Effects of Short-Term Low- and High-Dose New Zealand Blackcurrant Supplementation on Exercise and Cognitive Performance in Resistance-Trained Adults: A Randomized, Double-Blind, Placebo-Controlled Crossover Study

Background: New Zealand blackcurrant (NZBC) is an anthocyanin-rich supplement with reported ergogenic effects in endurance exercise; however, its effects in resistance-trained adults remain largely unexplored. Objective: This study aimed to examine whether seven days of low- or high-dose NZBC supplementation improves resistance exercise performance, anaerobic capacity, and cognitive function in resistance-trained adults. Methods: Twenty resistance-trained adults completed a randomized, double-blind, placebo-controlled crossover trial with four conditions: no-capsule control (CON), placebo (PL), low-dose blackcurrant (LDBC; 250 mg·day−1), and high-dose blackcurrant (HDBC; 600 mg·day−1), each for seven days. Outcomes included bench press and leg press 1RM, total lifting volume, Tendo-derived bench press power, 30 s Wingate performance, Stroop Color–Word Test scores, readiness, perceived exertion, hemodynamic responses, and adverse events. Results: LDBC and HDBC increased bench press 1RM versus CON and PL, with increases versus CON of +3.33 kg (ES = 0.72; p = 0.005) and +2.34 kg (ES = 0.49; p = 0.041), respectively. Leg press 1RM was higher in PL, LDBC, and HDBC versus CON, with the largest effects observed for LDBC (+37.2 kg, ES = 1.33; p < 0.001) and HDBC (+25.8 kg, ES = 1.11; p < 0.001). Leg press total lifting volume was substantially higher with LDBC (+2627 kg, ES = 1.56; p < 0.001) and HDBC (+1025 kg, ES = 0.74; p = 0.004) versus CON. Bench press volume showed no significant overall treatment effect (p > 0.05). For Tendo-derived power, HDBC exceeded PL for peak (+79.5 W; p = 0.006) and mean power (+46.2 W; p = 0.026). Wingate outcomes did not differ across conditions (all p > 0.05). LDBC exceeded PL on Stroop Color, Color–Word, and total scores (all p < 0.05); HDBC exceeded PL on Color–Word only. Hemodynamic responses and adverse events were comparable across all conditions. Conclusions: Short-term NZBC supplementation improved selected resistance-exercise and cognitive outcomes, with the strongest evidence observed for outcomes that exceeded both CON and PL. The PL response relative to CON suggests that non-specific capsule, expectancy, repeated testing, or period effects may have contributed to some of the lower-body improvements; therefore, placebo-controlled contrasts should be emphasized when interpreting NZBC-specific efficacy. Wingate performance was unaffected, and both doses were well tolerated over the short-term supplementation period.