The two forms of cardio that is often disputed is anaerobic vs. aerobic. Now, there are advantages and disadvantages to both, so why the dispute? Whichever one you need really depends on a variety of certain factors, such as: metabolism, goals, time and diet. On a ketogenic diet style diet towards the end of the week you want to avoid anaerobic cardio; This is because it can only burn glucose during the exercise. If you're doing a program that calls for loads of carbs, you can get away with doing either one.
"Aerobic" conditioning is enhancing the bodies ability to deliver oxygen to the mitochondria of a muscle on a protein molecule called "hemoglobin". When you perform a certain exercise that uses the oxidative (aerobic) energy system you are predominately using triglycerides as fuel. Your body produces energy inside of a muscle. When oxygen is present at the muscle, it is produced inside of the mitochondria of a muscle. As your body is being exercised, the heart begins pumping this blood to be delivered to the mitochondria through hemoglobin and the gas exchange (release of carbon dioxide and deliverance of oxygen) is made in the capillaries. The farther you go, the more the heart has become efficient at delivering this oxygen. These results in the inferior dimensions of the ventricles of the heart increasing; which in turn, increases the stroke volume of the heart and lowers the resting heart rate.
Even though the resting heart rate decreases, the bodies maximal heart rate would decrease too. In turn, the body instead just pumps more blood (stroke volume) instead of doing more work. Stroke volume refers to the amount of blood that leaves each ventricle during systole, which is the contraction phase of the cardiac cycle. The cardiac output, which is the amount of blood that leaves the ventricles in one minute, lasts the same. Basically, aerobic energy results in your body being able to become more efficient at pumping and delivering blood. When hemoglobin reaches the muscle, not all of the oxygen is released. Infact, the heart does not even pump all of its blood when it is resting. The "ejection fraction" of the heart explains how much blood leaves the ventricle during systole. Usually, if you are resting or not doing vigorous exercise, then it is about 50%. When you begin exercises, it becomes more than that and sometimes reaching 100%.
Lets take a look at how this benefits you. During rest time between sets of exercise, the body is more efficient at pumping blood; then, in return gives you more endurance and a faster recuperation rate between sets. Anaerobic exercise, like HIIT or weights, allows the muscle to store more glycogen as a form of endurance. What does this tell us? That both are beneficial and should be used. When your body stores more glycogen, you can squeeze out more reps. Think about it, why do you become more efficient at higher rep cardio training? Because you're using anaerobic glycosis to produce energy and your body realizes that it needs to continue producing energy and begins to store more glycogen, resulting in you being able to increase your reps as you become accredited to that activity.
Now, in a caloric deficiency HIIT can sometimes burn more muscle than LISS. This is because HIIT can only use glucose as energy. When your body reaches 50-85% of its maximum intensity, the anaerobic threshold kicks in and the body has to use glucose only. Glucose is simply known as blood sugar, or "carbohydrates" when they enter the body. When a deficiency of carbohydrates are present, then the body turns to the amino acids or phosphate creatine to make ATP (energy). On the other hand, HIIT lasts nowhere near as long as LISS, so you could argue that HIIT actually burns an insignificant amount of muscle in the comparison.
When you start using HIIT as cardio, you can easily notice 10 lbs lost in the first month, but do not mistake it all for fat loss. Glucose is a polar molecule, meaning it holds excess water in the muscles and results in water weight bloat. Seeing you're burning glucose during this specific exercise, you begin to go into a deficiency of the bodies normal supply of glycogen and pounds are dropped fast; it is water weight. The benefit to HIIT outside of using the anaerobic energy system, which burns more calories per unit of time, is the metabolic rate increase after the activity is over. Imagine doing cardio all day long, your body is constantly in a demand of energy and breaking down fat. You go in a glucose deficiency during HIIT, only to burn what is left (fat) through the rest of the day. That is what HIIT is all about. It is about making the equivalent results of doing lower intensity cardio all day long, by increasing metabolic activity, and resulting in better fat loss in half the time. This is because HIIT claims hormonal response from energy enhancing hormones like epinephrine and norepinephrine.
The conclusion of the topic is that you should do both if you are debating on which one. If you're in a glucose deficiency through calorie restrictions – like the anabolic diet or the keto, then perhaps LISS is better during the late week, but on the other hand, if you're confident in your reserve of muscle, HIIT is the better option most of the time. Try utilizing both, you'll love the results.