LISS Cardio is the opposite end of the spectrum from HIIT cardio. LISS is primarily used when your glycogen stores are already low (think of after sleeping all night or after an intense weight training session). Long as the intensity is relatively light (65%-75% of max heart rate) your body will use FAT for fuel.
This type of cardio can be done any time of day but as I mentioned before there are 2 areas that are prime: immediately after waking up and after an intense weight training session. Another time would be 1.5 -2 hours after having some carbohydrates (this puts you in a reasonably fasted state). You don’t want your body using the previously eaten carbohydrates for fuel. Wait the additional time and force your body to use our fat.
Now why would you want to do LISS vs HIIT cardio? Think about it this way. LISS is very easy on the body. Almost like taking your car out for a leisurely stroll. Just cruising down the highway and enjoying the weather. HIIT is like running your car in a quarter mile at the track. It is a very intense session (no more than 20 minutes) and very rough on the body. Both have different goals in mind.
HIIT cardio primary function is to help RAISE your metabolism for the duration of the day. Therefore you would burn more calories throughout the day (hopefully causing a caloric deficit). LISS cardio, on the other hand, is designed to use FAT as fuel during exercise. There isn’t any “after burn” like HIIT. Lasts only during the duration of the exercise – that’s why LISS normally lasts 30-45 minutes whereas HIIT only lasts 20 minutes.
So I’m sure you are now asking yourself: which one should I do?? Well I would suggest using both. Use LISS cardio on days that you are going to weight train: first thing in the am and even potentially after you weight train. I would recommend starting off with three 30-minute sessions of LISS a week and gradually either add more sessions or longer duration to those existing sessions. But do not, I repeat do NOT, go over 1 hour for LISS in one session. As for HIIT, do it on days that you are not going to weight train. Eat a small meal 30-40 minutes before the session and be sure to eat immediately after completing HIIT.