Nutritional strategies undeniably contribute to recovery from one training bout to another; crucial to achieving training adaptations. Athletes often train multiple times per day, requiring well considered eating strategies to optimise recovery; specifically replenishing muscle glycogen stores. Post-exercise carbohydrate availability is a key factor determining the rate of glycogen resynthesis. Nutritional guidelines suggest to consume carbohydrate at 1.2 g/kg body mass in the hours following exercise, however many athletes struggle to consume this.  Therefore, it is important to maximise the availability of glucose to support muscle glycogen resynthesis in the face of sub-optimal carbohydrate consumption.
Blackcurrants, one of the richest sources of polyphenols, includes high concentrations of anthocyanins, a major flavonoid subclass. New Zealand blackcurrant (NZBC) extract supplementation has shown potential ergogenic properties that induce physiological and metabolic responses. Cell culture investigations revealed that anthocyanins enhance glucose transporter protein 4 (GLUT4) translocation to the plasma membrane, increasing cellular glucose uptake. We have shown that one week of supplementation with anthocyanin-rich NZBC extract improves glucose clearance both following a mixed meal and in response to an oral glucose load. Supplementation with NZBC extract could therefore augment the uptake of exogenous carbohydrate during exercise recovery.  Thus, we aimed to investigate the effects of 7 days of NZBC supplementation on blood glucose responses following post-exercise carbohydrate ingestion.
Eight amateur cyclists (mean ± SD: age, 22 ± 4 years; body mass, 77.4 ± 4.7 kg; height, 1.81 ± 0.09 m; VO2max 55.5 ± 5.6 ml/kg/min) completed a randomised, cross-over, double-blind, placebo-controlled study. On day 6 of supplementation, participants performed a cycling protocol lasting ~90–120 min to deplete muscle glycogen, followed by a low-carbohydrate evening meal. The following morning (before breakfast), participants cycled for 45 min at 50% Wmax before performing repeated intervals to exhaustion to measure exercise capacity. Following exercise, participants consumed a sub-optimal dose of carbohydrate (0.8 g/kg BW) for 4 hours, every 15 min.
Exercise capacity was not different (P=0.095) between trials (placebo, 415 ± 44s; NZBC 454 ± 38s). There was no significant difference in blood glucose concentrations following exercise (P=0.702) between trials (placebo, 3.17 ± 0.69 mmol/L; NZBC, 3.24 ± 0.77 mmol/L). Glucose area under the curve was also not different between trials (Placebo, 1363 ± 72; NZBC, 1334 ± 56). However, NZBC lead to noticeably lower glucose concentrations 30 min post-exercise (P = 0.091 ± 1.4 mmol/L). although this did not reach significance.
Currently, the results show little difference between conditions in the blood glucose response during 4 hours of carbohydrate feeding in a glycogen-depleted state. However, blood glucose concentrations appear to be greater in the placebo trial at 30 min post-carbohydrate ingestion. This could indicate greater rates of blood glucose, augmenting the availability of glucose for muscle glycogen resynthesis. To investigate this hypothesis, we will now analyse muscle biopsies obtained at 0, 60- and 240-minutes following exercise to determine the rate of glycogen resynthesis. If this data supports our hypothesis, NZBC supplementation could be used to aid faster glycogen resynthesis, benefiting athletes who require short term recovery.