss of muscle function in both treatment groups) and that the recovery in muscle function observed by 36 hours in the blueberry condition is independent of the fruit’s inherent antioxidant capacity. Since exercise-induced ROS / inflammation, and especially muscle-derived IL-6 [37
] activate down-stream adaptive processes that facilitate skeletal muscle recovery [38
], it is feasible that blueberry-derived polyphenolic compounds (such as anthocyanins) may also facilitate these events, which may include the up-regulation of both muscle-specific adaptive processes and overall immunity. It is controversial as to whether an increase in circulating IL-6 correlates with skeletal muscle damage, since eccentric skeletal muscle contraction has been shown to elevate circulating IL-6, as well as other myokines, such as IL-15, IL-8, fibroblast growth factor [37
], which in turn, have been shown to facilitate anti-inflammatory, energy production and adaptive processes (e.g. anabolic action) and thus facilitate muscle performance and recovery. Although it is quite feasible that the initial increase in circulating IL-6 observed post eccentric exercise in the blueberry condition may be due to skeletal muscle contraction rather than damage, the overall increase in circulating levels of this myokine may serve to promote down-stream muscle recovery events. We have not measured here other myokines or down-stream anti-inflammatory cytokines associated with IL-6 (such as IL-10), but the notion that blueberry consumption facilitates eccentric exercise-induced anti-inflammatory (via IL-6) events is supported by McAulty et al.
] who showed that both acute and long-term blueberry feeding prior to exercise causes an increase in anti-inflammatory cytokines, such as IL-10 and facilitates recovery.
In this study we observed a rapid decline in oxidative stress blood indices that coincided with the increase in plasma antioxidant capacity in the blueberry condition supporting the notion that an increase in plasma antioxidant capacity may be involved in the reduced exercise-induced oxidative stress observed. However, it is currently unclear whether an increase in plasma antioxidant capacity facilitates [41
] or hinders the activation of muscle adaptive events aiding muscle recovery. The efficacy of dietary antioxidant supplementation in facilitating recovery following strenuous muscle damaging exercise is under debate. Recent reports indicate that dietary supplements rich in antioxidants, attenuate oxidative stress [42
], whilst other reports either show that antioxidants have no action [44
] or have the ability to induce pro-oxidant effects [45
]. Moreover, although elevated plasma antioxidant capacity post antioxidant supplementation consumption has been found in many studies [47
] have failed to demonstrate an effect or relationship to muscle function recovery following an eccentric exercise-induced damage. Goldfarb et al.
] recently showed that ingestion of whole fruit and/or vegetable extracts may attenuate blood oxidative stress induced by eccentric exercise but no significant effect on functional changes relating to pain and muscle damage were observed. Our findings here concur as all correlations of indices of muscle performance with plasma antioxidant capacity were insignificant; 0.09 and 0.190.
Several studies report the effectiveness of plant-derived phytochemicals at accelerating the recovery from exercise-induced muscle function after damage [30
]. The health promoting properties of plant-derived phytochemicals are being debated and evidence is building that any benefits are likely independent of their inherent antioxidant capacity [17
]. Hence it is feasible that polyphenolic compounds derived from blueberries may support muscle repair and recovery through a similar process that is unrelated to the fruit’s antioxidant capacity. Preliminary results from another study we have conducted show that blueberry-derived anthocyanins induce an up-regulation of phase II antioxidant enzymes (unpublished observation) supporting others that report plant-derived anthocyanins activate redox-sensitive transcription factors that lead to the up-regulation of phase II antioxidant enzyme systems [20
]. Since exercise-induced ROS have also been shown to activate redox-sensitive transcription factors - nuclear erythroid-related factor 2 (nrf2) [50
] and heat shock factor [52
], these adaptive mechanisms may serve to facilitate the repair and functional recovery of skeletal muscle. Furthermore, given that anthocyanins also have been described to active the nrf-2 transcription factor [20
] and induce heat shock proteins [52
] it is feasible that blueberry-derived anthocyanins may activate similar and/or parallel adaptive mechanisms within damaged muscle and underlie the findings observed here and by others. It is also unclear whether particular anthocyanins or other phytochemicals from fruits (or other sources) are responsible for or synergistic to the benefits reported here. Studies using isolated polyphenolics indicate that they potentially possess diverse functional efficacy within the body, which may not necessarily complement each other. It is feasible that certain fruit species or even certain cultivars (or combinations thereof) may provide the combination of polyphenolics that synergistically act together to most optimally deliver a specific biological action or actions that complement the adaptive events desired by exercise training athletes.