The effect of New Zealand blackcurrant on sport performance and related biomarkers: a systematic review and meta-analysis

The popularity of perceived naturally occurring plant extracts and phytochemicals to enhance physical performance, exercise recovery and maintain health has increased dramatically in recent years. Interest in the health properties and benefits of blackcurrants for active individuals started over 10 years ago [1, 2]; however the last 5 years has seen a surge in research interest [3]. In 2018, the International Olympic Committee (IOC) released a statement on the efficacy of dietary supplements for athletes, in which polyphenols were reported to increase mitochondrial biogenesis and endurance performance, at least in mice [4]. The IOC highlighted the importance of conducting reviews using a systematic process and in their hierarchy of scientific evidence, systematic reviews and the process of meta-analysis were considered the gold standard [4]. A semi-recent systematic review and meta-analysis investigated the effect of polyphenols, albeit a very broad dietary group, on athletic performance in humans [5] and demonstrated a significant improvement when taking polyphenols for 7 days or more. Given the review included all polyphenols and accepting the broad nature of the inclusion criteria, translating this finding into a recommendation for athletes is problematic. In the current review we investigate blackcurrants (Ribes nigrum), which are naturally high in a particular range of polyphenols and may provide an opportunity to be specific regarding optimal dosing strategies.

Berries are a brightly coloured fruit which have been investigated for their health-promoting effects, particularly blackcurrant, blueberries (Vaccinium spp), and blackberry (Rubus spp). Berries contain high concentrations of a particular class of flavonoids called anthocyanins, but each berry type has a specific make-up of anthocyanins [3, 6]. Anthocyanins are natural pigments responsible for the blue, purple, red and orange colours of many fruits and vegetables [6]. While the total daily consumption of total anthocyanins has been estimated to be between 3 and 215 mg/day [6], the optimal intake of total and specific anthocyanins is uncertain. In general, blackcurrant contains between 130 and 460 mg/100 g fruit weight of total anthocyanins [6] and the predominant type is delphinidin-3-rutinoside. In contrast, blueberries contain 62–300 mg/100 g fruit weight of total anthocyanins [6] with the predominant type being malvidin-3-monogalactoside [7]. Blackcurrant anthocyanins have shown to alleviate inflammation and oxidative stress, while preventing the depletion of mitochondrial content and damage [8], not reported with blueberry anthocyanins. Thus blackcurrant anthocyanins (delphinidin glycosides) appear to be a more effective antioxidant than blueberry anthocyanins (malvidin glycosides) [9, 10]. The physiological effects of the different berries suggests not all polyphenols and anthocyanins have the same bioactivity.

While the general health benefit claims of blackcurrants are warranted, there are few studies comparing the anthocyanin content of different blackcurrant varieties grown in a range of countries. New Zealand blackcurrants (NZ BC) have been reported to have higher concentrations of anthocyanins and other phytochemicals than those grown in other countries, which is a likely consequence of an environment with high UV and long, sunny days. The anthocyanin content of juice produced from NZ BC has previously been shown to be between 336 and 850 mg/100 mL, in comparison to non-NZ blackcurrants with an anthocyanin content ranging from 170 to 310 mg/100 mL of juice [11]. While acknowledging the total amount of blackcurrant anthocyanins consumed is likely to be a key factor, for pragmatic reasons a concentrated dose might be preferable for athletes, particularly prior to an event. As such, the purpose of the review was to determine whether blackcurrant anthocyanins, particularly from NZ BC, alter direct and indirect aspects of athletic performance. In particular, does NZ BC improve athletic performance and modulate oxidative and cognitive effects?

A total of 964 articles were identified through searching databases and other sources. Following removal of duplicates, 952 articles remained. After screening by title and abstract, 45 articles remained. Following application of specific inclusion and exclusion criteria, critical appraisal, and quality checking of articles, 16 were deemed acceptable for inclusion. See Fig. 1 PRISMA Chart.

A 1% difference in athletic performance is relevant to athletes and of sufficient magnitude to affect medal rankings in an Olympic-level competition, with the potential to elevate a medal from fourth to a podium position [30]. The aim of the current review was to systematically evaluate the literature and pool the available evidence to determine the potential benefit of NZ BC on athletic performance and whether this is sufficiently relevant to matter. The meta-analysis shows a significant improvement in performance (effect of 0.45) which is quantitatively deemed a small, significant improvement. Interestingly, the magnitude of the effect we report for NZ BC is not dissimilar to caffeine (effect of 0.41 (95% CI: 0.15–0.68), p = 0.002) [30], which has long been touted as the most potent legal ergogenic food or supplement available.

Whilst acknowledging the majority of the performance research included in the meta-analysis were conducted on sub-elite athletes, one article conducted a sub-analysis on participants based on training load [16]. The athletes with higher training loads had a greater response to NZ BC compared with the low training load group, which might suggest both sub- and elite athletes could benefit.

A recent narrative review highlighted the physiological mechanisms behind the potential of NZ BC [3] stating acute intake may influence cardiovascular alterations such as vasodilation and increased peripheral blood flow, but longer intake durations may be required to result in changes in cellular signaling and mitochondrial adaptations. Of the nine studies included in the performance data, eight used supplement protocols of 7 days, the one remaining study supplemented for 3 weeks, suggesting 7 days is a sufficient length of time to be effective. Alongside the consideration of the ideal anthocyanin supplement length of time is the effective daily dose, which in the performance studies is generally 105–210 mg daily, and in the oxidative and cognitive inclusive studies ranges from 20 to 500 mg daily. One of the performance studies provided participants a daily dose of 300 mg of anthocyanins, and reported minimal performance effects and some minor gastrointestinal side effects, suggesting the ideal dose should be less than 300 mg daily [16]. Recent studies have investigated the dose response of NZ BC, indicating a minimum of 120 mg of blackcurrant anthocyanin taken acutely reduces oxidative stress [25].

The bioavailability of polyphenols and anthocyanins are generally thought to be poorly absorbed with 5–10% occurring in the small intestine with key metabolites reaching the bloodstream between 0.5 to 1.5 h after consumption [31], and peak anthocyanin levels reaching the blood stream 2 h post consumption [25]. As such, the timing of anthocyanin intake to the exercise is a consideration in defining an optimal consumption. Of all included studies, 7 report providing the final dose 2-h before the relevant testing took place, 2 report 3-h, 4 report ≥1-h, the remaining did not report when the participants consumed the final dose.

The summary of included studies outlined in Table 1 indicate that an NZ BC anthocyanin intake of 105–210 mg taken for 7 days with the last dose 1–2 h before activity to be most effective for performance gains. With regards to the management of exercise-induced oxidative stress, data from Table 2 demonstrate effects after a single intake of NZ BC with the minimum dose required of 120 mg of blackcurrant anthocyanins. Further studies are required to determine whether performance benefits are achieved after a single intake.

The NZ BC was provided in various forms which varied between studies, including juice concentrate, powdered juice concentrate, powdered whole fruit and powdered extracted anthocyanins. One study investigated both juice (‘BlackAdder’ blackcurrant cultivar) and powdered extracted anthocyanins, reporting slightly more anthocyanin content in the powder when made up to an equivalent 200 mL serve [28]. The improvements in cognition were slightly better with the powdered extract than juice, which also had a lower blood glucose response. The predominant form of NZ BC used in studies was powdered product in capsules to help reduce placebo bias and maintain a consistent anthocyanin content.

To provide insight into the mechanistic action of NZ BC, we investigated the oxidative and inflammatory markers when taking NZ BC. The traditional approach to interpreting oxidative stress and inflammatory markers are to define a reduction as “good”, suggesting the underlying oxidative stress and inflammation are “bad.” However, it has been identified that oxidative stress can initiate signaling to support improved training adaptations and nutrients with very high antioxidant capacity may not be in the best interest of the athlete if taken chronically [32]. In the analysis, we attempted to define whether NZ BC altered the oxidative or inflammatory response to exercise and failed to see a consistent response outside the inherent variability of the biomarkers (demonstrated as the factor standard deviation presented in the results to support Fig. 3). However, of significant interest are the studies including an acute and chronic component, clearly showing an increase in inflammatory markers (TNFα, IL-6, IL-10) and an oxidative stress marker (FRAP) while ingesting NZ BC prior to exercise, but not after. It is possible the NZ BC is priming the antioxidant and inflammatory responses. NZ BC anthocyanins appear to interact with cellular antioxidant systems and mediate enhancement of antioxidant defenses and mitochondrial adaptation which are likely to be far more important than the oxidative scavenging activity [33]. Oxidative stressors that are proposed to induce mitochondrial adaptation include exercise, calorie restriction, and polyphenols acting as pro-oxidant primers [34]. The central signaling molecule likely to be important in these adaptive responses is nuclear factor (erythroid-derived 2)-like 2, also known as Nrf-2 [25]. Nrf-2 is a transcription factor and is regarded as a master regulator of antioxidant, inflammatory and mitochondrial responses. The Nrf family activate genes that encode mitochondrial respiratory chain and translocation proteins [35]. Under normal homeostatic conditions, Nrf-2 is suppressed and becomes active upon exposure to oxidative stress whereby it translocates into the nucleus and binds to the antioxidant response element of the genes of antioxidant enzymes. This results in the initiation of the transcription of the genes that control adaptive enzymes which in turn manage the oxidative stress [25, 33]. While the data we extracted in this review failed to see a difference in oxidative markers when taking NZ BC compared with control, we do acknowledge the number of studies included was very low, too low to meta-analyse or make meaningful conclusions. However, in theory, the provision of a moderate dose of BC anthocyanins may be priming the antioxidant and inflammatory processes, while activating mitochondrial growth to support peak performance.

The oxidative and inflammatory outcome measures included in this review are widely studied, but it should be acknowledged that more sophisticated measures may better differentiate changes with dietary intervention. Modern approaches include in vitro assays, metabolic and microRNA approaches, which require more research before results can be used to guide evidence-based practice. One of the limitations of the current review is the reliance on relatively crude oxidative and inflammatory markers which may in part explain the lack of clarity in the oxidative outcome measures.

Aside from the performance response, the cognitive and mood attributes appear trivial, with improvements in cognitive test score and monoamine oxidase (MAO) inhibitory effects thought to enhance cognition and mood [27, 28]. Studies have reported monoamine oxidase enzyme inhibition in humans after taking blackcurrant which may maintain neurotransmitter function, leading to improvements in cognition and mood [27, 28], and as such is a biomarker of cognition. Cognitive effects of polyphenols may result from the vasodilator activity which aligns with the short time frame of action [28]. Our results did not support a cognitive improvement.

To our knowledge, no single study has investigated nutrient supplementation and timing over a competitive season, let alone investigated the personalised approach of NZ BC. Certainly for athletes competing multiple times in a year, thought should be given to an ideal, personalised, supplement regime.

New Zealand grown blackcurrants mediate a small, significant improvement on athletic performance, particularly when consumed for 7 days at a dose of 105–210 mg anthocyanins, with a final dose 1–2 h before exercise. The predominant form of NZ BC used in studies was powdered product in capsules. While the evidence for a direct effect on sport performance was clear, the mechanism is not. We found only a small number of studies that have investigated oxidative, cognitive and adverse outcomes, with no known detrimental side effects reported.