Polyphenol-Rich Snack Consumption during Endurance Exercise Training Improves Nitric Oxide Bioavailability but does not Improve Exercise Performance in Male Cyclists: A Randomised Controlled Trial

Background: Antioxidants and nitric oxide (NO) precursors may improve endurance exercise performance by reducing oxidative stress and increasing NO production. Almonds, dried grapes, and cranberries (AGC) are good sources of antioxidants and NO precursors. Objectives: To determine whether AGC consumption improved physiological responses and endurance cycling time-trial performance in response to training. Methods: After 1 wk of light training (LT), 96 male recreationally trained cyclists consumed 125 g of AGC or control (CON: isocaloric oat bar) daily during 2 wk of heavy training (HT) and a 2-wk taper (T). At the end of LT, HT, and T, endurance exercise performance (5-min cycling time-trial; 5CTT), NO bioavailability (plasma and urine nitrate and nitrite), oxidative stress [plasma F2-isoprostanes (F₂-Isop)], muscle damage (creatine kinase) and subjective measures of wellbeing were assessed, as well as physiological responses during exercise at 70% maximal aerobic power output. Results: Compared to LT, 5CTT performance was impaired at HT (d = –0.27, P = 0.01) and improved at T (d = 0.79, P < 0.001), with no difference between treatments (P > 0.81). Compared with CON, during submaximal exercise at 70%, maximal aerobic power output AGC demonstrated higher oxygen consumption (HT: d = 0.46; T: d = 0.38, P < 0.001) and lower respiratory exchange ratio (HT: d = –0.61; T: d = –0.23, P < 0.032). At HT, urine F₂-Isop was higher compared with LT (d = 0.21, P = 0.036), but plasma F₂-Isop was lower (d = –0.22, P = 0.008), with no difference between treatments. At HT, AGC had higher subjective energy concentrations (d = 0.21, P = 0.02) and urinary nitrite (d = 0.23, P = 0.03) compared with CON and higher creatine kinase (d = 0.24, P = 0.02) and less fatigue (d = –0.20; P = 0.05) at T. Conclusions: Although not beneficial for 5CTT performance or exercise efficiency, AGC increases fat oxidation during exercise, NO bioavailability, and subjective energy concentrations, which may confer benefits for health and wellbeing. This trial was registered at www.anzctr.org.au as ACTRN12618000360213.