HIIT cardio is any cardio training involving short periods of using full burst of energy for about 15 seconds up to a minute and then lower the intensity for 1-2 minutes recovering your energy level enough to do another burst. HIIT sessions generally run 10-30 minutes and is a great for people who work on their cardio but do not want to lose much of their lean muscle mass. It has been shown that doing steady state cardio will burn fat as well as lean body mass if done for an extensive period. Lean body mass is very slow and hard to develop so you could be burning up all the muscles you built doing steady state cardio. This is why you see so many marathon athletes looking skinny.
It is more energy efficient to do HIIT cardio after weight training as opposed to before because you want to use up all your glycogen stores on helping you lift heavier weights to develop muscle growth. You can still have a good cardio workout with your glycogen stores depleted.
Another reason why you want to train to have more lean body mass is because the more muscles you have the more calories you will burn. Taking BCAA’s before and during cardio workout would help against the catabolic effect.
HIIT cardio allows you to burn more calories in a short period of time. Studies have shown that 15 minutes of HIIT burns more calories than jogging for an hour. When doing cardio on an empty stomach it is beneficial to supplement with MCT oil to try to get your body to use fat as fuel. Go for MCT oil with mostly C8 caprylic which is better quality and more true of being a medium chain triglyceride as opposed to C12 lauric acid which is more of a long chain triglyceride.