Iíve been doing a lot of research about HIIT and decided that I would post some of it since there are often a lot of questions about HIIT.
HIIT is a type of cardio where a cycle of short (30-60 seconds) intense work intervals followed by a recovery interval is used. Work intervals are at maximum intensity. For example, 30 seconds sprinting followed by 30 seconds of light jogging. This work/rest cycle is usually repeated for ~15-20 minutes. HIIT is very intense and can only be carried out for short periods of time (15-20 min. is usually considered optimal). Because HIIT sessions are shorter than traditional cardio sessions, the energy expenditure during a HIIT session is considerably less than that of a longer moderate intensity cardio session. The benefits of HIIT are reaped after the session is over, not during it.
HIIT is a unique form of cardio in that it increases post-exercise energy expenditure (it increases your metabolism). The only other way that basal metabolic rate (BMR) can be increased is by increasing muscle mass (eg weight training).
HIIT is a very effective way of reducing body fat. When compared to traditional longer duration moderate intensity cardio, fat loss in subjects using HIIT was 9 fold greater. Also, the value of (change in subcutaneous skinfolds)/(calories burned during exercise) is much higher for subjects using HIIT. In short, this means that you are burning more fat doing less cardio.
The effect of HIIT on energy metabolism is quite similar to that of many thermogenic supplements (ephedrine, caffeine, capasaicin, and Yoshioka). The thermogenic effect of these supplements is due to stimulation of the sympathetic nervous system. Thermogenic supplements affect every cell in the body, and thus have some undesirable side effects such as increased heart rate. There is evidence that HIIT increases the number of B-adrenergic receptors in skeletal muscle cells while having the opposite effect in cardiac muscle. This makes the skeletal muscle cells more sensitive to sympathetic stimulation while not affecting cardiac muscle. Therefore, the thermogenic effect of HIIT occurs in skeletal muscle and is due to the upregulation of B-adrenergic in skeletal muscles.
~(Tremblay &Doucet 1999) (http://fiordiliji.ingentaselect.com/...999mn=Febpg=99)
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HIIT and nutrition:
HIIT is a type of anaerobic exercise. The primary source of energy for short intense bursts of energy such as the work intervals in HIIT is creatine phosphate and muscle glycogen. Creatine phosphate is usually used up in the first ~10 seconds. After that, muscle glycogen is used. For this reason, HIIT should be done when muscle glycogen levels are high (not on an empty stomach first thing in the morning). Post workout should contain adequate carbohydrates and protein.
Since the only energy source HIIT can effectively rely on is muscle glycogen it is also very muscle sparing. Therefore it would make sense to incorporate HIIT if trying to loose fat without loosing muscle in the process.
The basics for designing a basic HIIT routine can be found at: http://www.exrx.net/FatLoss/HIITvsET.html
It should also be noted that HIIT is very demanding and has a higher risk of injury and overtraining than other forms of cardio. This should be taken into consideration when developing a HIIT routine.
Hopefully that will help some people. I know I learned a ton doing the research.