Metabolic Responses to Carbohydrate Ingestion during Exercise: Associations between Carbohydrate Dose and Endurance Performance

Abstract

Carbohydrate (CHO) ingestion during exercise lasting less than three hours improves endurance exercise performance but there is still debate about the optimal dose. We utilised stable isotopes and blood metabolite profiles to further examine metabolic responses to CHO (glucose only) ingestion in the 20–64 g·h−1 range, and to determine the association with performance outcome. In a double-blind, randomized cross-over design, male cyclists (n = 20, mean ± SD, age 34 ± 10 years, mass 75.8 ± 9 kg, peak power output 394 ± 36 W, VO2max 62 ± 9 mL·kg−1·min−1) completed four main experimental trials. Each trial involved a two-hour constant load ride (185 ± 25 W) followed by a time trial, where one of three CHO beverages, or a control (water), were administered every 15 min, providing 0, 20, 39 or 64 g CHO·h−1. Dual glucose tracer techniques, indirect calorimetry and blood analyses were used to determine glucose kinetics, exogenous CHO oxidation (EXO), endogenous CHO and fat oxidation; and metabolite responses. Regression analysis revealed that total exogenous CHO oxidised in the second hour of exercise, and suppression of serum NEFA concentration provided the best prediction model of performance outcome. However, the model could only explain ~19% of the variance in performance outcome. The present data demonstrate that consuming ~40 g·h−1 of CHO appears to be the minimum ingestion rate required to induce metabolic effects that are sufficient to impact upon performance outcome. These data highlight a lack of performance benefit and few changes in metabolic outcomes beyond an ingestion rate of 39 g·h−1. Further work is required to explore dose-response effects of CHO feeding and associations between multiple metabolic parameters and subsequent performance outcome.

Association of Daily Dietary Intake and Inflammation Induced by Marathon Race

“…In the 1990s, many researchers demonstrated that ingestion of carbohydrate supplements (30–60 g/h) during endurance/intense exercise attenuated increases in blood neutrophil and monocyte counts, stress hormones, IL-6, IL-10, and IL-1ra and improve immune function. It is also a consensus that utilization of carbohydrate during endurance exercises improve performance and endogenous glycogen stores [18, 24]. Endogenous glycogen stores and circulating plasma glucose are fuel sources for energy provision during endurance exercise, and fatigue coincides with depletion of glycogen store.…”

“…Endogenous glycogen stores and circulating plasma glucose are fuel sources for energy provision during endurance exercise, and fatigue coincides with depletion of glycogen store. Carbohydrate intake during exercise seems to suppress endogenous carbohydrate oxidation by around 10% [24]. However, the amount of muscle glycogen oxidation during prolonged exercise is high, around 100 g per hour.…”

Regulation of Muscle Glycogen Metabolism during Exercise: Implications for Endurance Performance and Training Adaptations

“…When taken together, it is currently thought that CHO feeding during exercise may, therefore, augment exercise performance via multiple mechanisms, consisting of muscle glycogen sparing [ 22 ], liver glycogen sparing [ 53 ] and maintenance of plasma glucose and CHO oxidation rates [ 8 ]. The role of CHO feeding in reducing liver glycogen breakdown as a performance enhancing mechanism is gaining increasing recognition [ 78 ]. In simple terms, a liver glycogen sparing effect ensures that more liver glycogen is, therefore, available late in exercise, thereby maintaining plasma glucose availability and delivery to the muscle to meet the CHO oxidation rates necessary to sustain the required workload.…”

“…As such, it is now advised that athletes should clearly practice their approach to in-competition fueling during training sessions of a similar intensity and duration as competition. As a general rule of thumb, it is suggested that 30–60 g·h −1 of CHO (glucose polymers e.g., maltodextrin) is consumed during events lasting < 60–150 min [ 78 ], whereas in events > 2.5–3 h, 60–90 g·h −1 (glucose/fructose blends) is the recommended rate [ 7 ]. Whilst beyond the scope of the present review, it is noteworthy that CHO ingestion (in either drink or gel format) during team sport type activity (i.e., <90 min duration) can also improve performance of technical skills (see reference [ 89 ] for an extensive review on this topic), thus providing further evidence for the ergogenic properties of CHO feeding during exercise.…”

Prolonged cycling exercise alters neural control strategy, irrespective of carbohydrate dose ingested

“…22,23 We have previously demonstrated in a parallel study of the same cohort that ingestion of CHO at a rate ≥39 g•h −1 leads to endogenous glycogen sparing during exercise. 22 CHO ingestion also preserves neuromuscular drive, as reflected by reduced EMG amplitude, attenuating peripheral fatigue during steady-state cycling to exhaustion 24 ; but there are few data that allude to what aspect of the neural strategy is altered in response to feeding CHO. Further research has proposed the existence of a muscle "glycostat" whereby chemoreceptor sensing of low glycogen stores could provide afferent feedback to the CNS to regulate the neural control strategy at the muscle, optimizing pacing strategy by regulating exercise intensity based on glycogen availability.…”