Carbohydrate ingestion 60 min before exercise

Introductory statement

Carbohydrates
Fig. 1 – Carbohydrates

An high-carbohydrate diet in the days before exercise, as well as ingestion of meals high in carbohydrate 3-4 h before exercise, better if with low glycemic index, can have positive effects on athlete’s performance.

For many years it has been suggested that ingestion of carbohydrates 30-60 min before exercise may adversely affect performance because it could cause hypoglycemia (blood glucose < 3.5 mmol/l or < 63 mg/l), a major contributor to fatigue. In fact, a typical athlete’s mantra is: “Avoid carbohydrate in the hour before exercise”!
What is the reason of that?
Glucose ingestion may cause hyperglycaemia followed by hyperinsulinaemina that may result in:

  • a rapid decline in glycemia 15-30 minutes after the onset of exercise, called rebound or reactive hypoglycaemia, most likely the result of:

I. an increase in muscle glucose uptake (due to the mobilization of GLUT-4 transporters by the action of insulin but also from physical activity itself);
II. the reduction in liver glucose output;

  • in addition, higher availability of carbohydrates to the muscle stimulates glycolysis and this, in combination to insulin-induced inhibition of lipolysis in both adipose tissue and muscle, results in a reduction in fat oxidation (apparently long-chain fatty acids, not medium-chain fatty acids). This may lead to premature glycogen depletion and early onset of fatigue (glycogen would be almost the only available fuel for working muscle).
    This effect is temporary, approximately lasting only for the first 20 min of exercise so, it is likely that this little glycogen breakdown has no significant effect on exercise performance.

Therefore, at least in theory, carbohydrate ingestion 60 minutes before exercise could affect performance but only two studies (Foster et al. 1979, e Kovisto et al. 1981) have reported a reduced endurance capacity while the majority of studies have reported no change or an improvement in performance.
To clarify these results, a systematic series of studies was done in trained subjects. The conclusion of these studies was that:

  • There is no effect of pre-exercise carbohydrate feeding on performance, even though in some cases hypoglycaemia did develop”.
  • There was no relationship between low blood glucose concentrations and performance”. (Jeukendrup and Killer S.C. 2010)

Conclusion

Ingestion of meals rich in carbohydrates 3-4 h before exercise is important for the increase of liver and muscle glycogen stores, or for their resynthesis in previously depleted muscle and liver.
Carbohydrate ingestion 30-60 min before exercise may be important in topping-up liver glycogen stores which serve to maintain blood glucose concentrations during exercise.
Based on the currently available scientific evidences, there is no reason to avoid carbohydrates 60 min before the onset of exercise, because they don’t seem to have any detrimental effect on performance.

References

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