Glycaemic Index

The glycaemic index (GI) is a ranking of foods between 0 and 100 according to the extent to which they raise blood sugar levels after being eaten. It should be noted that the glycaemic load of a food should also be taken into consideration when determining the effect on blood sugar levels after ingesting carbohydrates. In this blog, I’ll discuss low and high glycaemic (GI) meals and when it’s appropriate to eat them.


Pre-exercise meal

It has often been quoted that a pre exercise meal is best chosen from low GI foods. The rationale for this belief is that, if a high GI carbohydrate (CHO) meal is taken before training or competing, the relatively rapid rise in blood glucose will cause a corresponding release of insulin.

The feared effects of high GI CHO were:

  • an increased rate of early glucose oxidation
  • a ‘rebound hypoglycaemia’ effect where the increased insulin causes a rapid fall in glucose levels possibly even before the session has begun

Research has shown that cyclists given a low GI meal consisting of lentils eaten one hour before intensive exercise, performed for longer before fatiguing, when compared to those fed on a high GI meal. The research suggested that glycogen sparing may have occurred with the low glycaemic trial, thus promoting better performance. However, post training glycogen levels were never measured, and subsequent studies have failed to prove any clear benefit from pre-feeding on a low glycaemic meal.

The majority of studies show that there may be slightly more favourable metabolic conditions with regards to insulin levels during exercise associated with low GI foods than with high GI alternatives; these differences are small and short lived. The conclusion is that athletes probably perform the same on both pre-race meals.

The real difference in performance appears to be related to carbohydrate feeding during exercise, which seems to override any metabolic or performance effects arising from the type of pre-event meal. Athletes should consume adequate amounts of carbohydrate drinks during endurance exercise, and may feel free to choose their pre-exercise meal according to their personal preferences.


Carbohydrates taken during exercise

Ingesting carbohydrates is acceptable if:

  • the session is longer than an hour
  • the match or race is longer than 90 minutes
  • if pre-exercise meal is not possible (early morning intensive training)

The consumption of isotonic drinks during exercise has been shown to delay the onset of fatigue and improve performance in endurance athletes. Many athletes find it difficult to consume even a light meal before exercise with causing discomfort, or they simply may not have time before their planned training session. Ingesting an isotonic drink during endurance training is as effective as the pre-training carbohydrate meal. The replacement of fluid provided by the isotonic drink is also a direct advantage.


Post-exercise meal

After intensive exercise, the muscles are more sensitive to the effects of insulin thus enabling more efficient replacement of lost glycogen. This process is particularly evident during the first two hours following the training session. The rapid synthesis of muscle glycogen stores is aided by the immediate intake of high GI carbohydrate. Studies have shown that the first intake of carbohydrate should be taken within 15 minutes of the workout.



We are not all athletes, but the guidelines mentioned above seem to work for the general gym goer. I would suggest that eating a balanced low GI meal 2 hours before training, followed by a high GI snack like a banana with a protein shake within the 15 minute post-exercise, followed by a suitable balanced meal within 2-3 hours of exercise. The reason for the high GI fuel within 15 minutes of exercise is to get your body out of the catabolism (using your muscle for fuel) and getting it back as quickly as possible into anabolism (building muscle).