Under normal circumstances (ketosis will be covered separately) and excepting the first few seconds of exercise, skeletal muscle utilizes glucose and fatty acids for fuel. A typical 70kg man has 1200 kcal of muscle glycogen (storage form of glucose), 400 kcal of liver glycogen, 100 kcal of glucose and 136000 kcal of triacylglycerols (storage form of fatty acids). [Cahill, Clin. Endocrin. Metab. 5:398] He burns 98 kcal per mile running at 12 min./mile and 102 kcal/mile at 8 min./mile. Therefore, he has enough glycogen to run at most 17 miles using only that as fuel, but enough fat to run more than 1300 miles. An elite marathoner having 4% body fat would still have enough fat to run 250 miles. There is never a need to ingest fat for fuel, though it may be needed for palatability.
Events of half-marathon or less in distance require less glucose than normally stored, so ingesting carbohydrates to supply glucose is not necessary. The argument that exogenous fuel allows for greater power generation, and therefore faster times, does not hold at this range of distances.
Resting muscle utilizes fatty acids almost exclusively. Muscle contracting at maximal force and rapidity utilizes glucose almost exclusively, as the amount of oxygen demanded outstrips the availability and fat cannot be burned anaerobically. In between these extremes, there is a smooth gradient, rather than an abrupt change between aerobic and anaerobic metabolism ("anaerobic threshold" being an imaginary construct). The faster one runs, the greater the demand for glucose. This glucose can be supplied by glycogen stores, by ingesting carbohydrate or by converting the amino acids of proteins (either ingested or from muscle and enzymes) - a process termed gluconeogenesis. Though glucose can be converted into fatty acids, fatty acids cannot be converted into glucose. Liver glycogen differs from muscle glycogen in that muscle glycogen cannot be replaced during exercise, so one limiting factor in distance running is the depletion of muscle glycogen.
If muscle glycogen depletion is limiting performance, it is logical to address it through training and/or diet. One can increase the initial level of glycogen by intentionally depleting it and then consuming a large amount of carbohydrate ("carbohydrate loading" "carbo-loading" "packing"). One can train to burn glucose more efficiently; in fact, nearly all run training does this to some extent. One can also try to circumvent glycogen utilization, either by ingesting large amounts of carbohydrate while exercising, by depriving the body of glucose for several weeks and forcing the body to adapt to ketosis, or by exercising at a very slow rate where glucose metabolism is negligible (i.e. walking). This last method contradicts the goal to finish as quickly as possible, but is often resorted to by those wishing only to finish an ultramarathon.
The next post will look at consuming carbohydrate while running.
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