THE NATURE OF THE FOODSTUFFS OXIDIZED TO PROVIDE ENERGY IN MUSCULAR EXERCISE

Abstract

The authors take up the question as to whether or not muscle can use fat directly as a source of energy or accepts it only after it has been changed to glucose. In the work of previous authors, if the period were short or the exercise mild the R.Q. approximated unity, while with more severe or prolonged exercise the R.Q. tended to fall. The authors'' own experiments are not in agreement with the statement that the R.Q. approaches unity in short or mild exercise in normal animals on any diet. They report experiments in which dogs were given mild exercise usually of short duration. This exercise caused a 3- to a 4-fold increase in O consumption and were, with one exception, carried on in the post-absorptive state. "Normal," "high fat," and "high carbohydrate" diets were used. The R. Q. was obtained during exercise and recovery until the latter was complete, and in rest periods before and after. The R. Q. of excess metabolism of exercise and recovery correspond closely to the resting quotients. On a 45- and a 15-min. exercise the percentages of carbohydrate oxidized in the excess metabolism were: On a "normal" diet, 61 and 55 %; on a "high carbohydrate" diet, 70 and 100 %; and on a "high fat" diet, 13 and 15%. They conclude that even on mild exercise of short duration carbohydrate is not exclusively used and that the direct oxidation of fat as a source of energy is more probable than its indirect utilization by first being changed to glucose.