CHO ingestion has been found time and time again to aid in intense and prolonged endurance performances. Why? Because two of the mechanisms behind fatigue are decreased blood [glucose] and muscle glycogen content. The two go hand in hand, such that when muscle glycogen is depleted, blood glucose will be metabolized -- and when blood glucose is metabolized and not maintained through ingestion of CHO, blood glucose will drop.
Remember glycolysis? Then you remember glucose is necessary to run glycolysis. And there are two sources of glucose for working muscle cells - glycogen and blood glucose. When an athlete has low glycogen and low blood glucose, his ability to run glycolysis will be limited by CHO availability and exercise intensity will decline (Wright et al., 1991).
Blood glucose - In summary, glucose provides substrate for glycogen synthesis and immediate energy through glycolysis. Keeping blood glucose elevated, or preventing the decline of blood glucose during exercise allows the athlete to maintain a greater intensity while training. Does training at higher intensities elicit greater training responses? No doubt. Could training in a glycogen depleted state improve endurance performance? Maybe, maybe not... I am doubtful. It may depend on the athlete's goals, but note there was no difference in the one variable that matters here -- cycling performance.
It's a lot like the research I've been working on lately with David Morris at ASU. Which yields greater performance gains? Hypoxic or hyperoxic training? Can training with hyperoxic gas (60% O2) improve time trial performance at a simulated altitude of 14,000 ft? Hyperoxic gas enables the athletes to train at a higher work rate (as does training with saturated glycogen stores) on the bike and training at a higher work rate may elicit a greater training response. Further, can training with hyperoxic gas improve TT performance more than training with hypoxic gas (13% O2). The decreased oxygen content significantly reduces the work rate the athletes can sustain during training (like training in a glycogen depleted state) -- but might they get other adaptations from training at altitude? Our preliminary results are intriguing.
But, getting back to maintaining glycogen stores: To maintain training intensity, how can one keep glycogen stores and blood glucose high?
Take a look at the figure above and note, it all boils down to carbohydrate intake. But not all carbohydrates are the same. Muscle metabolizes glucose, and that glucose comes from a number sources in the diet:
1) Glucose and/or glucose polymers
2) Fructose, sucrose, galactose, and other sugars not glucose
3) Amino Acids
4) Fatty Acids
As you might have guessed, numbers 3,4, and 5 are not favorable substrates for raising or maintaining blood glucose levels -- but they can be converted to glucose in the liver in dire situations. Fructose is similar. Fructose cannot be metabolized by working muscle as fructose. Instead, it must be transported to the liver and converted to glucose. But, some of that fructose will not be converted to glucose. Some of it may be converted to lactate. Not only can conversion of fructose to lactate increase blood [lactate], the CHO available to working muscles will be lower than if the same amount of glucose had been ingested. Note, I lumped sucrose with fructose, because it is not technically glucose. Sucrose is a disacharride made of one molecule of fructose bound to one molecule of glucose. Despite being 50% glucose, when the same amount is consumed, sucrose will not elevate blood glucose as much as glucose alone (Murray et al., 1989).
Another problem with fructose and sucrose is that they do not stimulate insulin secretion to the degree of glucose does -- even when equal amounts are consumed. This will be more important following exercise, when insulin is need to help transport glucose into muscle cells and activate glycogen synthase.
Further, glucose has a faster intestinal absorption rate (and helps with water absorption). Glucose is actively transported out of the small intestine. Fructose, however, must be passively absorbed -- delaying intestinal absorption rate, and possibly leading to gastric distress. (Murray et al., 1989)
In summary - glucose is the superior form of CHO to ingest in and around exercise. It is actively transported out of the gut, it raises blood glucose directly and can be metabolized by working muscle immediately, and after exercise, it is a better secretagogue of insulin - enabling transport into muscle cells, activating glycogen synthase and restoring glycogen more rapidly than other forms of carbohydrate (Murray et al., 1989; Bowtell et al., 2000).
What does this mean for the athlete?
When training, pass on the sodas, pass on the fruit juice, pass on the Powerade... And go for the drinks that contain maltodextrin, glucose, dextrose, and maltose.
Bowtell, J. L., Gelly, K., Jackman, M. L., Patel, A., Simeoni, M., & Rennie, M. J. (2000). Effect of different carbohydrate drinks on whole body carbohydrate storage after exhaustive exercise. J Appl Physiol, 88(5), 1529-1536.
Murray, R., Paul, G. L., Seifert, J. G., Eddy, D. E., & Halaby, G. A. (1989). The effects of glucose, fructose, and sucrose ingestion during exercise. Med Sci Sports Exerc, 21(3), 275-282.
Wright, D. A., Sherman, W. M., & Dernbach, A. R. (1991). Carbohydrate feedings before, during, or in combination improve cycling endurance performance. J Appl Physiol, 71(3), 1082-1088.