During swimming workouts which last for about an hour or more, it is very important to ingest carbohydrate during the exertion. The ingested carbohydrate provides the fuel that muscles are looking for as they begin to run low on glycogen during the hour-plus effort, and research convincingly shows that such "exogenous carbohydrate" (carbs taken in during exercise) can increase the quality of the training session. With added carbs pouring into your system, you are simply able to swim faster: your muscles don't have to turn to speed slowing fat for energy as their internal carbohydrate stores diminish.

For the last 15 years or so, individuals who are knowledgeable about sports drinks have been saying that the practicalities of carb intake during swimming are no big problem. You simply find a good sports drink, a quaffable with a 5 to 9 percent carb concentration ( a drink with a less-than 5 percent content won't speed enough carbs to your muscles; at the other end, a beverage checking in at more than 9 percent might drag water into your tummy and increase  the risk of gastric distress). You take in about eight to 10 ounces (eight to 10 "regular swallows") of the stuff 10 minutes before you begin to swim, and you ingest five to six regular swallows every 12 to 15 minutes during your exertion. As a result, the carbs are absorbed at a steady rate and stream amply through your blood to your muscles, keeping them as happy as possible during your strenuous effort.

The eight- to 10-ounce pre exertion bolus is part of an effort to solve what has been believed to be the key limiting factor associated with carbohydrate intake during exercise: the limited rate at which fluids can pass from the stomach into the small intestine. Remember that no carbohydrate can be absorbed across the wall of the stomach, so getting sports drinks from the gullet into the small intestine is an essential part of moving carbs from their starting point in a sports-drink bottle to their demolition site in your muscles. Research has shown that the movement rate from tummy to intestine is dependent on the amount of fluid in the stomach; the more liquid present, the faster the movement rate. However, too much aqua in the stomach can produce gastric distress, so eight to 10 ounces has been viewed as an advantageous "load" to which most swimmers can become adapted.

The eight- to 10-ounce bolus also serves another purpose: it kick-starts the breakdown of exogenous carbohydrate. Some reports have indicated that the carbohydrate in the liquid bolus can begin to be metabolized for energy by muscles within five minutes after ingestion. In other words, the exogenous carbohydrate will start furnishing fuel for your muscles before your long workout even begins. This of course is great from the standpoint of glycogen-depletion prevention, and it is also "insurance policy" in case you begin your workout at too-fast a pace. Buzz-saw beginnings to long workouts tend to have a severely negative effect on muscle-glycogen stores, but this can be counteracted if the muscles are gulping sown exogenous carbs.

Until now, many scientists interested in carbohydrate intake during exercise believed that the bolus-plus-six-swallows-every 15 minutes strategy had solved the final problem associated with sports-drink ingestion, ensuring that the best-possible rate of carbohydrate delivery could be achieved. Now, however, there is an exciting new development in carbohydrate intake research. Outstanding investigator Asker Jeukendrup and his colleagues in the Human Performance Laboratory at the School of Sport and Exercise Science at the University of Birmingham in the United Kingdom have noted that another key limiting factor (for carbohydrate absorption during exercise) may be related to what are called intestinal transport mechanism. As you are probably aware, glucose, fructose, sucrose, and other carbohydrates can not move willy-nilly across the wall of the small intestine. Their movement from inside the hollow, coiled tube which call the small intestine into the small blood capillaries which will carry the carbs into the general circulation and thus to the muscles depends on "transport proteins" embedded in the walls of the small intestine. These proteins in effect help give carbohydrate molecules an inward directed "ride" through the wall of the gut and thus into the circulatory system (this process is a "transport mechanism").

Glucose absorption, for example, depends on a sodium-dependent glucose transporter called SGLT1 (sodium-dependent means that sodium must be present for SGLT1 to do its job, which is one key reason why sports drinks contain, sodium). Fructose, another simple sugar, seems to depend entirely for its absorption on something called GLUT-5 transporter, which is quite different from SGLT1. Somewhat surprisingly, the mechanism underlying the absorption of sucrose (aka table sugar), which is a disaccharide composed of one part glucose and one part fructose, is controversial. Some gutsy scientists argue that sucrose is simply hydrolyzed to glucose and fructose at the small-intestine's inner membrane, followed by absorption of the two constituents using SGLT1 and GLUT-5 transporters. However, there is some evidence that disaccharides-related transporters which are independent of SGLT1 and GLUT-5 (1).

Why go into all of that? Of course, there are not an infinite number of SGLT1 transporters lying around in the inner walls of the small intestine, nor is there a stupendous quantity of GLUT-5 carriers. In fact, the densities of these carriers appear to be rather moderate - good enough for the sofa spud who is watching the game of the week on television but perhaps not ample enough for the endurance athlete who wants to maximize the carb exit rate from the gut and subsequent entry into circulatory system during exercise. What may happen if a swimmers sports drink contains only glucose, for example, is that all of the SGLT1 carriers may become "busy" (i.e., attached to glucose molecules) as the swimmer moves along during his/her long workout. Other glucose molecules wait impatiently in the gut, nervously looking forward to their speedy passage to the muscles, but they can't move in because all of the transport "vans" in the intestinal wall are fully booked.

If this truly happens, then a drink which contains glucose and an additional carbohydrate (and thus which relies on both SGLT1 and a different type of transporter) Should work better (remember that the idea with sports drink ingestion is to maximize the rate, the higher the intensity of exercise which is permitted and the lower the risk that intramuscular glycogen depletion will hurt performance). A sports drink with two different types of carbohydrate might permit a much-speedier passage of carbs into the blood (since an increased number of transporters would be available). In theory, a beverage with three different types of carbohydrate would be better still.