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.
