When one considers a strategy for supplying the body with what it needs for any type of sustained activity (by sustained, I mean activity in excess of 4hrs), it seems almost second-nature to include complex foods, rather than something more basic. The rationale for choices like this among the endurance athlete population is ubiquitous. If you need further evidence of this phenomenon, I encourage you to take a stroll through the transition area of an Ironman and pay particular attention to the rolling buffets attached to arrow bars and draped over top tubes – what you’ll find is a collection of anything and everything from PowerBars to pizza (and I’m not joking) all patiently waiting to be ingested at some point late in the race. As a physiologist, I have made a simple career from begging the question of “why” especially in terms of health and performance, but no matter the broken chain of logic that defines the answers I normally receive, I will not be convinced that complexity overrides simplicity when it comes to nutritional selection over the course of an endurance event– and I have one important item at my disposal when forming a counter argument…. Science.
The digestive system by and large is one seemingly endless network of tissues that depend on many degrees of consistency.
Anytime you ingest a food, a very important cascade of events begins from the second whatever it is you’re eating enters your mouth. In the case with “the” key nutritional element of performance, the carbohydrate, salivary amylases in the mouth, upon contact, begin the sequential deconstruction that eventually leads to the energy formation. The process of degrading this energy substrate could be metaphorically likened to filling a furnace with firewood for heat. In this case, the individual stick of firewood takes a dramatically different form from its original status as part of a complete tree. Anyone who’s ever had the privilege of cutting firewood knows that the formation of heat doesn’t come without a substantial investment of energy on the front end. Think of this: If you desire to heat a large space for several hours on nothing but firewood you had better plan your timber harvesting accordingly. What I mean by that is simple: It would behoove you to cut down larger trees first that would serve as more long term sources of energy by yielding a larger volume of firewood initially especially if you had a small amount of precut wood keeping the furnace hot while you were busy dige… errr creating firewood from a much larger source. You would then, as the night continued to get colder and thus increasing your firewood demand, you want to aim for progressively smaller trees so as to limit your energy investment because, let’s not forget, this IS NOT an easy, or “free” process. What you are seeing develop here is picture of the enormous responsibility placed upon the digestive system to deliver a continually increasing supply of energy SUBSTRATE for use in the metabolic furnaces that are human muscle tissue. Just as in the case of chopping firewood, who’s process is only a means to an end, not the end in itself, presenting firewood to the furnace in a size, shape, and density necessary to keep it producing maximal amounts of heat is crucial… if you want to stay warm, of course.
There’s also another layer of complexity: Blood flow. As we tied in via metaphor just above, the human digestive system doesn’t work for free. It’s made of muscle; different muscle than your biceps I grant you, but it still requires energy to carry on the business of doing work. Not only that, the digestive system doesn’t create it’s own energy. It’s dependent upon blood flow to deliver the energy substrates required for activity. As in the case with the firewood harvesting metaphor, the digestive system is responsible for making this large, unusable energy substrate more “bite-sized” for muscle metabolism. In order to do that it requires energy substrates of it’s own whose presence is not always guaranteed as you’re about to see. When exercising at ~80-85% Vo2max, your body’s circulatory network strategically redirects blood-flow away from the systems that aren’t immediately required to create movement. Unfortunately, the digestive system is on the short list of momentarily expendable tissues that draw the short straw in terms of blood-flow. This means that at these intensities (which are quite common in most endurance sports), whatever you’ve just eaten will sit in your stomach and wait for your digestive tract to reshuffle its internal priorities due to limited resources. It would be exactly just like the security line during peak airport hours with only one TSA agent working. What makes matters worse is the level of complexity of the food waiting to be digested. Under circumstances of low blood-flow, your digestive system will show preference towards smaller particles that require less mechanical work to breakdown, thus keeping energy status relatively constant even as energy demand (based on exercise intensity) starts to rise.
Let’s move back towards the original argument. Complexity versus simplicity. During the first 90mins of activity, your body is quite happy making due with what you have stored within the complex fabric of your muscle fiber. This means that the importance of the digestive tract is momentarily reduced. SO, if you were ever going to burden your digestive system with a complex meal, this would be the perfect (and only) time (Figure 1). As you continue, hour after hour, your reliance upon internal stores of energy starts to diminish and nutritional supplementation gains importance. At this point, especially if you are exercising at or above ~80% Vo2max, the simplicity of your food source must, absolutely and unequivocally, increase if you want remain active.
What’s more important is the shunting of blood flow away from the digestive tract when you are at race intensities. As if the situation couldn’t get any more dire, now there is a 40% reduction in blood flow leading to (and from) the digestive tract which is responsible for transporting vital nutrients away from the stomach and to the working muscles. This means that if the meal you just consumed is complex, it’s going to languish inside the digestive system until it can be shuttled away by the circulatory system. In a lot of cases this is “the” mechanism behind major gastrointestinal upset. Because most athletes tend to select more complex foods late in a race, the stomach simply isn’t equipped to handle the increase load and often reacts… sometimes violently. This, again highlights the need for simplicity without overburdening an already overburdened system.
In conclusion, keep your carbohydrates simplistic in nature. Avoid maltodextrins and other glucose polymers when exercise intensities are high and instead opt for simple sugars like honey, Coke, and other liquid based carbohydrate sources. These foods are incredibly simple to digest and are often broken down and absorbed before they ever reach the digestive tract. This lightens the workload placed on this critical resource without putting in jeopardy your ever-increasing need for energy.