How should I refuel between two hard sessions on one day?
Start straight after the first: the highest rates came with 1.0-1.85 g/kg/h of carbohydrate every 15-60 min for up to 5 h; waiting hours may halve it (B+, abstract).
Jentjens and Jeukendrup review what decides how fast muscle glycogen is rebuilt in the first hours after glycogen-depleting exercise.
In the abstract the process has two phases: a rapid phase lasting about 30-60 minutes that does not require insulin and is driven by exercise moving the glucose transporter GLUT4 to the cell surface, followed by a much slower phase that can last several hours.
Muscle contraction and insulin both activate glycogen synthase, the rate-limiting enzyme, and a low glycogen concentration itself speeds glucose transport and storage.
The highest synthesis rates were reported with large amounts of carbohydrate (1.0-1.85 g/kg/h) eaten immediately after exercise and at 15-60-minute intervals for up to 5 hours; a delay of several hours "may lead to ~50% lower rates".
Protein or amino acids added to carbohydrate raise the rate, probably through more insulin, but only when carbohydrate is insufficient (below 1.2 g/kg/h); at 1.2 g/kg/h or more, taken regularly, they add nothing to glycogen.
The authors name possible limits: much of the ingested glucose is taken up by the liver, other muscles and fat, and intestinal absorption may cap synthesis when more than 1 g/min of glucose is eaten.
Our reading: this is the short-term companion to Burke, van Loon and Hawley's later review (REF-1336, which cites it as ref 52), and its "~50% lower" fits Ivy 1988 (REF-056, 45% slower once the delayed group ate).
The two reviews draw the "early" phase differently — 30-60 min of insulin-independent synthesis here, 0-4 h in REF-1336 — and put the protein threshold at 1.2 g/kg/h here and about 1 g/kg/h there; both say protein helps only when carbohydrate is short.
Protocol
Key Results
What the abstract states (rates in the abstract are relative; no absolute rate is given):
| Point | The abstract's statement |
|---|---|
| Phase 1 | rapid synthesis, no insulin required, lasting about 30-60 min; exercise-induced translocation of GLUT4 to the cell surface |
| Phase 2 | much slower synthesis that can last several hours |
| Regulators | muscle contraction and insulin raise glycogen synthase activity; glycogen concentration is a potent regulator of the enzyme; low glycogen goes with faster glucose transport and storage |
| Highest rates | large amounts of carbohydrate (1.0-1.85 g/kg/h) immediately post-exercise and at 15-60 min intervals, for up to 5 hours |
| Delay | carbohydrate delayed by several hours "may lead to ~50% lower rates" |
| Protein / amino acids | can raise rates, most probably through a larger insulin response; no further effect when carbohydrate is ≥1.2 g/kg/h at regular intervals; may help when carbohydrate is insufficient (<1.2 g/kg/h); may also stimulate net muscle protein anabolism |
| Limits proposed | uptake of ingested glucose by liver, other muscle groups and fat; intestinal glucose absorption when more than 1 g/min of glucose is ingested |
Mechanisms
Practical Application
Limitations
How this entry was checked and changed
authoritative narrative review from a leading exercise-nutrition group (Jeukendrup, then at the University of Birmingham), the rubric's line; read from the abstract only, so the studies behind its numbers, their size and their populations could not be checked