Wednesday, August 31, 2011

Fat Metabolism for Better Health

After summarizing years of research into carbohydrate structure and metabolism, I had occasion to turn back to fat metabolism this week after running into a very knowledgeable Paleo enthusiast.

I discovered a couple of interesting branches of research on the Google Safaris that followed. One suggests that it is not caloric restriction in general that promotes longevity, but restriction of protein, and the other, restriction of carbohydrate. Both of these findings have been published in just the last 5 years.

What was more interesting to me though, was to understand how fat, or fatty acids in particular - as opposed to the glycerol that's also part of triglycerides - is actually metabolized by cells (Cellular Uptake of Fatty Acids), both adipose cells that store fat, and muscle cells that power the large skeletal muscles, through a pathway that requires zero insulin.

Also of interest is that 18 of the 20 amino acids in proteins can be converted to glucose via gluconeogenesis, and are the major source of energy in large skeletal muscles when catabolic muscle destruction takes place. When you eat too much protein, or incomplete protein, it's burned as carbs. By contrast, only the glycerin part of triglycerides can be converted to glucose.

Here's the exciting thing for cyclists and other endurance athletes. Fatty acids, which supply up to half of total energy needed for endurance athletes, have a completely separate, and parallel metabolic pathway from glucose, and can transition through the cell wall and into the cell mitochondria WITHOUT the aid of insulin. The implications of this are so far-reaching that I am still coming to grips with it. But already a few things are clear.

First, super-surges in strength almost certainly occur when energy demands are fed both through fatty acids AND high glucose levels while insulin levels are very high. This condition  would occur after 5-10 minutes of relative rest, say to 50-60% of max HR, when liver glycogen, and ride fuel digestion products, have been added together to create high blood glucose levels, and then the 6-minute insulin release cycle starts by dumping huge amounts of insulin into the blood. If this were to occur when maximal fatty acids are available to the muscle cell, and sufficient oxygen is available, the muscle cells have as much fuel and oxidizer as they can ever hope to use.

Second, this explains why so many are suffering with metabolic syndrome, obesity, and type II diabetes. When you eat either carbs, or protein (via gluconeogenesis) your body cannot immediately use, you use the carbohydrate, small intestine, insulin pathway to fuel the cell's mitochondria. Worse, there is some good evidence from research at UCSF that insulin is the primary cause of aging via the "Grim Reaper" gene.

When  you eat, or use fat stored in adipose tissue, only the small percentage of the triglycerides that is glycerol can be converted to glucose, and digested via the aqueous environment of the small intestine. Almost all of the energy in fat, in the form of fatty acids, is digested into the bloodstream by the liver with the help of bile salts, and those fatty acids are available to the cell directly, in a completely separate pathway, where insulin plays no role. Fatty acids are always a ready fuel, ready to go, delivered by their own private digestion pathway, and they never become toxic like glucose does in hyperglycemia.

Since the energy density of fatty acids is much greater than glucose, and elevated fatty acid levels are not toxic like glucose, the fat-based energy cycle doesn't have to be as tightly controlled as the glucose-based cycle. The glucose-based cycle is regulated against excess by adipose tissue mopping up glucose when there is no immediate demand from muscle, so sedentary consumption of carbs must result in storing fat to prevent hyperglycemia. The required speed of adipose mitigation of glucose levels depends of the GI of the carb.

By contrast the liver digests fat into the bloodstream, pulls it back out when VLDLs get too high, can convert stored glycogen into glucose, can convert fat's glycerin to glucose, and can absorb up to 2,000 calories worth of glucose in the form of glycogen. In short, it's a much more complete and flexible organ for regulating energy levels than is the pancreas, but you have to give it some fat to work with.

The push to move people to high carb, low fat diets has led us to run almost an entirely insulin-driven metabolism, both for powering muscle, and for storing fat. This is in contrast to our historical diet of mostly fat and protein driven diets. As a result our pancreases are dangerously overworked for most of our lives, are failing in many people, and the muscle and fat cells in our bodies are forced to consume glucose and insulin instead of primarily fatty acids - with only small amounts of glucose from liver glycogen, converted amino acids, and glycerine to support exclusively glucose-fueled organs like the brain. Wild swings in energy levels has created an epidemic of ADD, ADHD, and other behavioral problems in the "bargain".

In short, we're trying to run a marathon on one leg!  It just doesn't work.

I'm always cautious when incorporating new information in my daily life, but I have been eating more omelets and have switched from skim milk to 1%. The effect on hunger is amazing. Tentatively, I intend to save carbs to fuel rides, and move more and more towards fat as part of the nutrient rich diet that I eat off the bike, sans carbo loading, and recovery.

On a personal note, I'm just stunned at the utter stupidity of the nutritionists who suggested this, or somehow believed a low fat diet was going to work out well. As Robert H. Lustig, MD, UCSF Professor of Pediatrics in the Division of Endocrinology in this excellent video shows, there were some strong dissenters from the stampede to carbs and away from fats.


When the entire population of the US is 25lbs heavier than they were 25 yrs ago, and we have an epidemic of childhood obesity in 6 month old children, it's hard to argue that obesity is caused by lazy slobs who just can't be bothered to get off their butts and exercise. Something more profound is going on, and this video lays it out pretty well.

Give your pancreas a rest. Eat some fat, and live a longer and healthier life.

Friday, August 26, 2011

Odds & Ends

First, I want to give a big shout-out to my friend Robin Blackburn who will be putting 9 months of training to good use this weekend competing in the IM Kentucky. For all of us who Father Time has robbed of that ability, get some for us too Robin!

Second, I wanted to give an update on my tire experiments after a few hundred miles. I have settled on Michelin again, because the sidewalls are just so much more robust than Conti tires, even the Gatorskin. I am now running my standard, long time favorite Pro3 Race 23mm tires in front, and Michelin Pro Optimum 25mm in the rear after skidding through 2 Pro3 Races in back in 6 weeks after single 10-15 ft skids.

The Pro Optimum don't roll quite as well as the Pro3 Race, but I can't notice any difference in wind drag. I'm not sure the Optimum roll any slower than a Pro3 25mm would either. What I get in return is a tougher tire (I did have a thorn flat, but the thorn forests go on for a thousand yards in places these days, and those dry thorns are hella hard) that is very hard to skid, AND stays hooked up extremely well in turns.

We have a lot of sharp hairpin turns along the SE side of Lake Natomas, and I am starting to trust the Pro Optimum tire now to stay hooked up even over light sand and gravel. The result is less braking and faster overall lap times. The ride is also better over large road cracks and medium stones, and pinch flats less likely. They also don't leak down as fast, having more internal volume than 23mm tires do.

The Gatorskin AKA Ultra Gatorskin tire isn't as bad as my 1st impression, because once the rather extensive pips wore off the crown the tire settled down a lot and stopped hopping all over the trail and looking for reasons to break loose in turns. It might have more puncture resistance than the Pro Optimum too, but the sidewalls are just too fragile to be safe when riding long distances. I don't want to spend 2-3 hours riding home praying some make-shift boot is going to hold. The Optimum's ride is much less harsh, and much more skid resistant.

The Pro3 Optimum tires are a dedicated front and back tire set, which Michelin claims will wear about the same number of miles. I've been so happy with the rear tire, I am tempted to try the front tire too now. At 25mm It might push more wind, a big consideration up front (irrelevant in back), but I am thinking it will likely have a softer ride, and I am on the cusp of being able to ride 100% of the time on the hoods now, after going to carbon bars, and getting the seat dialed in.


 I had an epiphany when looking for some detail on carbohydrate, and then water digestion. It's pretty clear that your body can absorb a maxed out carb-water mixture, like Gatorade's 6% solution, faster than it can digest the carbs in this mixture on hot days. This is because both the large intestine, and the small intestine can digest water, but only the small intestine is capable of digesting carbs.

No matter what the source of carbs, when the mix of carbs and water in the small intestine exceeds some threshold level, the mix, carbs and all, is swept into your large intestine where the remainder of the water is absorbed very efficiently. Unfortunately, the undigested carbs can no longer be digested by you, but are instead fermented by bacteria in your large intestine (colon). The required intestinal micro-structure, nor amylase to break down carbs, are anywhere to be found in your colon. The bacteria's fermentation creates the gas and bloating we all know and hate.

Even with your entire GI track working at max capacity, your ability to sweat still exceeds your digestion rate. If this disparity persists, your body will take water from inter-cellular, intra-cellular, and finally, blood, to make up the difference. 

As your body pulls water out of your blood, reducing blood volume, it puts a tremendous strain on your heart and cardio system to maintain adequate blood pressure. At some point near death, your body will attempt prevent unconsciousness by closing off your capillaries to maintain blood pressure, causing your skin to go dry and your core temps to soar. This condition is known as heat stroke.

To prevent internal organ damage, and even death, you must cool the body in a way that does NOT depend on sweating. Immersion in water, and ingesting cold water is about as good as it gets. Having someone hose you down until your body temp is below 100F is great, pouring water down your back and over your head good, and/or a sock full of ice around your neck and between your legs a potential life saver. Rehydrate as rapidly as possible by ingesting huge quantities of sodium and water. It's impossible for you to absorb water without sodium. Manage accordingly

Nunn and water, especially distilled water, is an excellent electrolyte protocol. Distilled water, having zero osmotic pressure, will support the maximum rate of  sodium and water digestion - the sodium absorption required to maintain an isotonic electrolyte balance as new water is absorbed into the bloodstream. 

Therefore, the limitation on athletic performance on hot days is neither muscle endurance, VO2max, forestalling glycogen depletion, nor even electrolyte management, but your ability to digest water fast enough to keep up with requirements for sweating sufficient to keep your core body temps under control while avoiding dehydration.

All of this is again raising the question in my mind as to whether synthetic fibers, which transport, but do not absorb water, are a major contributor to dehydration on hot days. Cotton and/or linen blends may well manage available digested water more effectively.

This speculation is informed by many studies that show that while men sweat more than women, women are less susceptible to dehydration and heat stroke on endurance events precisely because they sweat less.

Monday, August 15, 2011

Experimental Ride Fuel

I have made it no secret that I am very impressed with Nishiki brand sushi rice, prepared with sugar while fluffing, as the ultimate ride fuel. I have been looking for a better way to carry it with me on rides though.

I had an epiphany tonight, and decided to try grinding up the cooked rice, as grinding up the dry rice and then cooking it didn't work at all. I think this is going to work. I'll use a Hammer flask to hold the gel, just like Hammer Perpetuem - a product I won't use because it contains carnosine, a snake oil from the Area 51 of nutrition.

Carnosine might work, it might work great, it might also leave your body in ruins. I'm not getting paid a boat load of money to ride, so I'd like to stick to food that's proven beneficial to mankind for a few tens of thousands of years.

I used a wand mixer and my stainless steel Starbucks cup, but think something larger is needed. I also was using 5-day old refrigerated rice, which was a bit dry and hard. In my next attempt I will grind the rice while it is still warm and soft.

I may add a little more sugar, and even try adding some coffee, as the rice I used today, while it tastes nice and sweet eating whole, tasted a lot more earthy once ground and in the flask. A good thing maybe, as on long events all those commercial ride fuels start to taste sickly sweet.

If you want to add a powerful, natural antioxidant to this rice, add acai powder or juice. I experimented with it quite a bit a few years ago, and it works. It does tend to bring on sudden cramps when you run out of it, but since it's been used for thousands of years by natives in the Amazon, I think it's fundamentally safe. The cramping may well be due to the increidble levels of strength and energy it supports. Wow, what a ride!

I have two goals in making my own ride fuel. The 1st, is to reduce its cost. Second, a friend challenged me a couple of years ago to find natural foods to use as ride fuel, as he was increasingly concerned with dangerous ingredients, mystery ingredients, and outrageous lies of all sorts made by sports nutrition companies. ( EFS, for example, claims sucrose (table sugar) and dextrose, a synonym for glucose, are complex carbs. A flat out outrageous lie, as a 10 second Google Safari will reveal.)

I could, of course, just order a 50lb bag of maltodextrin from GPC in Mollines, Iowa, but there's nothing natural about corn flour that's been treated with enzymes and then cooked in high heat to break the complex carbs down into something technically not sugar, but very close. As we've seen with HFCS, these seemingly harmless chemical changes can have some nasty and unexpected side-effects.

For pre-ride fueling and on ride fueling, the rice allows me to go as hard as I like and never get indigestion, cramps, or bloating. Pretty great stuff!

Monday, August 8, 2011


Do you wear a sweat cap when you ride, but wish there were a lighter, smaller, better breathing option that would still keep the sweat off your glasses and out of your eyes? Me too!

Sweat band inspired by Andy Schleck
My 1-size-fits-all-the-budget-we-have Halo sweat cap has a nice rubber strip that runs across the forehead, and does a good job of forcing the sweat that gets channeled from the Gishallo helmet's  sweat pads to run down the sides of my face, not all over my glasses The problem is, it doesn't breathe as well as my hair, and it's too large, so it, my helmet straps, and sunglass stems all conspire to make my scalp break out above my ears.

I had a moment of inspiration, and created the Schleck-Band. It's shown here as Frank, although Andy's brilliance can be seen in the foreground. It's a piece of cotton string that is tied behind my head, just like a normal Cadel Evans sized one. It really works too!

There's a Thor Hushovd version too, but it's made of 10mm steel cable. ;)

Isn't tongue-in-cheek  fun?

Friday, August 5, 2011

Michelin vs Conti Tires

Tired of twice ruining a Pro3 Race Michelin tire on the back wheel from a short skid avoiding a clueless pedestrian on the ARPT (We need a new rule making it legal to Tazer clueless peds who walk 4-5 abreast across the Sunrise foot bridge. It's a bridge, not your living room!) I decided it was time to stop running the same tire out back as up front. After all, they do completely different jobs.

I started by removing the Pro3 Race from the front and mounting a Conti GP4000s in front, and UltraGatorskin in back. (now renamed just Gatorskin). It seemed like a good place to apply sticky and tough respectively.

I flatted about 2 blocks from home when the GP4000s blew completely off the rim on one side. After a little head-scratching, I think that's because Conti tires, and especially the GP4000s, are kind of greasy when new. A tip about mounting new tires. Rub the sidewalls around on a dusty carpet or use some talc to mop up some of the oil.

A couple of days later, while doing routine maintenance, I happened to be looking at the rim and noticed the sidewall of the GP4000s was punctured, and the tube was coming through in a half-dozen places. I'm also disturbed that the GP4000s tires have no structure. They look like a sausage, or balloon, with no discernible sidewall or crown shape. They also turned out to be pretty bouncy at pressure, and gave a harsher, and less predictable ride than my Pro3 Race had. I returned them to Performance Bike Shop for a refund.

Remounting the Pro3 Race in front, I rode with that in front and the Gatorskins (folding, Kevlar bead, of course) in back. The Gatorskins were incredibly harsh, very twitchy, and when rolling over small twigs, pebbles, or cracks caused by erupting roots under the ARPT asphalt, they hopped all over the place. It really killed my speed coming home on the SE side of Lake Natoma, which has lots of twisting, hilly turns. I just didn't trust that I could lean into a turn and know where I'd come out of it with the back end hopping all over the place.

I've put about 150 miles on the Gatorskins now, and after the center dipples wore off, AND I let the pressure leak down to 80 psi, they are reasonably stable, but obviously, at that pressure they are a bit of a drag on performance. While, perhaps, a little tougher than the GP4000s sidewalls, the Gatorskin sidewalls don't inspire much confidence either. The tread seems pretty stout, but the sidewalls are thin and don't seem to have anything offering sidewall protection comparable to the Pro3 Race - which I've flatted on 6-8 times on crushed rock without issue.

I just got through mounting a new Pro3 Race on the front, and a Michelin Krylion on the back. I also have a new Pro Optimum rear tire. They only come in 25mm, but are a set with a dedicated rear and front tire. I was able to buy the rear separately, but having only one tube that will fit a 25mm, I decided to try the Krylion first.

One of the riders at SBHs claims to get ~ 3,700 miles on the rear and 6,700 up front with the Krylions, so I'm really hoping the Krylion in back and Pro3 Race in front will make an excellent combo, well matched for mileage. (It appears Michelin hasn't decided how to spell Krylion - or Krylon - who knows)

 At this early juncture I have only one observation. If you're riding Conti tires, there's a much more compliant, supple ride, with better grip and sidewall protection waiting for you on a Michelin tire.

UPDATE: 8/5/2011

Michelin Krylion tires on the back roll a little slower than the Pro3 Race, and about the same as Gatorskins at full pressure, but without the lumber-wagon ride at full pressure. Feel is like the Gatorskins at around 85psi, but impressively stable, even while still being broken in. They also went on the rim easier than the Pro3 Race, as Michelin continues to tighten up the bead on the Pro3 based on the last 3 sets I've used.

 Krylions are a tad bit harsher ride than the Pro3 Race, but not objectionable. Fairly supple, they ballistically ejected a stone to the side of the road when I rolled over it at ~ 25mph, but the back wheel stayed pretty well planted. They are narrower than the Gatorskins, and slice through the wind better. Both the Krylion and Pro3 Race are exactly 23mm by my digital caliper. Both were run at 115psi tonight.

I've also become aware that when riding in aerobars, tires that guarantee you a sure line when you initiate a turn are much more important. You just don't have the margin for error in aerobars that you do riding up  the blocks.  You also don't want blowouts or pinch-flats in front, and with a lot less weight on the rear tire, it has to be supple or it's going to be moving around all over the place, which is completely unnerving.

I'm going to do one more ride on the Krylions, and then try the Pro Optimum. I just have a hunch they are going to feel a lot like the Pro3 Race tire, but with just a tiny bit more wind drag at high speeds. I read an online review of a guy who put the Optimums on his TT bike and beat his prior 40km time by 3 minutes. Part of that may have been improved training, but he didn't think the Optimums slowed him down at all. We shall see.

Tuesday, August 2, 2011

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 are 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. In fact, some 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.

Sweetness is NOT correlated with speed of digestion, as the GI of various sugars makes obvious, but does indicate when the GU count is getting down into sugar's range, as starches, and high-GU maltodextrins, are not sweet. 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.

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.
Amylose: Short, slow, and unbranched
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, 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.

The 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 tend to 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 "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 how fast you could burn fuel, 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 pump 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.

Knowing what you do now 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 end-breaking amylase to do though, relying solely on linear polymer reduction. 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 for kindling. 
Glycemic Index and Insulin Index of Selected Rices
We're not quite done with our story though. 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 22 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.