HIIT is becoming more and more popular for weight-loss each and every year. Hundreds of people are switching from long, boring jogs lasting around 45 minutes to an hour and instead doing HIIT for 10-20 minutes at a time. Most people when starting HIIT end up losing about 8-10 pounds in the first two weeks, they are significantly smaller and therefore, people are making the assumption it is “better” for fat loss and I won’t disagree.
HIIT, however, relies primarily on the anaerobic glycosis energy system; this energy system is dominant when the body can no longer produce energy inside of the mitochondria of a muscle; thus, aerobically. This results in anaerobic threshold occurring, usually within the 50-85% range of maximal effort. The anaerobic metabolism, regardless of current calories, cannot burn fat. Despite popular belief, regardless of whether or not you’re in a calorie deficit, the anaerobic energy system can only use glucose, which produces ATP outside of the mitochondria and is not the primary energy system for creating ATP.
If you are in a calorie deficit, the first couple of weeks HIIT is recommendable, due to the fact that is creates a lot of beneficial hormones like epinephrine and norepinephrine, it will significantly increase your metabolic rate and put you in a fat burning mode far beyond that of aerobic exercise – i.e., walking on an incline, slow jogs, etc. It will also burn away any exercise glycogen that may be causing bloat to your muscles; however, it can also burn muscle and it usually will.
When you get into about 2-3 weeks of cutting, you would ideally want to switch the cardio methods up. I always recommend the cyclical ketogenic diet for cutting phases, so early in the week after your carb up day you would ideally want to do HIIT, after-all, it is much funner and more convenient, right? Due to the anaerobic glycosis being dominant with HIIT, glucose can only be used to create ATP. Since glucose is stored in the form of glycogen in the muscles and liver, it creates extra water weight. Ever noticed how bloated you are after eating pasta? Brown rice? Oatmeal? It is because those are high in complex carbohydrates, that convert to glucose and later to glycogen. Glycogen is a polar molecule, which means it holds excessive water in the muscles, which is why water weight is always the first weight that is lost when you begin exercising. So in a physiological standpoint, you would ideally want to do HIIT your first few weeks of cutting.
It depends more-so on the goal and metabolism. If you have a naturally slow metabolism HIIT is probably preferable for you. It will burn more muscle, but it will also burn significantly more fat in a 24-hour period than lower intensity cardio. The downside is you will not receive the same benefits as conventional aerobic exercise, it will not increase your stroke volume, decrease your resting heart-rate, or increase your cardiac output under maximal effort exercise. It will, however, create stronger walls of your heart and allow you to train longer without a sufficient supply of oxygen to your exercising muscles. It is overall better for fat loss at the end of the day, but can only burn glucose during the activity; therefore, resulting in more energy expanded but at the expense of muscle being burned too. For a quick gradual cut, it’s superior.