Nutritional and Health Aspects of Sweeteners - Sweeteners: Nutritional Aspects, Applications, and Production Technology - page 353

11.6 hUMaN perFOrMaNCe appLICatIONS
The classical study of Christensen and Hansen in 1939 demonstrated the importance of carbohy-
drates in the diet of athletes (Christensen and Hansen 1939). In the 1960s, when the biopsy technique
of obtaining muscle samples was made easier (Bergstrom 1962), several studies showed that exercise-
induced reductions in the body's limited muscle glycogen stores correlated well with the development
of fatigue (Ahlborg et al. 1967; Hermansen et al. 1967; Saltin and Karlsson 1971; Gollnick et al.
1973). As a consequence, “carbohydrate-loading” strategies were developed (Astrand 1967; Sherman
et al. 1981), and the ingestion of carbohydrate-electrolyte drinks (sports drinks) before, during, and
after exercise became a common practice for athletes. It was also the beginning of a new challenge
for the food industry to develop sports-speciic food products and drinks. Such sports drinks should
be designed to provide luid and energy in the form of carbohydrates to the exercising individual
without causing any gastrointestinal discomfort, which is sometimes a problem during exercise
(Brouns and Beckers 1993). Currently, many products have been launched in the market in liquid,
semiliquid (gel), solid (bars), or powder form. Speciic products with different formulations accord-
ing to the intended use are offered to the athlete—products for optimizing fuel (body carbohydrate)
levels before exercise, maintaining blood glucose levels and postponing the onset of fatigue during
exercise, and speeding up recovery and enhancing muscle development after exercise.
Various natural and artiicial sweeteners are used in all these products. Glucose (dextrose),
sucrose, fructose, glucose syrup, HFCS, and glucose polymers such as maltodextrin are some com-
mon natural sweeteners used in sports drinks for providing energy. Low-calorie sweeteners like Ace
K, sucralose, aspartame, and truvia (rebiana) are also used in some “low-calorie sports drinks,” for
example, Powerade Option by the Coca-Cola Company and All Sport Naturally Zero by PepsiCo.
In this section, the claims “performance enhancement,” “maintenance of euhydration,” “improved
recovery,” and “increased muscle mass” made by manufacturers of sports beverages will be briely
examined using a nutrient timing approach. Extensive analysis of these issues is beyond the scope
of this topic and can be found elsewhere (Austin and Seebohar 2011; Burke 2007; Jeukendrup and
Gleeson 2010).
11.6.1 Ingestion of Carbohydrates before exercise
The reasons for consuming carbohydrate before exercise are to enhance the availability of gly-
cogen in the muscle and liver and of glucose in blood. However, in an early study conducted by
(Costill et al. 1977), it was found that the ingestion of 75 g of glucose in 300 mL of water 45 min
before the start of submaximal treadmill running caused a greater rate of muscle glycogen utiliza-
tion than when exercise was performed after drinking water. This, of course, is not favorable for the
athlete, because it is well known that body glycogen stores are limited and an elevation in the rate of
glycogenolysis during prolonged exercise could lead to an early onset of fatigue. In fact, researchers
from the same laboratory conirmed this hypothesis in a study where cyclists fatigued sooner when
they ingested a similar glucose solution 30 min before exercise compared to water ingestion (Foster
et al. 1979).
The indings of these two studies were explained by the hyperglycemia and hyperinsulinemia
caused as a consequence of the preexercise ingestion of high-GI carbohydrates such as glucose.
This hyperinsulinemia before exercise depressed fatty acids during exercise, denying in this way
this important substrate to exercising muscle. As a consequence, the body relied more on the lim-
ited glycogen stores, which in turn caused premature fatigue (Costill et al. 1977; Foster et al. 1979;
Hargreaves et al. 1985; Horowitz et al. 1997). However, since the early study of Foster et al. in 1979,
many studies have reported either improvements in endurance capacity and performance or no effect
as a result of ingesting various types of carbohydrates such as glucose, fructose, and maltodextrin
in liquid or even in solid form (Coombes and Hamilton 2000). Furthermore, several attempts have