At first glance, ketosis sounds like something to avoid at all costs, as it is the state seen in diabetes and in starvation. Yet, it is also the state seen in the depletion stage of carbohydrate loading (last post) and has been repeatedly touted in various form for various reasons since the Atkins diet craze. Can it be used to bypass the running limitation of glycogen depletion?
The body has an absolute need for glucose (red blood cells, for example, can use no other fuel), but if it is not supplied by diet, there are few reserves; muscle glycogen, once formed, cannot be converted to glucose and exported to other tissues. In one day, all the reserves of glucose are depleted. Yet the brain requires 150 grams of glucose. To get this, the body converts protein into glucose through a process called gluconeogenesis. Because historically survival depends upon being able to move rapidly, which requires muscle mass, using protein for fuel can only be a temporary fix.
After three days of glucose depletion, the fatty acids being used for fuel begin to overwhelm the usual pathway of degradation and a secondary process begins, which generates products called ketone bodies (from which "ketosis" and "ketogenic diet" are named). The brain is able to use ketone bodies for fuel, though glucose is preferred and by the third day, one third of the energy usd by the brain comes from ketone bodies. By six weeks, the brain's need for glucose is only one-third what it was before glucose deprivation.
Muscle can also use ketone bodies for fuel in place of glucose. So, by several weeks of adapting to having little glucose, fat is replacing sugar, even for anaerobic needs! This does obviate the glycogen storage problem, but there are considerable drabacks. For one, diet is limited, consisting largely of meats and vegetables, and has to be carefully monitored to ensure essential vitamnis and minerals are being consumed in adequate amounts. Second, one has to stay on the diet for a very long time without straying. Third, the first days on the diet are often miserable, as the brain, deprived of its usual food, causes one to become irritable and sluggish.
So, in conclusion of the series, continuous high carb fuel during races is best for fast 100 mile races and longer, glycogen loading is preferred for marathon and 50K, a combination of the two is preferred for slow 100s and 50 milers, ketosis may be best for fast 50 milers and none of these are important for races of half-marathon or less.