HIIT has been shown to increase both fat and carbohydrate metabolism. In under 20 minutes, HIIT generates increased fat metabolism that extends well beyond the period of workout. To achieve the same effect doing steady state cardio, it will take upwards of 60 minutes.
To understand why HIIT is so effective we need to look at how the human body generates energy during exercise. There are basically three methods:
1) ATP-PC System, 2) Glycolitic System, 3) Aerobic Systems.
The details are complicated and best saved for another more detailed post, but for the sake of the question at hand its sufficient to know the function of these systems during exercise at a high level. First thing to note is that ATP (Adenosine Triphosphate) is the energy currency of the body and to supply energy during an activity, the body relies on creation of ATP molecules. These ATP molecules are acted on by the enzyme ATPase to break it down into ADP and Inorganic Phosphate molecule and release 7.3 Kcal per ATP molecule. Before ATP can release additional energy again, it must add back another phosphate group to ADP.
When you start an exercise, the ATP-PC system is triggered first and is capable of supplying energy for about 15-30 seconds. This system is anaerobic (does not rely on oxygen) and is fast, so the body prefers it at the onset of an activity. In this process, ADP (Adenosine Diphosphate) combines with a Creatine Phosphate molecule to form ATP and Creatine.
If the exercise continues beyond that time, the Anaerobic glycolytic system is triggered next, which uses Glucose (sugar) or Glycogen (stored carbohydrate) to release ATP. This leads to creation of Lactic Acid as a byproduct, which gives the burn feeling in the muscles. This system can provide energy for 30-50 seconds. Most traditional workouts with 6-12 reps fall within this time period.
At this point the Oxygen intake catches up to the energy demands and Aerobic processes for energy generation are initiated.
In the presence of Oxygen, instead of Lactic Acid, the byproduct is Pyruvic Acid which is broken down into Acetyl CoA. Acetyl CoA enters what is called the Krebs Cycle where it undergoes Oxidation and produces ATP molecules and byproducts Carbon Dioxide and Hydrogen. These Hydrogen ions work with Enzymes and lead to phosphorylation of ADP to form ATP.
Complete metabolization of 1 glucose molecule produces 35-40 ATP molecules. After about 90 minutes, muscle glycogen stores are fully depleted. Athletes can with training store up to 50% more glycogen for longer sustained activity.
When glycogen metabolization stops, the exercise intensity begins to sow as the primary energy supply switches from glycogen to fats.
Fat metabolization is also an Aerobic process. Triglycerides stored in fat cells are converted to Free Fatty Acids (FFAs), which is converted to Acetyl-CoA through the beta-oxidation process. This then enters the Krebs cycle described above to result in ATP molecules.
Different fat molecules from our diet may result in different number of ATP molecules but its typically 3-4 times as much as that produced from glucose.
Metabolism of fat takes more oxygen than Carbohydrates, so Carbohydrates are the preferred fuel for oxidative metabolism. In turn, the Aerobic metabolism of Carbohydrates and Fats take much longer than Anaerobic glycolysis and ATP-PC systems, but can produce energy for indefinite time.
Since Fat metabolism takes more oxygen than Carbohydrates, one way to estimate what is contributing to the fuel during an exercise is to measure the respiratory quotient (RQ), which is the amount of Carbon dioxide expired divided by the amount of Oxygen consumed using a metabolic analyzer. When the value is 1.0, it indicates that Carbs are supplying 100% of the fuel, while when it is 0.7 then Fats are providing 100% of the fuel.
An RQ between 0.7 and 1.0 indicates a mixture of Fats and Carbs. Note that Protein provides minimal contribution to energy production so it is ignored here.
This brings the concept of Fat burning zone, which basically says that we can burn more fat by exercising at lower intensity as this won't require energy to be produced quickly and won't trigger Carbohydrate metabolization, focusing instead on using more oxygen for a lower respiratory quotient and being Fat dominant as fuel.
Although it must be noted that by increasing the exercise intensity one can increase the energy demand and caloric expenditure, so that even with proportionately higher Carbs contribution (high RQ), the Fat contribution also is higher thereby leading to increase fat loss potential.
At the beginning of the article we posited that HIIT has been found to give the same fat metabolism as steady state cardio in one third the time. How does that work?
To understand this, we need to look at the findings of the research done by Larsen, Maynard, and Kent below.
Larsen, R. G., Maynard, L., & Kent, J. A. (2014). High-intensity interval training alters ATP pathway flux during maximal muscle contractions in humans. Acta physiologica (Oxford, England), 211(1), 147–160. doi:10.1111/apha.12275
I will leave you to read the details of their study but would like to draw your attention to this relevant discussion.
" The results of this study indicate that short-term HIT not only increased VO2peak and whole-body work capacity, but also altered skeletal muscle ATP flux in vivo during maximal-intensity contractions, such that a greater proportion of total ATP turnover was supported via aerobic metabolism. Using 31P-MRS, we showed that, while 6 training sessions had no effect on force production and overall ATP synthesis during the 24-s MVC, oxidative ATP production increased and the relative contributions from net PCr breakdown and non-oxidative glycolysis decreased. "
This basically says that by doing HIIT for a few sessions, the body is conditioned to prioritize ATP production through oxidative (Aerobic) systems, decreasing the contribution from non-Oxidative (Anaerobic) systems. We know from our earlier explanation of metabolic pathways that Aerobic systems lead to more fat metabolization.