Interesting Article On Best Carbs To Use During Workouts.

I thought this article was very interesting. If nothing else it helps explain why some people prefer waxy maize over dextrose.

What is your take on this Biochem?

October 2012: Cyclic Dextrins – the Ultimate Intraworkout Carb

By Bill Wills

The next rage in carbohydrate supplementation involves the use of designer glucose polymers called highly branched cyclic dextrins (HBCDs). A few supplement companies are now including HBCDs as a major carb source in their products, and some companies are even offering HBCDs as stand-alone product. A lot of claims have been made, but do these “designer” carbs really live up to all the hype? What are HBCDs, how do they work, and how can we use them as a tool to take our training, nutrition, and performance to the next level?

To answer these questions and more, we need to take a look at carb supplementation in general. It’s a good idea to include some type of carbohydrate supplement in your intra-workout nutrition. This serves two purposes: to deliver a rapid and steady supply of blood glucose to hard working muscles, and to harness the power of insulin, the most anabolic hormone. This keeps performance up, protein synthesis on, and minimizes the inherently catabolic effects of intense training. To that end, it is a common practice to use a combination of quickly digesting proteins such as casein or whey hydrolysates along with a carb source in the intra-workout nutrition shake. While protein is obviously important, the choice of the carb source is also key. The simplest way to go is to use simple carbs such as glucose (aka dextrose). After all, all complex carbs are broken down into glucose before they are absorbed, and glucose is absorbed very rapidly.

The problem with dextrose/simple carbs:

Although dextrose/simple carbs look good on paper, things are not so simple in reality. The human body is a finely tuned machine. Nutrient density is sensed in the small intestine by special types of receptors called “osmoreceptors”, which sense the concentration (also referred to as “osmolality”) of stomach contents as they exit the stomach. This information is relayed back to the stomach through neural and hormonal messages, controlling the rate of gastric emptying. If these osmoreceptors sense that contents exiting the stomach have a high osmolality, gastric emptying is delayed. This is a problem with dextrose/simple sugars in general: In spite of warp-speed absorption in the small intestine, glucose/simple sugar solutions (unless they are VERY dilute) have an extremely high osmolality. This delays gastric emptying into the small intestine, where carbs are actually absorbed. Worse, in the context of an intra-workout nutrition shake, you aren’t only delaying the delivery of carbs; delivery of those fast acting (and generally pricey) protein hydrolysates will also be delayed. Use too many of the wrong type of carbs, and you might as well be gnawing on a steak during your workout, which wouldn’t be a terrible thing, but good luck getting any amino acids actually delivered to muscle tissue during in a timely fashion!

To understand how this works, it helps to understand osmolality. Osmolality is a measure of the concentration of a solution in terms of osmoles solute (Osm) per kilogram of solvent. For our purposes here, the “solute” is the stuff being dissolved (carbs, for the sake of our discussion), and the “solvent” is the stuff that does the dissolving (the water in your intra-workout shake). To avoid overloading the digestive system, the stomach senses the concentration of stomach contents as they pass into the small intestine, and regulates gastric emptying accordingly.

What about glycemic index?

Concentrated dextrose/simple carb solutions have a high osmolality, which delays gastric emptying. But doesn’t dextrose have a high glycemic index? Generally when a “fast” carb source is desired, we choose one with a high glycemic index, which is a measure of the blood glucose increase from carb consumption. Dextrose has a very high glycemic index, so it does seem a bit paradoxical that high GI concentrated dextrose/simple carb solutions also delay gastric emptying. To illustrate how this works, think of a horse race, where each different type of carb is a different horse, and glucose is the fastest horse out there. The problem is, after the starting gun is fired, when all the slower horses are off to the races (to the small intestine), glucose is stuck at the gate (in the stomach). Glucose isn’t worried though. It’s the fastest horse out there, and can easily make up the lost ground. Before the race is over, the gate finally opens for glucose, which flies around the track, leaving all the other horses in a cloud of dust as it crosses the finish line (i.e. absorption from the small intestine into the bloodstream). This nicely illustrates second problem with dextrose as a carb source in your intra-workout shake. Blood glucose levels are not sustainable. Although delayed, glucose absorption in the small intestine is extremely rapid, sending a huge bolus of sugar into the bloodstream. Naturally, this causes a big insulin response to deal with all that blood sugar. The problem is that when insulin is released in large amounts, it almost always does too good of a job. The increase in blood sugar is transient. After insulin spikes, blood sugar decreases, usually causing hypoglycemia.

Of course a solution to the problem would be to constantly sip simple carbs throughout your workout. This fixes the “sustainability” issue, but not the delay in gastric emptying. Unless you are dealing with an extremely dilute solution, simple carbs in ample amounts will significantly delay gastric emptying. Not only will protein absorption be delayed, but this is also a common cause of stomach cramps. This is one of the major reasons why people complain of stomach issues when drinking these intraworkout drinks actually. Not good.

The fix: high molecular weight glucose polymers

Fortunately, there is a way to “trick” the stomach into releasing large amounts of carbohydrate into the small intestine for rapid absorption. Rapid absorption isn’t our only concern though. Going back to the horse-race analogy, we a need a horse that runs slow, but is also very fast out of the gate. This would provide a rapid, but sustained increase in blood sugar. High molecular weight glucose polymers are ideal in this respect. When glucose molecules are linked together to form high molecular weight polymers, one molecule can consist of hundreds to thousands of individual glucose molecules linked together. These high molecular weight polymers have a much lower osmolality in solution. As a simple example, compare a solution consisting free glucose molecules (i.e dextrose) to another solution of equal volume with an equivalent amount of glucose, but in the form of high molecular weight glucose polymers. (Remember, that osmolality = molecules of solute/ kg of solvent.) Because there are much more solute molecules in the dextrose solution, a dextrose solution will always have a much higher osmolality than an equivalent solution of glucose polymers. Now you understand the value of high molecular weight carbs; they have a much lower osmolality in solution compared to free glucose, so they are emptied from the stomach extremely fast.

Although emptied from the stomach rapidly, high molecular starches are also very large, so they need to by hydrolyzed into free glucose by digestive enzymes in the small intestine before they are absorbed into the blood stream. This property makes them an ideal carb source; high molecular weight glucose polymers generally provide a rapid, but very sustained release of glucose into the bloodstream. One of the most popular high molecular weight starches in recent years has been waxy maize. The carbs in waxy maize consist almost exclusively of amylopectin, a glucose polymer with a highly branched molecular structure. Because of an extremely long glucose chain length and extensive branching, the starch in waxy maize has a high molecular weight. As mentioned above, high molecular weight = low osmolality in solution. As a result waxy maize passes through the stomach much faster than an equivalent glucose solution. The highly branched structure of the amylopectin starch in waxy maize also provides a slower, steadier release of glucose into the bloodstream. This gives a rapid, but also sustained release of glucose.

Enter HBCDs

The current state of the art in carb supplementation involves taking natural starches like the amylopectin in waxy maize and modifying their molecular structure to increase molecular weight as well as the extent of branching/crosslinking. Increased molecular weight reduces osmolality in solution, speeding up gastric emptying. Increased branching/crosslinking controls the access of intermolecular glucose linkages to digestive enzymes, which extends absorption time in the small intestine. The result is an ideal carb source; one which passes through the stomach very rapidly, providing a quick but also sustained release of glucose into the blood stream. With these properties in mind, highly branched cyclic dextrins (HBCDs) were created. HBCDs are a new type of glucose polymer that is produced by reacting waxy maize starch with a special branching enzyme, forming a cyclical structure. The result is a glucose polymer with some ideal properties: HBCDs have an average molecular weight of 160,000 Da, so they have an extremely low osmolality in solution and a rapid gastric emptying time. (*Compare HBCDs to dextrose, which has a molecular weight of around 180 Da!) The highly branched/cyclical structure of HBCDs also provides a rapid, but very sustained release of glucose into the bloodstream (1, 2).

HBCDs don’t just look good on paper; preliminary animal research suggests they can actually increase athletic performance (1). As you can see in the figure below from Takii et al. 1999, HBCD supplementation in mice significantly increased swimming time to fatigue compared to glucose or water. These results are not actually surprising; it makes sense that sustained glucose release provides a steadier supply of carbohydrates to burn when glycogen stores are depleted by intense exercise.


HBCDs are one of a number of new “designer” carbs that are definitely worth checking out. There are two very big advantages to using these high molecular weight glucose polymers in your intra-workout nutrition. First, they provide a rapid, but very sustained release of blood glucose. How rapid vs. how sustained depends not only on molecular weight but also on the overall molecular structure, which determines how quickly enzymes in the small intestine are able hydrolyze these large glucose polymers into free glucose for absorption. The use of a particular branching enzyme with HBCDs resulted in a high molecular weight glucose polymer with some ideal properties. The second advantage to using high molecular weight glucose polymers is that that they have a very low osmolality in solution. Unless you are an endurance athlete, you probably include some type of protein isolate or hydrolysate in your peri-workout nutrition. The last thing you want to do is delay absorption of this “fast” protein by delaying gastric emptying with high osmolality carbs. More likely than not, there will be many more advances in the science of carbohydrate supplementation in the future, and we’ll be here to keep you posted.

Until next month,


Reference List

1. Takii H, Ishihara K, Kometani T, Okada S, Fushiki T. Enhancement of swimming endurance in mice by highly branched cyclic dextrin. Biosci Biotechnol Biochem 1999;63:2045-52.

2. Takii H, Takii NY, Kometani T, Nishimura T, Nakae T, Kuriki T, et al. Fluids containing a highly branched cyclic dextrin influence the gastric emptying rate. Int J Sports Med 2005;26:314-9.


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