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HIIT's afterburn doesn't change fat loss
The afterburn
Picture ten one-minute sprints separated by one-minute jogs that burn about 400 calories. In the hours that follow, there seems to be a small increase in metabolic rate. It isn't enough to matter. In meta-analyses and tightly controlled studies that equate work between high-intensity intervals and moderate or low-intensity cardio, there are no differences in loss of body fat.
Mechanisms are snapshots
A mechanism says the afterburn should lead to more fat loss, but a mechanism is a snapshot in time. One possible explanation is a rise followed by a decrease that washes it out, though that is speculation.
Changing his mind
This is a case where Layne Norton changed his mind. With a biochemistry degree he was drawn to mechanisms, and his nutrition advisor kept asking what the outcome would be.
How big is the afterburn effect?
The afterburn is formally called excess post-exercise oxygen consumption. In a 2003 review in Sports Medicine, Elisabet Børsheim and Roald Bahr found that it adds roughly 6 to 15 percent to the energy cost of the workout itself. After a session that burns 400 calories, that is on the order of 25 to 60 calories, spread across the following hours. Intensity and duration both raise it.
Why do people still use sprint intervals?
In a 2016 study in PLOS ONE, Martin Gibala's group at McMaster University tested a ten-minute workout with three 20-second all-out sprints. After six weeks, cardiorespiratory fitness matched that of a group doing 45 minutes of steady cycling. The sprint group spent about a fifth as much time exercising. Fitness benefits are separate from the fat loss question.
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