TYPES OF CARBOHYDRATE
Studies that have directly measured the effects produced by the ingestion of glucose, sucrose and maltodextrins, either alone or in combination, have found little differences in their ability to maintain blood glucose concentration, and carbohydrate oxidation or to improve performance (2,14,23,24). Studies that depart from this conclusion have been observed, but the amounts as well as the types of carbohydrate were altered (3). Recently maltodextrins have become a popular form of carbohydrate in sport drinks. Probably the major reason for this lies in the fact that this type of sugar is not very sweet tasting. Accordingly, solutions containing >10% carbohydrate are more palatable for most people (3). Contrasting with the sugars mentioned previously, the ingestion of fructose during prolonged exercise might not improve performance as a result of its slower rate of absorption by the gut and subsequent conversion to glucose in the liver (24). Therefore, its ingestion will result in lowered blood glucose and insulin concentrations and consequently reduced rates of carbohydrate oxidation. Also, it may be worth noting that fructose ingestion may increase the incidence of gastrointestinal distress in certain athletes.

LIQUID VERSUS SOLID CARBOHYDRATE
Few studies have examined the effects of solid carbohydrate ingestion on energy metabolism and endurance exercise performance. Fielding et al. (13) and Hargreaves et al. (16) both compared the metabolic effects and endurance performance times of a solid carbohydrate candy bar, which contained some protein and fat, with the ingestion of a placebo solution during intermittent exercise. Both studies showed that solid carbohydrate ingestion every hour during 4 hours of exercise elevated blood glucose and delayed fatigue, thus increasing performance. Mason et al. (22) and Ivy et al. (19) further determined that solid and liquid carbohydrate feedings during 2 to 3 hours of exercise were equally effective in maintaining blood glucose and insulin responses (22).

More recently, Robergs et al. (25) have evaluated the blood glucose responses and exercise performances resulting from solid carbohydrate ingestion with added protein and fat during prolonged steady-state exercise. This was of interest since most carbohydrate bars on the market contain some protein and fat. Although the stomach emptied solid carbohydrate feedings containing fat and protein more slowly when compared with liquid carbohydrate ingestion, the results of the study indicated that blood glucose levels and exercise performances remained similar between both trials. Collectively these studies, in my opinion, do not provide a complete evaluation of solid carbohydrate ingestion due to differences in exercise protocols, types of carbohydrate used and additional amounts of fat and protein in solid bars. Nonetheless, in the light of these studies, it can probably be concluded that liquid and solid carbohydrate feedings during exercise are equally efficient in maintaining blood glucose level, carbohydrate oxidation and performance.

PRACTICAL IMPLICATIONS FOR ATHLETES
On the basis of the most recent findings highlighted in this brief article, the following recommendations are made to athletes participating in endurance and ultra-endurance training or competition:
1. Because the rate of muscle glycogen utilization in working muscles is unaffected by the ingestion of carbohydrate, athletes are therefore encourage to maximize their muscle glycogen store before prolonged exercise, irrespective of the decision to ingest or not carbohydrate during that exercise;

2. As only about 20 grams of consumed carbohydrate are oxidized in the first hour of exercise, athletes are consequently invited to ingest a carbohydrate solution containing 3-5 grams 100 ml -1 every 10 minutes for the first 60-75 minutes of exercise. Ingesting more than this quantity of carbohydrate may possibly retard fat mobilization, thus increasing reliance on muscle glycogen utilization;

3. After the first 60 to 75 minutes of exercise, athletes must increase their carbohydrate consumption to match the peak rates (~1 gram minute -1) of plasma glucose utilization. Table 1 lists the volume as well as the concentration of various carbohydrate solutions to consume to match this recommendation;

4. To assure a high rate of gastric emptying, athletes must continuously maintain their stomach fluid volume at approximately 300 ml to 500 ml. This may be achieved by consuming 400 ml at the start of exercise and by repeated drinking of 100-150 ml every 10 minutes. Moreover, athletes are encouraged to consume a carbohydrate solution concentrated at no more than 8g 100 ml -1 and to ingest no more than 1000 ml of fluid/hour;

5. In the light of the studies, both liquid and solid carbohydrate seem to be equally effective in maintaining blood glucose level and carbohydrate oxidation during exercise. Consequently, athletes may choose to use a combination of liquid and solid carbohydrate consumption during exercise for i) the replenishment of lost water and electrolytes from the body and ii) reasons of satiety, respectively;

6. Finally, while carbohydrate ingestion during exercise decreases the rates of liver glycogen transformation to blood glucose and therefore preventing or retarding hypoglyceamia, it can not prevent fatigue or exhaustion that corresponds with critically low level of muscle glycogen (20 mmol/kg -1 wet weight muscle)

Eric

Additional background information and references to this article now follow.

BACKGROUND INFORMATION
It was as early as the 1920s during the prestigious Boston Marathon that exercise scientists first demonstrated that carbohydrate ingestion during prolonged exercise (>90 min.) enhanced performance in runners by preventing the development of hypoglycaemia (15,21). This finding in the field was then confirmed by Dill et al. (12) in 1932 during researches conducted in the distinguished Harvard Fatigue Laboratory in Boston. In fact, they showed that when their dogs, Joe and Sally, were fed 20g/carbohydrate/hour, their treadmill running endurance times to fatigue were improved by about 15 hours. Unfortunately, these results were largely ignored by the athletic community up until the early 1980s.

Between the 1930s and the early 1960s, fundamental research conducted in exercise physiology focussed primarily on the importance of replacing lost water during exercise to prevent dehydration and hyperthermia rather than consuming carbohydrate to enhance performance. In the 1960s, which coincided with the appearance of the muscle biopsy, Scandinavian researchers showed that the depletion of muscle glycogen was closely related to the development of fatigue and exhaustion (1). Accordingly, their primary concerns during these years were to find ways to increase pre-start exercise muscle glycogen concentration to increase time to fatigue; from there came the development of the glycogen loading technique known today by every ultra-endurance athlete.