New Zealand blackcurrant (NZBC) is a supplement made from a berry naturally rich in plant compounds called anthocyanins. Researchers have studied these compounds for years and found that they may help the body by improving blood flow, reducing post-exercise stress on the body, and supporting energy production. Most of that research, however, has focused on endurance athletes like runners and cyclists. Almost nothing was known about whether NZBC could benefit people who do strength training. That is what this study was designed to find out.

Twenty healthy, resistance-trained men and women between the ages of 18 and 40 were recruited. Each participant completed four different conditions over the course of the study: taking no supplement at all, taking a placebo (a dummy capsule with no active ingredient), taking a low dose of NZBC (250 mg per day for seven days), and taking a higher dose of NZBC (600 mg per day for seven days). Neither the participants nor the researchers evaluating them knew which capsule was being taken at any given time, a design that helps ensure the results are as reliable as possible.

At the end of each seven-day period, participants came into the lab and completed a full battery of tests. Measurements included how much weight could be lifted for one maximum effort on the bench press and leg press, how much total volume could be completed across multiple sets, and how fast and powerfully the bar could be moved. Participants also completed a 30-second all-out cycling sprint to test anaerobic fitness, and a paper-and-pencil color-word test to measure focus and mental sharpness.

Strength increased noticeably. After taking either dose of NZBC, participants were able to lift more weight on both the bench press and leg press than when taking the placebo or no supplement. The lower dose had a particularly striking effect on leg training: participants pushed through roughly 38% more total volume in their leg press sets. For bar speed and explosive power on the bench press, the higher dose had the edge. Cognitive performance followed a similar pattern. The lower dose sharpened mental performance across several parts of the color-word test, while the higher dose made a more modest but still meaningful difference on one section. The sprint cycling test showed no differences between conditions. Importantly, both doses were safe and well-tolerated, with no meaningful side effects reported throughout the study.

One week of supplementation was sufficient to produce real differences in strength, power, and mental sharpness, and neither dose raised any safety concerns.

Background and Rationale Polyphenol-rich supplements have attracted growing interest in sports nutrition research, largely because of evidence that anthocyanins can influence several physiological processes relevant to exercise. These include improvements in vascular tone and blood flow, reductions in exercise-induced oxidative stress, changes in substrate utilization during activity, and improvements in fatigue tolerance. New Zealand blackcurrant (NZBC) extract has emerged as one of the more studied anthocyanin sources in this area, with prior research documenting benefits in high-intensity cycling, intermittent running, and fat oxidation during exercise.

Despite this growing body of evidence, nearly all existing research on NZBC has focused on endurance or aerobic exercise contexts. Whether NZBC could influence maximal strength, total lifting volume, or barbell power output in resistance-trained individuals had not been previously examined. Additionally, the potential for NZBC to influence cognitive function during resistance training is biologically plausible given its monoamine-related bioactive profile, but had not been rigorously tested in this population. This study was designed to address both gaps using a well-controlled crossover design.

Study Design This was a randomized, double-blind, placebo-controlled, four-period crossover trial conducted in the Exercise Physiology and Nutrition Laboratory at Jacksonville State University. The four conditions were a no-capsule control, a placebo, a low-dose blackcurrant condition (250 mg/day), and a high-dose blackcurrant condition (600 mg/day). Each capsule condition lasted seven consecutive days, with laboratory testing on day seven. The no-capsule control was always administered first; the three capsule conditions were assigned using a balanced 3×3 Williams design to minimize carryover effects. Randomization was computer-generated by an investigator independent of data collection. All capsule conditions were separated by washout periods, and capsules were identical in appearance and mass to maintain blinding.

Interventions The placebo capsule contained 600 mg of maltodextrin per day. The low-dose condition contained 250 mg of NZBC extract plus 350 mg of maltodextrin per day, providing an estimated 87.5 mg of anthocyanins daily. The high-dose condition delivered 600 mg of NZBC extract per day, providing approximately 210 mg of anthocyanins daily. Capsules were taken once daily, timed two hours before training on workout days or at noon on rest days. On laboratory testing days, the final capsule was taken on-site two hours before testing, with staff present to confirm ingestion.

Dietary standardization was implemented throughout the study. Beginning two weeks before the first test session, participants discontinued all dietary supplements and over-the-counter medications. In the 72 hours preceding each laboratory visit, participants abstained from structured exercise, caffeine, alcohol, dietary nitrate, polyphenol-rich foods, and antibacterial mouthwash. Dietary intake was monitored using four separate three-day recording windows via MyFitnessPal to verify consistency across conditions.

Statistical Analysis Each outcome was analyzed using a repeated-measures general linear model with treatment (four levels) as the within-subject factor and sex as the between-subject factor. The model tested the main effect of treatment and the treatment-by-sex interaction. Sphericity was assessed using Mauchly's test, with Greenhouse-Geisser corrections applied where needed. Significant overall treatment effects were followed by pairwise comparisons using estimated marginal means with the least significant difference procedure. Effect sizes for pairwise comparisons were calculated as Cohen's d_z and interpreted as trivial, small, moderate, or large. All analyses were conducted in IBM SPSS Statistics, version 31.

Inclusion Criteria:

• Age 18-40 years
• At least 2 years of regular resistance training that included the bench press and either the leg press or squat
• Body mass index (BMI) of 18.5-24.9 kg/m²

Exclusion Criteria:

• Diagnosed with a metabolic, cardiovascular, or thyroid disorder
• Current use of prescription or over-the-counter agents with potential cardiovascular or neurocognitive effects (e.g., beta-blockers or stimulants)
• Tobacco use, vaping, or smokeless nicotine use
• Self-reported blackcurrant sensitivity
• Habitual alcohol intake greater than 12 standard drinks per week
• Any recent musculoskeletal injury likely to impair testing
• Resting heart rate exceeding 100 bpm at familiarization
• Resting blood pressure ≥140/90 mmHg at familiarization after repeat confirmation
• Body fat percentage exceeding the study's predefined threshold at familiarization

Deidentified individual participant data (IPD) for all primary and secondary outcome measures, including resistance exercise performance, anaerobic exercise performance, cognitive function (Stroop Color-Word Test), hemodynamic responses, rating of perceived exertion, and adverse events, will be made available upon reasonable request to the corresponding author beginning 6 months after publication.

IPD and supporting information will be available beginning 6 months after publication and will remain available for a minimum of 5 years following the publication date."

Deidentified IPD will be made available to researchers who submit a methodologically sound request to the corresponding author. Requests should include the proposed use of the data, the research question, and the requester's institutional affiliation. Data will be shared in a standard format (e.g., Excel or SPSS) via secure file transfer.

Supplements were coded and prepared by a third party not involved in data collection or analysis, ensuring that neither the participants nor the investigators were aware of the treatment assignment during each session. The statistician performing data analysis was also blinded to group allocation until after all analyses were completed. Unblinding occurred only after the final dataset was locked.