Found this link on Supertraining-- it points to an MFW discussion regarding interval training specifically for fat loss.
I've been saying for awhile now that shorter, harder intervals are better for fat-burning than using 20 or 30 second bursts, and this seems to lend to that argument, but with a bit of a twist.
The study notes that, given equal treadmill speed and work duration, fat oxidation was 3 times lower in the group that used 24 second work intervals and 36 second rest intervals, compared to a group using 6s/9s ratios.
"The decline in muscle oxygenation during work was correlated
with mean lactate concentration (r = 0.68; n = 12). Lower
levels of fat oxidation occurred concurrent with accelerated
carbohydrate metabolism, increases in lactate and pyruvate and reduced
muscle O(2) availability. These changes were associated with
proportionately longer work and recovery periods, despite identical
treadmill speed and total work duration. The proposal that a metabolic
regulatory factor within the muscle fibre retards fat oxidation under
these conditions is supported by the current findings."
The discussion goes on to mention myoglobin, the oxygen-binding protein in muscle. Myoglobin provides short bursts of oxygen in anaerobic conditions. That seems to be the key here; lactate blocks fat oxidation, leaving myoglobin to fuel the oxidative process. Once it gets used up, fat utilization is shut off in favor of glycogen metabolism.
Now, I'm not exactly sure what protocol was used in the study mentioned; I also don't know what "optimal" work:rest ratio exists. However, it is compelling information to keep intervals short and hard, using ample rest periods.