Some scientific studies have suggested, and anecdotal evidence supports, the idea that muscle glycogen can be completely spared by ingesting carbohydrate during exercise. There are two conditions: 1) Energy must be consumed at the same rate as it is being used and 2) A minimum of 70-100 grams of carbohydrates (roughly 300-400 calories) per hour must be consumed. How much of this range is due to experimental error is unknown, as is how much of the variation is trainable. From the few people who have told me exactly what they consumed in 100 mile or 24 hour races, these numbers appear correct for the fastest runners; finishers of the Superior Sawtooth 100 Mile manage 6000-7000 calories consumed during the event regardless of finish time. For a 150 pound runner, who expends 100 calories per mile and is consuming only packaged gels (which range from 90-130 calories each, depending on brand), this is one gel per mile, or three per hour for the winner and two per hour for the middle of the pack finisher.
If one consumes less than this amount, one impedes but does not stop glycogen depletion. If one consumes more than this amount, the excess glucose is converted to fatty acids (if one is accustomed to consumption during exercise) or can lead to vomiting or diarrhea (if one is not). Because there appears to be a limit to the amount of glucose that can be absorbed by muscle of 1 gram per minute (according to Noakes), much of the glucose ingested is converted to fatty acids even when calorie intake and expenditure are balanced; because muscle can use fatty acids for fuel in aerobic conditions, this is not a problem for the athlete.
This leaves questions about the type of carbohydrate to ingest and what becomes of fat and protein that are ingested. Starvation studies have shown a maximal loss of 75 grams of protein per day, which means that if one included no protein, one could lose 3/4 pound of muscle in one day, which would have little effect in a one day event, but could be catastrophic in multiday events. 300 calories of protein in one day among 7000-10000 of carbohydrate is negligible and will have little effect upon carbohydrate absorption; it is not known at what level protein intake would interfere with carbohydrate absorption, but it is unlikely to occur. There is a requirement for about 16 grams of omega-6 fatty acids and 1.7 grams of omega-3 fatty acids per day, which is about 200 calories total; omitting this for one day would not affect race outcome. Greater fat intake slightly slows carbohydrate uptake, but is not likely to be problematic. In short, eat what is enjoyable, but make sure to get sufficient carbohydrate.
Not all carbohydrates are absorbed the same way. Fiber is carbohydrate that is not absorbed at all. Other starches and sugars get absorbed at different rates, the rates being compared by "glycemic index." If one is ingesting only slowly-absorbed carbohydrates, one should begin hours before the race. Runners prefer the fastest absorption possible, which is found in converted corn starch (maltodextrin), cooked potato starch, sucrose (table sugar), maltose, glucose (corn sugar) and some baked goods (such as white bread). These carbohydrates require water to be absorbed, which due to the large amounts being ingested, can lead to problems of osmolality; maltodextrin has beome the fuel of choice, as it is most rapidly absorbed and has less effect on osmolality than simple sugars; its greatest drawback is its low solubility, which means that it must be mixed with water long before being consumed - this is why it is packaged in gel form. Flavor is also a consideration; maltodextrin has minimal sweetness even at high concentration, glucose/dextrose has a slight sweetness (which I find "artificial" and unpleasant), sucrose and fructose are very sweet and can be cloying when consumed in large amounts for long periods.
Fructose is not converted to glucose by the body except when it is 10 to 20-fold more prevalent than glucose, which does not normally occur, and therefore does not get incorporated into glycogen (though it can be used as fuel by muscle); it is largely taken up by fat cells, which incorporate it into triglycerides, which can then be used by muscles. Sucrose is cleaved in half by enzymes or stomach acid into glucose and fructose, so one half is not put into glycogen. As glycogen is not created during exercise, this is only a consideration when preparing for a race, not for the event itself.
Next post: changing glycogen levels and efficiency of its use through training and diet.