Monday, December 8, 2008

Mixing up some medicine

Wompatuck: 4+ hours in the cold and snow, incredible riding. The woods quiet except for the crunching of the snow beneath our wheels and the clatter of a chain skipping on a frozen pulley. I was doing fine until about mile 15, when the cumulative effects of the cold and riding started to take their toll. It was as if I had just crossed the line into frustration with no way back. We'll have a full cover of this story up shortly.

The loss of coordination was the result of a drop in blood sugar levels. Avoiding this state is especially important for mountain biking, where a small loss of coordination can mean a large increase in frustration, difficulty picking lines and managing obstacles, which in turn causes one to be stopping more frequently and moving into a downward spiral. It's best not to go there.

I wanted to relay some thoughts Keith and I had about on-the-go feeding. The basic problem is this: relying on Power/Cliff bars in the sweltering heat of a summer stage race is a sure way to not eat enough. You have the simple choice between choking to death on a bar as it gets lodged halfway down your parched esophagus, burning 100 calories in a prolonged masticating effort all the while not breathing efficiently, or surviving on a liquid diet. It's one thing to deal with bars on the road, it's altogether another thing to deal with them on the trail where your attention is demanded on a second by second basis. In a stage race like the Breck Epic, one could wait for the aid stations, but overall performance and digestion would be better for a more steady ingestion. The basic task is to replenish, or at least mitigate the loss, of approximately 500 calories per hour on the trails.

As a general strategy, we need to be sure to maintain as best as possible our caloric reserves in the first half of each stage, and that means eating regularly. As the stages can be 6+ hours each, our bodies will be using both fats and carbs in fair proportion. A few basic concepts will guide us through the analysis of the options. Mind you, I'm no expert in this, so please speak up if I am wrong on anything here.

I. Energy Sources - I long ago gave up on Joe Friel and his training cartoons. If you want a plan based on science rather than imagination, try reading something like Tim Noakes' "Lore of Running", it has a great account of physiology of endurance athletes. While the literature suggests that fats can comprise a good portion of the energy supply, about 30% at 50% VO2_max, studies show that the regulator of this process is the muscle glycogen stores. Thus, ingestion of fats immediately prior to or during competition are likely to have little effect, assuming one is not hypoglycemic. The point then is to eat a balanced diet, say something like 40% carbs, 30% fats, 30% protein, and then use carbs during the event to keep up muscle glycogen stores. The question now is what kind of carbs to use?

II. Glycemic index (GI) - a measure of the rate at which a carbohydrate form causes the blood sugar levels to increase. High glycemic index foods quickly enter the blood, low glycemic index foods more slowly. Our guiding principle will be to have an eating/hydration strategy that attempts to keep the blood sugar level as close to optimal and constant as possible. As a rule of thumb it takes about half an hour for a high-glycemic index food to raise your blood glucose level, which again reinforces the idea that we need to eat regularly and often to maintain.

III. Insulin Index - a measure of the rate at which foods (in general, not restricted to carbs) will induce the release of insulin, a hormone which signals cells to take in glucose, depleting the blood of its glucose stores. Foods with a high insulin index are often, though not always, associated with a high glycemic index. High levels of insulin, signaling the muscles to use glucose, can essentially shut down the fat metabolic pathways. Finding a food that keeps a moderate insulin level should allow for a more even utility of fat and carbs.

Fats contain 9 kcal/g, compared to 4 kcal/g for both carbs and protein. Additionally the carbs require 2g of water per g of carb in the metabolic pathway, so in terms of effective energy density per unit mass, fats have about an 8 fold benefit over carbs. This basic fact tells us that when we're operating well below our LT, we should be trying to maximize our utility of fats. Ideally then, we should try to avoid spiking our insulin levels, but keep a steady flow of carbs during exercise.

In summary: we want a food that has a moderate to high glycemic index, moderate insulin index, high calorie/mass ratio, high concentration of carbs perhaps with some fats, and is decently edible. Below are four race-time diet strategies (other than bars) and a brief discussion of each.

Syrup-like sports drink
One could say double or triple the concentration of powdered sports drink like, Accelerade, Gatorade, or whatever. A shockingly dense mixture of sports drink, say 3x, should give a caloric content of 200-300 calories. We plan on carrying only two bottles to minimize weight on the trails. A super concentrated drink requires a chaser of water - a dry, sticky mouth is certainly worse than a plain dry mouth. Two bottles over a space of two hours means that this technique can at most give us about 100-150 calories per hour, seemingly far short of the goal. This suggests the need for a more calorie dense solution.

While certainly refreshing, especially ripe plums and crisp apples, the high fiber content of fruit on long rides can be a downside. Additionally the relatively low calorie/mass content of fruit is a drawback. A few pieces of fruit on a hot day can certainly be a comfort, but it is not a winning strategy. I think we might have progressed a bit in the science of nutrition since the days of the 1960's TdF where fruit and wine were the standards of the day.

In lieu of completely solid food, one might compromise and go for the partially hydrated food source, that mysterious fifth state of matter with fractional phase space dimension, the gel. A single packet of GU contains 100 calories. The gels are essentially all carbs, in the case of GU it is maltodextrin (glucose polymers) which tend to have high GI's around 90-100. The insulin index of these foods can also be high. Benefits: easy to eat, good caloric density. Downsides: expensive per calorie cost, relatively high insulin index. All in all, putting down 2-3 gels between aid stations may be a simple and solid strategy. I am a bit surprised that there is not a gel (that I know of), that contains a more diverse carb distribution, but then again, I am no expert in sports physiology.

---- Edit From The Wrench ----

Adding a couple of things:

There are definitely Gels out there that approach the Insulin Index problem directly. In particular, Honey Stinger is basically pure fructose, which has a low insulin index.

PowerBar claims that the there are separate intestinal uptake pathways for the uptake of fructose and glucose, so that a blend of these two types of simple sugars allows calories to be absorbed faster. Though I am inclined to believe this is true (research supports it), I have to ask the question whether or not sugar uptake is a limiting factor (see next section)? I don't have the answer, but a survey of the different gel products out there suggest that multiple sugars are at least good marketing, as all but Honey Stinger and Hammer Gel seem to use a fructose / maltodextrin mix.

Something else to consider is that the rate at which the stomach contents are released into the intestine (were uptake of the sugar occurs) is slowed by the solute concentration of the mixture. This means complex carbohydrates can be digested at higher energy densities than simple sugars for a given amount of liquid. Furthermore, the concentrations that make the body happy are very low, around 300 mOsm/L, so I would be inclined to believe that this is potentially a limiting factor in the real world where you have limited water and are particularly sensitive to digestive distress. Hammer Nutrition uses this theory as the basis for their gel products, which are based on the complex carbohydrate maltodextrin alone, although the article I linked below concludes that the complex carbohydrate theory is flawed because the receptors that measure the solute concentration reside in the duodenum, where the complex sugars are broken down. Maybe the truth lies somewhere between the two theories? Where's a nutrition scientist when you need one?

Here is a really comprehensive survey of all sorts of things affecting digestion and metabolism of sugars.

------End Edit------------

Home Brew
Over the next few weeks I'll experiment with home brew medicines for race day feeding. We will attempt to develop a food that combines the desirable traits of being calorie dense, palatable in hot conditions, easily transportable, and containing a somewhat diverse set of carbohydrate sources to keep the insulin spikes down. We might look for a food that combines aspects of the immediate and longer term insulin trigger, something akin to a buffered aspirin.


Richard said...

Thanks for the great articles on endurance racing. This one clears up a lot of confusion for me.

Anonymous said...

A good book is Monique Ryan's Sports Nutrition for Endurance Athletes. I find her quite knowledgeable and competent and she also seems to be a proponent of keeping things as simple as possible (as opposed to authors who support all sorts of expensive marketing gimmicks).

PS: Here is the link to her book/website:

Anonymous said...

Another source that came to my mind: Prof. Asker Jeukendrup is supposedly also a leading expert on nutrition and physiology. There is an interview with him on the SRM blog, but sadly it is only in German. But in summary, his conclusions are close to what you wrote. He notes that everybody is slightly different so there is no one nutrition strategy. But he recommends a mix of glucose/fructose (preferably via gels; he squeezed his gels in a bottle with water; 16 for the Hawaii Ironman), a mix of bottles with energy drinks (with carbs) and bottles with plain water, as well as an energy bar to have some solid food in the stomach.

Anonymous said...

PS: In terms of quantity, Prof. Jeukendrup recommends 80-90g of a fructose/glucose mix per hr.

The Pedal said...

We're working on a more in depth analysis of this. It certainly is a complex problem. My latest reading suggests that fructose is not the way to go, because fructose is rapidly converted to lipids (fats) in the liver. We really want to be using glucose, the primary energy source for our muscles. So, while fructose can be a good energy source in the long run, it has to go through the process of first become a lipid and then hits the fatty-acid cycle and is therefore less efficient than glucose/maltodextrin/starch.

Thanks for your comments and links, I'll try to read over these when I get some more time.

Anonymous said...

As for fructose vs glucose. I should add that Jeukendrup asserts that a mix of fructose and glucose is better than glucose alone, since such a mix (he does not specify a ratio but noted that he used Power Bar gels, which may give an indication) is faster absorbed than just glucose.

I am no expert in this matter, neither theoretically nor practically (since I have never done long endurance races), and just objectively re-state what I read; without endorsing or opposing it. I am definitely interested in your analysis!

Anonymous said...

I just discovered that Jeukendrup has just recently written an academic article on the matter (I have not read it. so I am in no position to assess its quality):

Currell K, and A.E. Jeukendrup. Superior performance with glucose and fructose ingestion during exercise. Medicine and Science in Sports and Exercise 40(2):275-81, 2008

He has also published numerous other articles on nutrition, to be found here: