As I mentioned in my previous post on time control plans, our intended objective in the Tetons may require 20+ hours of sustained effort. In order to fuel this effort, we will need a steady supply of calories, ample fluids, and a whole lot of psych. Precisely how much of each of these ingredients is still a bit unknown, though, as there is not a whole lot of data out there on the nutrition appropriate for endurance alpine climbing. However, there is ample data on other endurance events, particularly things like ultra-running and Ironman triathlons. Both of these events last far longer than the typical marathon, and therefore require the athlete to push past their limits of stored glycogen, just as in alpine climbing. Consequently, I have selected them as complements to the limited data that exists regarding climbing nutrition. Using this data, I have formed a few conclusions about the best way to fuel while on the move.
Fueling Your Cells: A Primer
I’ll start with a brief lesson on how your body fuels itself. The body’s most readily available source of energy is glycogen, which is simply stored glucose (sugar). The typical person has enough stored glycogen to fuel around 2 hours or so of sustained aerobic effort. Once glycogen is depleted (the dreaded “bonk”), the body has to burn fat for energy. Unfortunately, the conversion of fat into cellular fuel is a much slower process than that of converting glycogen into glucose. Consequently, this energy pathway is only viable for moderate exertion, up to about 70% VO2 max or 80% max heart rate. Surges of high exertion require glycogen. This is why many endurance athletes “carbo-load” before their races and events, to maximize their stock of glycogen (carbohydrates are broken down into sugars and stored as glycogen).
This means that in order to sustain effort beyond the body’s stored glycogen supply, carbohydrates must be consumed during the activity to replenish the glycogen stores. This comes with a caveat, however. Digestion requires energy, but typically the athlete wants to devote as much energy as possible to the event, not digestion. Therefore, there is an optimal threshold at which calories can be consumed, digested, and absorbed by the intestines without detrimentally impacting performance while still maximizing caloric intake.
When operating at steady-state moderate exertion, the current recommendations are to consume about 250 - 350 calories an hour. Beyond that threshold energy must be dedicated to gastric motility instead of moving, and as an athlete on a mission in the mountains, you presumably want all power to the engines. These calories should come in the form of carbohydrates, though there is some evidence to suggest that small amounts of protein may be beneficial as well, especially for activities like climbing.
Fluid Intake Requirements
In addition to these carbohydrates, water needs to be consumed at a rate of between 16oz and 40oz an hour, depending on temperature, sweat output, and body size. Anywhere from 8 to 12oz of this water should be consumed in conjunction with the aforementioned carbohydrates. In addition to water, 200-300mg of sodium an hour will replace the sodium lost in sweat as well as encourage both water and carbohydrate absorption in the small intestines. Finally, small amounts of caffeine can help with alertness and increase athletic performance.
Although it is possible to intake fluids and calories at the appropriate rate, this does not guarantee that once ingested they will be emptied from the stomach rapidly or absorbed effectively by the small intestines. The most effective means for ensuring rapid gastric emptying and maximal intestinal absorption is to consume the carbohydrates as a fluid in a solution with water. This is the premise behind most sports drinks and some sources even suggest that endurance athletes should perform on purely liquid diets.
The caloric drink consumed should be between a 6% and 8% solution of carbohydrates in water, depending on the particular carbohydrate(s) used for the beverage. This solution mimics the make-up of fluid in the body (it is isotonic) and will result in the most efficient absorption. Solutions that are less concentrated (hypotonic) slow absorption because the body must first add electrolytes to the fluid in the intestines. Hypertonic solutions, or those with greater concentrations of carbohydrates, require the body to first add water to the solution before absorption, encouraging dehydration. If solid food is ingested, it should always be taken with water to help combat the effect of a hypertonic substance entering the small intestines.
These are the basic principles behind nutrition while on the move. In my next post I’ll discuss what this all means for my particular nutritional plan while in the Tetons.