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High Intensity Interval Training to Maximize Fat Burning

Updated: Apr 30, 2023

Medium read.

There are many methods people use to try and maximize fat burning. Manipulating diet and traditional steady state training are very effective, however, there are other tools that can be used in the fat burning arsenal. One of which is high intensity interval training (HIIT).

During exercise, we switch between anaerobic metabolism that breaks down phosphocreatine (PCr), glucose, or glycogen to produce adenosine triphosphate (ATP). This type of metabolism lasts up to 3-minutes, and when more ATP is needed for exercise, our aerobic metabolism (aka oxidative phosphorylation) takes over and uses carbohydrates and fat to produce ATP. And this is the energy system we need in order to burn and loose fat.

During the work portions of HIIT, the ATP needed is derived from anaerobic metabolism using carbohydrates. At intensities of at least 85% of our maximal oxygen uptaek (VO2max), up to 97% of the energy is obtained from carbohydrates, while fat use decreased drastically to the point where it was almost 0 (2).

I know what you may be thinking, how can working at such a high intensity that does not burn fat help you burn fat? The answer to this lies in the muscle adaptations from HIIT.

What separates HIIT from steady state cardio training is the greater magnitude to which our muscles are used, how much ATP is produced, and how all of this takes place over such a short amount of time. With such high-power outputs needed to perform such intense exercise, our muscles are highly taxed to a point much greater than a treadmill run of 30 minutes at a constant pace. In addition, our post exercise oxygen consumption is elevated in order to supply oxygen to the muscle during the recovery portion, to promote the res-synthesis of usable ATP. This gives the appropriate stimulus we need to adapt so that we can better handle the next HIIT session.

Some of the adaptations happen in our muscles after only a few weeks of HIIT training and include the following. Enhanced oxidative capacity, enhanced fat oxidation, and increased time to exhaustion in performance. These adaptations are attributed to factors that include, better anaerobic and aerobic enzyme functioning, increased mitochondrial enzymes, proteins, and biogenesis; and reduced lactate production (1).

Oxidative metabolism takes place in the mitochondria, and when ATP levels are low within the muscle, as what happens after 30 or 60 seconds of an all-out sprint, a wide range of proteins and enzymes trigger the mitochondrial protein peroxisome proliferator activated receptor y co-activator (PGC-1a). This protein is considered the master regulator for mitochondrial adaptations. The adaptations that come from the activation of PGC-1a enhances the ability of mitochondria to be able better perform oxidative metabolism using both carbs and those fatty acids.

In a study that used a 6-week HIIT protocol of 10 4-minute intervals with 2-minute rest at 90% of their VO2max for 3 days a weeks and showed a variety of adaptations, all of which were beneficial (3). Perry et al. (2008), measured muscle enzymes, metabolites, glycogen, fatty acid transporters, hormone sensitive lipase, mitochondrial proteins and enzymes, energy levels, and blood markers. They looked at these values at three points after the 6 weeks of HIIT: at rest, cycling at 90% VO2max, and cycling at 60% of the participants VO2max.

At rest, they found more fat utilization. During the 90% VO2max cycling test, time to exhausting increased, increased ATP/ADP ratio (indicating more available energy), along with reductions in creatine, inorganic phosphate, and glycogenolysis (which can all promote fatigue). These adaptations help to regulate the amount fatigue producing byproducts of anaerobic metabolism, which helps to increase more oxidative phosphorylation and a greater oxidative capacity during high intensity exercise, improving time to exhaustion. Even at exhaustion, a major limiter of fat burning (acetyl-carnitine) was reduced, increasing the potential to burn fat after high intensity exercise.

It is well known that fat gets burned during lower intensity exercise. The protocol used in this study was able to increase the fat oxidation during cycling at 60% of the VO2max by 12% while reducing the amount of carbohydrate utilization by 16%. The proposed mechanism was that HIIT protocol increase the fatty acid transport proteins FAT/CD36, FABPm, and reductions in acetyl-carnitine content within the muscle. These help to increase the amount of fatty acids in the muscle as well as the amount of fatty acids that can enter the mitochondria so that the fatty acids can be broken down for ATP production.

In addition, even lower lactate was found in the blood, which is important because high concentrations of lactate in the blood actually promotes triglyceride synthesis from the broken down fat, which is the opposite of what we want (3). The more-fatty acids in the blood, the more we can use and burn within the muscle.

HIIT should be a part of everyone’s training program in some way, and it can be tailored to fit into your program based on your goals and personal fitness level. However, it is imperative to go through the work bouts with max effort to be sure to obtain the adaptations. Unfortunately, this type of workout requires you to suffer for it work, and this is not the type of exercise that can be done for long training blocks or cycles. Doing so would increase the risk of overtraining. Fortunately, it seems that only 2 weeks is needed to produce adaptations (1).

During this cycle, performing 1-3 days of HIIT training can be impactful. During those sessions, opt for 30-60 seconds of work in which your heart rate is at least 80% of your estimated heart rate max, separated by between 1-4 minutes of recovery (1). These numbers can be adjust based on experience and goals, but the principle is simple, put yourself through a short period of hard and high intense work followed by a period of rest.

Another great method that can be incorporated is polarized training (1). This method has 75-85% of the training volume at low to moderate intensities, and the remaining 25-15% of the training volume using high intensity intervals. This is a great way to still obtain adaptions without running the risk of excessive overtraining since a majority of the training volume with this method is low to moderate intensity training.

In summary, HIIT, when performed and programmed correctly, can be a great way for you to help increase your fat burning capability while also improving performance during short term high intensity exercise.


  1. Gibala MJ, Little JP, Macdonald MJ, Hawley JA. Physiological adaptations to low-volume, high-intensity interval training in health and disease. The Journal of Physiology 2012;590(5):1077–84.

  2. Peric R, Meucci M, Nikolovski Z. Fat Utilization During High-Intensity Exercise: When Does It End? Sports Medicine – Open 2016;2(1)

  3. Perry CG, Heigenhauser GJ, Bonen A, Spriet LL. High-intensity aerobic interval training increases fat and carbohydrate metabolic capacities in human skeletal muscle. Applied Physiology, Nutrition, and Metabolism 2008;33(6):1112–23.

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