The form of carbohydrates ingested before exercise may have different effects on both metabolism and performance. Moreover, the ingestion of solid foods slows gastric empty, digestion and absorption rates compared with liquid foods and this has a different impact on glycemia.
For these reasons, several studies have investigated the effects of the form of carbohydrates on glycemic responses, oxidation rates and performance.
Studies comparing solid versus liquid carbohydrates and solid versus gel carbohydrates have found no difference in glycemic responses between groups.
Studies that have investigated difference in performance effects have found no differences.
Furthermore, no differences are found in carbohydrate oxidation rates between the carbohydrate ingestion in the three forms during exercise.
Hypoglycemia: strategies to limit it in susceptible subjects
From several studies it appears that the risk of developing hypoglycemia (blood glucose < 3.5 mmol /l or < 63 mg/l) is highly individual: some athletes are very prone to develop it and others are much more resistant.
A strategy to minimize glycemic and insulinemic responses during exercise is to delay carbohydrate ingestion just prior to exercise: in the last 5-15 min before exercise or during warm-up (even though followed by a short break). Why?
Warm-up and then exercise increase catecholamine concentrations blunting insulin response.
Moreover, it has been shown that ingestion of carbohydrate-containing beverages during a warm-up (even if followed by a short break) does not lead to rebound hypoglycemia, independent of the amount of carbohydrates, but instead increases glycemia. When carbohydrates are ingested within 10 min before the onset of the exercise, exercise will start before the increase of insulin concentration.
Therefore, this timing strategy would provide carbohydrates minimizing the risk of a possible reactive hypoglycaemia.
In addition, it is possible to choose low glycemic index carbohydrates that lead to more stable glycemic and insulinemic responses during subsequent exercise.
An intriguing observation is the lack of a clear relation between hypoglycaemia and its symptoms (likely related to a reduced delivery of glucose to the brain). In fact, symptoms are often reported in the absence of true hypoglycemia and hypoglycemia is not always associated with symptoms. Though the cause of the symptoms is still unknown, it is clearly not related to a glycemic threshold.
Some athletes develop symptoms similar to those of hypoglycemia, even though they aren’t always linked to actual low glycemia. To minimize these symptoms, for these subjects an individual approach is advisable. It may include:
carbohydrate ingestion just before the onset of exercise or during warm-up;
choose low-to-moderate GI carbohydrates that result in more stable glycemic and insulinemic responses;
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”.
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.