Optimal Size and Structure of Sports Carbohydrate

Like many amateur athletes, when I took up cycling again 3 years ago after a long hiatus, I came to the sport with a lot of poorly-informed ideas about sports nutrition, and specifically, what the characteristics of optimal sports carbohydrates were, and how they are digested and used in the human body.

While omitting oceans of details in writing this, I will attempt to illuminate the most relevant points, while avoiding overwhelming you with detail. This often-promised post has taken so long to write, because this is such a difficult balancing act to achieve.

Before we go any further, lets make this page a lot more readable and agree on an abbreviation for saccharides, or glucose units. Lets abbreviate that as GU.

Length and Branching of Carbohydrates

If you're old enough to consume carbohydrate as alcohol, then your mother probably warned you to avoid sugar, and stick to complex carbs like bread, rice, pasta, and potato. That advice is well-intentioned, but misinformed (So long as you're burning that sugar. Otherwise stay away from fructose and sucrose). All carbohydrate ultimately is digested into simple sugars, and then into the stuff that's flowing through your veins - glucose.

There is no mechanism in digestion to absorb any carbohydrate other than glucose into the bloodstream. ALL carbohydrate is reduced to glucose for digestion. Carbohydrate that cannot be reduced to glucose for absorption by the small intestine is either fermented by bacteria in the large intestine, producing heat and gas, or is excreted as waste.

The rate at which this occurs, if it occurs at all, is measured by the glycemic index (GI), and is NOT determined by the size of the glucose polymer you are ingesting, simple or complex. I say if at all, because cellulose and inulin, and for some people, lactose, is not digestible.

Glycemic Index of common sugars

 It's also important to understand that your sense of sweetness doesn't indicate anything useful about how suitable a particular kind of carb is for sports nutrition. Fructose tastes 2X as sweet as glucose, but takes over 15X as long to digest. This difference in sweetness explains high-fructose corn syrup, which until converted, is almost entirely glucose. Other carbs that taste rather sweet, like the inulin in bananas, cannot be digested by humans, and serve only to feed bacteria in your large intestine, creating gas and bloating.

So sweetness - a subjective sensory phenomenon - is NOT correlated with speed of digestion, as the GI of various sugars makes obvious, but does indicate when the GU count of starch (the shortness of the glucose polymer) is getting down into sugar's range, as starches, and high-GU maltodextrins, are not sweet. An interesting exception is Asian people have been eating rice for so long that their saliva breaks down rice starch in the mouth so fast it tastes sweet to them, and them only.

Fruit trees have maximized their inducement to animals to eat their fruit, and thereby spread their seeds, by producing the maximum amount of sweetness for the minimum investment in energy. If trees could walk, this wouldn't be necessary. A lot less energy is needed when "sweet" is 2-5X as great for the same amount of carbohydrate/energy.

The glycemic index of anything ingested is established by the simplest of tests. Healthy humans are fed the test food, and then have their blood glucose levels measured every 15 minutes, usually for 3 hours. You can easily perform you own glycemic index tests for the modest cost of a blood glucose tester. The result is a graph like this.

Venous and Capillary Blood Glucose levels after ingesting 5 different grains

The much lower peak, and longer tail of the left graph is the result of insulin's effect on skeletal muscle's rapid uptake and metabolism of blood glucose. In creating this graph pair, I took great care to insure the Y (vertical) axis were the same. By following the colored lines at 60 and 90 minutes you can get a feel for how dramatic these differences are.

Carbohydrates known as sugars are typically either mono or disaccharides, having 1, or 2 GUs  respectively.  There are also trisaccharides, with 3 GUs, which you need Bean-O to digest, oligosaccharides with 3-10 GUs, maltodextrins with 5-33 GUs, amylose with 300-3,000 GUs, glycogen with 30,000 GUs, and anylopectin with up to 2 million GUs. Plant starch is either amylose, or amylopectin. Here's a great discussion of polysaccharides from Sacramento City College.

Amalyose: a linear chain of glucose molecules

Glycogen, containing ~ 30,000 GUs branched every 8-12 units. A masterpiece.
Liver glycogen's branching is novel in that it has more branches near the core, and fewer, larger branches on the periphery, reminding us that these are physical structures that must fit neatly in 3 dimensions.  

Amylopectin's massive 2 million GUs branched every 15-30 units.
Contrary to proponents of the Paleo Diet, humans are uniquely adapted among primates to digest starches, so starch has been part of the human diet long enough to change our genetics.

Glycogen is often referred to as "animal starch" because it has a very similar chemical structure to the huge amylopectin glucose polymer in plants. It's "shorter", having fewer GUs, but is more branched, and that branching turns out to be the 800lb gorilla in the room sports nutrition mfgs don't seem to want to discuss.

Human Metabolism of Carbohydrates

Digestion of carbohydrates takes place in the mouth, duodenum, and about the first 40cm of the small intestine. Except for alcohol, the stomach is incapable of absorbing anything, doesn't have enough surface area even if it tried, and for the most part, is a special-purpose organ for breaking down proteins - especially meats - and its polar opposite Ph balance arrests digestion begun in the mouth.



Polymers of carbohydrate present in starch and complex sugars are first attacked by salivary amylase in the mouth. Interestingly, the sweetness of certain grains, like waxy rice, is due to some of the starch being broken down into small enough polymers - nominally glucose - to be perceived as sweet.

Maltodextrin commonly used in sports nutrition have a DE of 9, where glucose is 100, so a GU of 100/9, or ~ 11. It is fairly easy to reduce that to glucose and maltose. The same is true for waxy rice's 3D "grape cluster" structure getting cleaved off the "vine" and attacked.

This brings up an important point. While many carbohydrates, such as starch, fructose, lactose, etc, have their own dedicated amylase that acts only on them, all amylase fall into 1 of 2 kinds. One kind attacks ONLY the branches of glucose polymers, and the other attacks ONLY the ends.

There isn't much for the latter to do with something long and unbranched, like amylose, which accounts for its much lower GI. Both amylases attack at random locations, but in highly branched polymers, breaking a single branch connection exposes dozens of ends. By contrast, breaking a linear polymer exposes only 2 ends.

This simple idea is why branching is much more important than polymer length in creating the high GI carbohydrates for optimal sports performance. (as a point of interest, the very best high explosives have this same, very complex branching structure, but with many oxygen atoms bound into the molecule, so that both the fuel and oxidizer are present in close proximity in explosives)

Intestinal villi's brush border has a surface area equal to a small 2 bdrm apt

Let's come at this conclusion from the other end. Since all carbohydrate digestion finally reduces glucose polymers to single glucose molecules, why not just ingest glucose. How can you beat that?

Well, first, you can beat that. Maltose, a disaccharide, has a GI of 105, higher than glucose's reference GI of 100. So do certain types of rice, potato, and dates, where certain varieties approach a GI of 140. This clearly indicates that the intestinal brush border is capable of simultaneously reducing glucose polymers, and absorbing the resulting glucose monomers.

(Dedicated amylase such as sucrase, and invertase, go unused if no sucrose is available, so adding sucrose to denser fuels, whose digestion occurs simultaneously, increases the total digestion rate, and that strategy is used by almost all commercial ride fuels, but these are supplements, not replacements for denser fuels)

For athletes, the real answer to why you can beat pure glucose is temperature. Cold, dense air increases VO2max, but minimizes electrolyte demands, so Gatorade's combined electrolyte, hydration, fuel strategy fails, and only denser fuels can provide the extra energy to fully utilize available oxygen. Heat is more insidious. It's a frontal assault on you ability to digest carbs, so optimal fuels are imperative.

Sugars are NOT very energy dense. Imagine how much straw you'd  have to burn to keep warm in a really cold climate. Straw burns very fast, but it doesn't produce much heat. You could also burn balsa wood, spruce, pine, douglas fir, fruit wood, or oak. You still might not care what you burn if there were no limits to the bulk/volume of fuel you could burn, but there are. Think of your carb digestion rate as the size of the hole in the wall you have to pull fuel through. Denser fuel is obviously better, especially since HEAT shrinks the size of the hole.

Sugar, like salt, increases osmotic pressure (as much as 300 psi) as more of it is added to liquids like Gatorade. Increase the strength to get more fuel, and the osmotic pressure becomes so great your small intestine can no longer pull salt and glucose into your blood, so it passes undigested into your colon, where bacteria are eagerly awaiting their next meal. The by-product of that bacterial digestion is gas, and its attendant bloating. This limits the amount of sugar to about a 6% solution - exactly what Gatorade has.

When it gets hot, things go from bad to worse fast, because your body has to keep your core cool, and to do that it has to open your capillaries and start devoting a lot of small intestinal surface area to absorbing water to support sweating. This has 3 negative impacts.

First, it starves your digestive tract for blood, so even if your intestine hasn't completely shut down, there isn't enough blood to properly absorb all available glucose from the intestine.

Second, at some point, your small intestine can no longer absorb enough water to support sweating. Not to worry, your large intestine (colon) is not only ready to absorb more water, it's actually more efficient at it, BUT, when processing so much water, there isn't much time for carbohydrate digestion and absorption while transitioning through the small intestine. Any carbs that get past your small intestine, feed bacteria in your colon, which will ferment them, but are downstream of the point where they are of any use to you.

Third, you need sodium, usually from salt (sodium chloride) or sodium citrate, to unlock any and all transport sites in your small intestine so glucose can be transported into your blood. While this does not consume sodium, sweating does, and in large quantities on hot days. Glucose, wrested from long-chain polysaccharides by the action of amylase,  ends up useless without adequate sodium, and ends up downstream in the colon, supporting fermentation with its attendant gas and bloating.

Initially, you can dilute your Gatorade mix, which is what I do, but in doing so, you have to add back electrolytes and carbs to make up for the dilution. There are lots of good electrolyte solutions out there, but that doesn't help make up for the lost carbs.

(With intense sweating, only salt and pure water will prevent gas and bloating, as carb digestion is completely shut down. The body's water absorption rate is the limiting factor for sports performance in intense heat, and that is very dependent on maintaining adequate sodium levels)

What's needed to prevent glycogen depletion is a denser fuel with a much lower osmotic pressure than sugar. Say hello to starch, or some derivative of it, like maltodextrin, with a GI of 105. While salt greatly enhances the rate of water absorption, with intense heat, carbs have to be consumed at 75% of max HR or less to reduce sweating. Salt, water, then food.

Now knowing what you do about the structure of carbohydrates, you'll be looking for a carbohydrate with a structure like liver or muscle glycogen. Highly branched, and very densely packed. Obviously, amylopectin meets this criteria, and accounts for the large difference in GIs amongst rice varieties with high and low percentages of amylose.

Maltodextrin squeaks by because it has ~ 11GUs, and a much lower osmotic pressure than sugar. It doesn't leave much for the branch-breaking amylase to do though, relying solely on linear polymer reduction by end-breaking amylase. I'm guessing, but I think all that type-specific, unemployed amylase becomes part of the problem.

High amylose starch is necessarily low amylopectin. All starch is one or the other. The former has GIs in the 60s, and the latter in the high 80s. In fact, short-grain waxy rice has zero, or very near zero amylose, and certain varieties have a GI of over 130. (item #293)

Relatively small amounts of fuels this dense, supplemented with small amounts of sucrose and fructose to make max use of all types of amylose, will produce as much glucose as a balanced attack by all varieties of human amylase can sustain, and the intestinal brush border can absorb. It can do so with NO osmotic pressure problems. This is like burning 400 yr old English Oak, with straw and pine sawdust blown into a fire mixed with compressed air. WOOSH!!!
 

Glycemic Index and Insulin Index of Selected Rices

We're not quite done with our story though. Until now this discussion has been entirely about how to maximize the sustained rate at which we can get glucose into the bloodstream, but this is only half of the problem. We still need to get that glucose moved through muscle cells' outer membrane, and into the cell's mitochondria. Without the action of the hormone insulin, all that glucose is locked out of the muscle, and essentially worthless, so how can we maximize an insulin response to use a maximized glucose delivery? Choose foods that solve both halves of this problem.

Look back up to the two GI graphs. One for venous, and one for capillary glucose. Remember the huge difference in those levels is due entirely to the effect of insulin on large skeletal muscles. Now look at the insulin index of Waxy rice above. Even though the GI of waxy rice is lower than Pelde white rice, the insulin index is a staggering 32 points higher! (we might also suspect that such a high level of insulin is lowering the observed GI by increasing the rate at which measurable glucose is being absorbed by either adipose tissues, or large skeletal muscles)

Have you ever had a 15-20 minute interval where your strength was super-human? We've all seen these displays by professional athletes. They're the stuff of legend. I'm speculating here, but I think those incredible moments of strength are due to a convergence of high blood glucose and high insulin levels. The pancreas does not release insulin on a continuous basis, but at approximately 6 minute intervals, or as short as 3 minute intervals in highly trained athletes. After this flood of insulin is released into the blood, it's monitored for depletion. For athletes, carbs that induce a larger release of insulin are better carbs.

In summary, looking at the GI of many foods, it seems clear to me that the gains made by sports nutrition companies in "predigesting" starch carbohydrate into shorter, linear glucose polymers (maltodextrin and brown rice syrup), has reached a plateau. It looks like the way forward is to find varieties of rice, or perhaps other starchy grains, which naturally have very high GIs, investigate their branching structures, and cross breed or genetically engineer "super fuels" - which may have GIs close to 150, and provoke intense insulin responses. Such a break-through is more likely to come from Monsanto than Hammer, as the research effort will surely be large and expensive.

Ironically, this investigation has already begun, but it's focused in the opposite direction - to find or create lower GI grains to address the obesity epidemic. The table above was taken from such a study. It may seem a frivolous endeavor to find a "super fuel", but imagine the benefit for infantry to have a fuel that will stave off glycogen depletion from sunrise to sunset on the longest day. It, of course, is also of great interest to those of us who compete against time and reason to find satisfaction and glory.