The article below by Mark J. Smith, Ph.D pulls together some extensive research on high intensity exercise as compared to low intensity exercise and proposes some interesting conclusions including…

You can lose more fat through sprint training compared to continuous running even if the total energy cost of the continuous running is higher than the sprint training.

Three 1 minute bouts of maximum intensity exercise with an hour rest in between produces 75% of the oxygen up take created from 20 minutes of low to moderate intenstiy exercise. This gives some scientific backing to the idea of doing sets throughout the day (or what Pavel calls Greasing The Grove GTG)

Good cholestrol improves with high intensity exercise but not low intensity exercise provides no such benefits. High intensity exercise alsoimproves plasma glutimine levels, and beta endorphin levels - something which low intensity exercise also fails to achieve.

Chronic over training is bad.

Quote:

“The effect of a 20-week endurance-training program (mean estimated energy cost - 120.4 MJ) upon body fatness and muscle metabolism was compared to a 15-week high-intensity intermittent-training program (mean estimated energy cost - 57.9 MJ)16. Despite the lower energy cost of the high-intensity program, it induced a more pronounced reduction in subcutaneous fat compared with the endurance program. When corrected for the energy cost of training, the reduction induced by the high-intensity program was nine-fold greater than the endurance program. Muscle biopsies taken before and after the training programs revealed that the high-intensity intermittent-training program increased the enzyme activity of a betaoxidation (fat metabolism) marker. It was concluded that for a given level of energy expenditure, vigorous exercise favors negative energy and lipid balance to a greater extent than exercise of low to moderate intensity.

another study that demonstrated that three 1-minute bouts of maximal intensity exercise, separated by 1-hour recoveries, constituted 74% of the oxygen uptake of 20 minutes of low- to moderate-intensity exercise. The idea of extending recoveries to allow for maximal performance in subsequent bouts of intermittent training may be useful for training anaerobic capacity. It has been shown that following 30 seconds of maximal isokinetic cycling, 4 minutes of recovery is sufficient to almost completely restore AdenosineTriphosphate (ATP) and mixed-muscle phosphocreatine (PCr) in type I muscle fibers, but not type II muscle fibers. Furthermore, the restoration of ATP and PCr correlated positively with total work production of a subsequent 30-second bout. Consequently, the inclusion of training sessions in ones overall program where recoveries are extended to allow for total restoration of type II muscle fiber ATP and PCr content may improve muscle training for highintensity performance.

It has been shown that increases in high-density lipoprotein cholesterol levels (the “good cholesterol”) have occurred as a result of intermittent exercise, but not as a result of continuous exercise. Beta-endorphin levels, which are associated with positive changes in mood state, have been shown to increase following incremental graded and short term anaerobic exercise, the extent correlating with the lactate concentration. However, in endurance exercise performed at a steady-state between lactate production and elimination, blood beta-endorphin levels do not increase until exercise duration exceeds approximately 1 hour with the increase being exponential thereafter. Plasma glutamine, an essential amino acid for the normal functioning of the immune system, was decreased in over-trained athletes and after prolonged exercise (intermittent and continuous) but increased after short-term, high intensity exercise. The total antioxidant capacity of marathon runners cannot prevent exercise-induced lipid peroxidation following a half-marathon run, and at rest have demonstrated significantly elevated levels of conjugated dienes (an index of lipid peroxidation) as compared to sprint-trained athletes and controls. These latter findings all lend support to some benefits of high-intensity exercise that might not normally be considered.”

Complete article here: http://xiser.com/Smith-HIIT.pdf