Journal Title (Medline/Pubmed accepted abbreviation): Sports Med
Page numbers: 941-959
Summary of article:
The authors have written a comprehensive review article (19 pages) summarizing the currently available studies examining muscle glycogen resynthesis in response to ingesting varying amounts of carbohydrate during a short-term (ie, > 2 to 6 hours) recovery in humans. The main purpose of the review is to consider the direct effects of ingesting carbohydrate protein mixtures on physical performance, whether or not related to muscle glycogen resynthesis. The review outlines the results of 33 major studies on this subject (Table 1 is an excellent overview of the studies). Studies using nonexhaustive running (n = 2) or resistance-type (n = 2) exercise protocols before recovery are analyzed as well as the more commonly employed cycling protocols, which are both exhaustive (n = 13) and nonexhaustive (n = 16) in nature. Additional topics of discussion include the metabolic responses (eg, glucose and insulin); recommendations about the optimal timing of supplementation; and type, form, and quantity of carbohydrate or carbohydrate-protein mixtures that should be ingested during short-term post-exercise recovery.
Summary of research findings:
- Timing of carbohydrate ingested:
- More rapid rates of muscle glycogen resynthesis are typically achieved when carbohydrate is provided immediately after exercise at relatively frequent intervals (ie, every 15 to 30 minutes).
- Delaying carbohydrate ingestion by just >= 2 hours following exercise can result in a 50% reduction in the rate of muscle glycogen resynthesis.
- Type/form of carbohydrate ingested:
- High glycemic index (GI) carbohydrates, as opposed to low GI carbohydrates, can accelerate muscle glycogen resynthesis over the first 6 hours of recovery.
- Lower GI carbohydrates during recovery can improve the capacity for continuous exercise either later the same day or on the following day.
- Whether carbohydrate is ingested in solid or liquid form does not appear to influence the rate of muscle glycogen storage during recovery.
- Amount of carbohydrate ingested:
- The ingestion of ~1g/kg/hour of carbohydrate was associated with highest reported rates of glycogen resynthesis (45 to 50 mmol glucosyl units/kg dry mass/hour) during short-term recovery (> 2 to 6 hours).
- The metabolic and/or ergogenic consequences of ingesting protein or carbohydrate-protein mixtures during post-exercise recovery:
- The protein intake of 0.3 to 0.5 g/kg/hour was linked with highest reported insulin concentrations.
- 13 studies have reported significantly lower blood glucose concentrations following ingestion of a carbohydrate-protein mixture rather than carbohydrate alone.
- Co-ingestion of carbohydrate (~0.8 g/kg/hour) and protein (>= 0.3 g/kg/hour) during recovery led to increased rate of muscle glycogen storage during recovery vs ingesting carbohydrate alone.
- Ingesting >= 1g/kg/hour of carbohydrate during recovery resulted in no changes in muscle glycogen resynthesis when protein was added.
- Combined carbohydrate-protein ingestion and physical performance:
- Several studies have subsequently confirmed that including protein in a carbohydrate supplement postexercise can indeed postpone fatigue during a subsequent bout of exhaustive exercise.
- Eight studies have reported no ergogenic effect of ingesting added protein in terms of either exercise capacity (ie, time to fatigue at 70% to 85% peak oxygen uptake [VO2peak]) or exercise performance (ie, time to complete 7 kJ/kg; 880 to 27 kJ; 80-km time trial; 6-km time trial).
Interpretation of findings/Key practice applications:
This review covers some aspects of the optimal nutritional strategy for recovery from prolonged moderate- to high-intensity exercise. The most effective way to rapidly replenish depleted glycogen reserves is likely to involve ingesting a high GI carbohydrate source at a rate of at least 1 g/kg/hour (when ingested alone) or <= 0.8 g/kg/hour (when >= 0.3 g/kg/hour protein is added), beginning immediately after exercise and then at frequent (ie, 15- to 30-minute) intervals thereafter. Some studies have reported improved physical performance with ingestion of carbohydrate-protein mixtures, yet many equally well-controlled studies do not support these findings. Far fewer studies have examined the relationship between carbohydrate intake and exercise capacity. Future research is warranted to determine the precise causal relationships between post-exercise carbohydrate intake, muscle glycogen resynthesis, and restoration of physical performance.