Horowitz et al (Horowitz JF, Mora-Rodriguez R, Byerley LO,and Coyle EF. Substrate metabolism when subjects are fed carbohydrate during exercise) found that when trained subjects exercised at 50 percent of their max heart rate, an intensity that equates to a slow walk, there was no difference in the amount of fat burned--regardless of whether the subjects had eaten. These results held true for the first 90 minutes of exercise; only after this period did fasted cardio begin producing a favorable shift in the amount of fat burned.
So unless you're willing and able to slave away on the treadmill for a couple of hours or more, fasted cardio provides no additional fat-burning benefits, irrespective of training intensity.
Fasted cardio makes even less sense when you take into account the impact of excess post-exercise oxygen consumption. EPOC, commonly referred to as the "afterburn," represents the number of calories expended after training. Guess what? Eating before exercise promotes substantial increases in EPOC.
More EPOC equals more fat burned. This favors eating prior to performing cardio.
Plus fasted cardio can also hacve a catabolic effect on lean tissue. At best, the effects on body composition won't be any better than if you trained in a fed state; at worst, you'll lose muscle and reduce total fat loss.
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