Investigating the Efficacy of Polyphenolic Bioactives on Athletic Performance in Endurance Athletes

In elite athletes, conventional performance determinants such as maximal oxygen consumption (V̇O₂max ) approach physiologic ceilings, shifting the focus toward marginal improvements in substrate utilization, oxygen delivery, and metabolic efficiency to gain competitive edge. Plant-derived bioactives such as anthocyanins and flavonoids may enhance nitric oxide bioavailability and mitochondrial function, supporting more efficient fuel use during exercise. This study evaluated the effects of a polyphenol-rich supplement (Supplement A) on aerobic capacity, substrate utilization, and pulmonary diffusion capacity (DLCO) in trained male endurance athletes, assessing both acute and chronic supplementation. Seven endurance-trained male athletes (31.3 ± 8.4 y; baseline V̇O₂max 53.8 ± 7.3 mL·kg⁻¹·min⁻¹) completed four laboratory-based cycling sessions: baseline V̇O₂max, pre-supplementation steady state + V̇O₂max, acute supplementation (1–2 h after first dose), and chronic supplementation (after 14 days of daily intake). Substrate oxidation was derived from indirect calorimetry, and pulmonary gas exchange was assessed via DLCO testing. V̇O₂max remained unchanged across tests (p = 0.55), and DLCO showed no significant difference between pre- and post-supplementation (p = 0.20). In contrast, substrate utilization displayed consistent shifts. Fat oxidation increased significantly during steady state (+25–100% acute, +14–70% chronic, both p < 0.001) and V̇O₂max stages (+81% at 340 W chronic, p < 0.001). Thus, as expected, carbohydrate oxidation declined in parallel, with significant steady state reductions during acute (p < 0.001) and chronic supplementation (p < 0.001), and significant reductions in chronic supplementation during V̇O₂max stages (p < 0.001). Respiratory exchange ratio (RER) decreased significantly across steady state workloads in both acute (p < 0.001) and chronic phases (p < 0.001) and V̇O₂max stages (acute p = 0.044, chronic p < 0.001). Recovery showed elevated fat oxidation after chronic supplementation (p = 0.05). Supplementation appeared to shift substrate use towards greater fat oxidation and reduced carbohydrate reliance across exercise intensities, with notable individual variability. Even in athletes near their physiological ceiling, these changes suggest that plant-derived bioactives may confer measurable benefits with potential competitive relevance. However, responses were not entirely uniform. While several athletes demonstrated clear and consistent improvements, others showed only marginal or phase-specific changes, and one presented a distinct atypical trend. These findings reinforce the need for individualized or tailored approaches to supplementation. Future studies should extend duration, incorporate mechanistic biomarkers, and diversify cohorts to clarify the durability and generalisability of these effects.