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At rest, muscle burns fat
Fuel At Rest
Muscle at rest uses only two to three grams of carbohydrate an hour, so it is not the most metabolically active organ, despite the common claim. The brain, with its obligatory fuel use, burns far more. Activity changes the picture: movement ranges from about 30 grams of carbohydrate an hour at a slow walk to about 70 grams an hour as heart rate climbs.
The Glucose Test
A sedentary person can dispose of around 40 to 50 grams of carbohydrate over two hours without sending insulin and glucose out of range. An oral glucose tolerance test uses a 75-gram load over those same two hours, and blood sugar that stays elevated counts as prediabetes or diabetes.
The Daily Carbohydrate Load
The average American eats about 300 grams of carbohydrate a day, while the RDA recommends 130. That works out to roughly three glucose tolerance tests a day, and about 75 percent of people are not meeting baseline exercise recommendations. Carbohydrate tolerance is the amount a person can eat without deranging metabolism, and it depends on how much the muscle is being used.
Why does resting muscle prefer fat?
In 1963, Philip Randle and colleagues at the University of Cambridge described a competition between fuels in muscle. When fatty acids are plentiful and being oxidized, the cell slows its own glucose burning. When fatty acids are scarce and insulin is high, glucose takes over. The back-and-forth became known as the Randle cycle, or the glucose-fatty acid cycle.
Why does the glucose test use 75 grams?
The World Health Organization standardized a drink of 75 grams of glucose in water, followed by a blood draw two hours later. A two-hour reading of 200 milligrams per deciliter or higher counts as diabetes, and 140 to 199 counts as impaired glucose tolerance. The fixed load lets results be compared across clinics and studies.
How does exercise move glucose without insulin?
Contracting muscle can pull in glucose through a route that bypasses insulin. Calcium release and the energy sensor AMPK trigger GLUT4 transporters to move to the cell surface, as shown in work from Erik Richter's lab at the University of Copenhagen. Muscle keeps taking up glucose this way during exercise even in people whose insulin signaling is impaired.
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