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.
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.
During endurance exercise, the most likely contributors to fatigue are dehydration andcarbohydrate depletion, especially of muscle and liver glycogen.
To prevent the “crisis” due to the depletion of muscle and liver carbohydrates, it is essential having high glycogen stores before the start of the activity.
What does affect glycogen stores?
The diet in the days before the competition.
The level of training (well-trained athletes synthesize more glycogen and have potentially higher stores, because they have more efficient enzymes).
The activity in the day of the competition and the days before (if muscle doesn’t work it doesn’t lose glycogen). Therefore, it is better to do light trainings in the days before the competition, not to deplete glycogen stores, and to take care of nutrition.
The “Swedish origin” of carbohydrate loading
Very high muscle glycogen levels (the so-called glycogen supercompensation) can improve performance, i.e. time to complete a predetermined distance, by 2-3% in the events lasting more than 90 minutes, compared with low to normal glycogen, while benefits seem to be little or absent when the duration of the event is less than 90 min.
Well-trained athletes can achieve glycogen supercompensation without the depletion phase prior to carbohydrate loading, the old technique discovered by two Swedish researchers, Saltin and Hermansen, in 1960s.
The researchers discovered that muscle glycogen concentration could be doubled in the six days before the competition following this diet:
three days of low carb menu (a nutritional plan very poor in carbohydrates, i.e. without pasta, rice, bread, potatoes, legumes, fruits etc.);
This diet causes a lot of problems: the first three days are very hard and there may be symptoms similar to depression due to low glucose delivery to brain, and the benefits are few.
Moreover, with the current training techniques, the type and amount of work done, we can indeed obtain high levels of glycogen: above 2.5 g/kg of body weight.
The “corrent” carbohydrate loading
If we compete on Sunday, a possible training/nutritional plan to obtain supercompensation of glycogen stores can be the following:
Wednesday, namely four days before the competition, moderate training and then dinner without carbohydrates;
from Thursday on, namely the three days before the competition, hyperglucidic diet and light trainings.
The amount of dietary carbohydrates needed to recover glycogen stores or to promote glycogen loading depends on the duration and intensity of the training programme, and they span from 5 to 12 g/kg of body weight/d, depending on the athlete and his activity. With higher carbohydrate intake you can achieve higher glycogen stores but this does not always results in better performance; moreover, it should be noted that glycogen storage is associated with weight gain due to water retention (approximately 3 g per gram of glycogen), and this may not be desirable in some sports.