At approximately 65 % V02 max, fuel is derived from approximately 50 % fats, and 50 % carbohydrates. Of this, fatty acids are derived in nearly equal measure from peripheral and endogenous ( intramuscular) TGs, while the majority ( 80 %) of glucose is derived from intramuscular fuels, with only 20 % from the periphery(Romijn et al. 1993). Of the 20 % glucose released by the liver, approximately 15 % of it is from gluconeogenesis (Ahlborg et al., 1974, Ahlborg and Felig, 1982, ). However, again such fuel use is also time dependent. For example, after two hours of exercise TGs become dominant from peripheral fuels compared to endogenous fuels. Twice the amount of peripheral fatty acids are used relative to intramuscular fatty acids which may be due to depleted intramuscular TG stores. Depletion of glycogen also increases use of peripheral fuels. Ahlborg and Felig (1982) performed a similar study to the Alborg et al. ( 1974) study, with the exception that exercise was performed at 59 % V02 max. Because exercise intensity was higher than low protocols( 30-40 performed in ALborg et al. 1974), 75 grams of glucose was released by the liver in 3 instead of four hours. After this amount of time, the level of liver depletion, like low intensity exercise, increases the rate of gluconeogensesis. This is in large part due to increasing cortisol levels due to lowered plasma glucose, as well as increased glucagon levels (see below). Up to
60 % of fuels from the liver at this time are produced by gluconeogenesis. During this study, plasma glucose levels rose from the onset of exercise and peaked at 90- minutes of exercise. However, by 3.5 hours of cycling, blood glucose levels had decreased to hypoglycemic levels. The decline was most rapid from 120 – 180 minutes, and was associated with the lowest outputs of glucose from the liver, due to the extreme and catabolic reliance on gluconeogenesis. Further, the fall in plasma glucose was 40 % lower after 3 hours than it was at low intensity exercise.